Bio-oil from mico algae

Micro algae
Microphytes or micro algae are microscopic algae, typically found in freshwater and marine systems. They are unicellular species which exist individually, or in chains or groups. Depending on the species, their sizes can range from a few micrometers (µm) to a few hundreds of micrometers. Unlike higher plants, micro algae do not have roots, stems and leaves. Micro algae, capable of performing photosynthesis, are important for life on earth; they produce approximately half of the atmospheric oxygen and use simultaneously the greenhouse gas carbon dioxide to grow photoautotrophically. Micro algae have higher photosynthetic efficiency, larger biomass, faster growth and higher content of components preferable for pyrolysis compared to those lignocellulosic materials and been suggested as very good candidates for fuel production.
Pyrolysis
Pyrolysis is a most efficient process for biomass conversion and it can compete and replace nonrenewable fossil fuels. Pyrolysis process can be tuned to yield char, liquid or gas as products. Liquid bio-oil obtained from pyrolysis can be readily stored or transported and has lower nitrogen and sulfur contents which favor its use as transportation fuel. In recent years fast pyrolysis process for biomass has attracted a great attention for maximizing liquid yields compared to slow pyrolysis as almost no flowing bio-oil products were directly produced from slow pyrolytic process.
Fast pyrolysis
Fast pyrolysis is a high temperature process in which biomass is rapidly heated in the absence of oxygen. The essential features of a fast pyrolysis process are very high heating and heat transfer rates, carefully controlled pyrolysis reaction temperature of around 500◦C, short vapor residence times of less than 2 s and rapid cooling of the pyrolysis vapor. Further a greater amount of high quality bio-oil can be continuously produced.
Mico algae bio-oil
Bio oil can be produced using fast pyrolysis from mico algae at temperature of 500◦C with a heating rate of 600◦Cs−1 and the sweep gas (N2) flow rate of 0.4m3/h and a vapor residence time of 2–3 s. The bio-oil yields vary from 17.5 to 23.7%. By continuously processing micro algae by the fast pyrolytic process high-quality bio oil can be obtained. The process is time saving and requires a lower energy input compared to the slow pyrolytic process. The contents of carbon and hydrogen of bio-oils from micro algae are higher than those of oil from wood. The bio-oils of micro algae are characterized by low oxygen contents with higher H/C ratios than the bio-oil from wood, sunflower bagasse and cotton straw and stalk. The decrease in the oxygen contents of bio-oils from micro algae compared to the bio-oil from higher plants such as wood is important because the high oxygen content is not attractive for the production of transport fuels Low oxygen content of mico algae bio-oils makes it more stable than the bio-oil from wood. Compared with the bio-oil from wood, the bio-oil from micro algae has higher heating value (about 1.4 times of that of wood), lower viscosity and lower density. These physical properties of bio-oil of micro algae make it more suitable for fuel oil use than pyrolysis oils from lignocellulosic materials.

Global biomass gasification research activities

Gasification
Gasification has been receiving attention throughout the world. Based on the current stage of development of thermochemical conversion technologies, gasification provides potential for near-term deployment, while pyrolysis will help to meet longer-term biofuels goals and in providing a route to renewable gasoline, diesel and jet fuel. Biomass gasification is a complex thermochemical process that begins with the thermal decomposition of a lignocellulosic feedstock. This is followed by partial oxidation or reforming of the fuel with a gasifying agent—usually air, oxygen, or steam—to yield raw syngas. The raw gas composition and quality are dependent on a range of factors, including feedstock composition, type of gasification reactor, gasification agents, stoichiometry, temperature, pressure, and the presence or lack of catalysts.
Work is being carried out on the gasification of variety of biomass such as municipal solid wastes, agricultural wastes and forest residues for different applications such as heat/power generation, production of syn-gas, methane, and hydrogen etc.It is reported that in the near future, the use of gasification to convert solid wastes into ready-to use fuel could literally solve several environmental problems at the same time.
A great number of small-scale fixed bed gasifiers are available around the world. So far successful applications have been seen in Finland, and Denmark where the gas is used for combustion in a boiler. Also other countries have demonstrated small fixed bed gasifiers with success for 1000 hours of operation in a year for power production. Biomass gasification devices aboard generally are large-scale, of high automation degree with complex techniques, and concentrated on power generation and thermal application. Their gasification efficiencies can reach 60-90%, and combustible gas has a caloric value of 17-25MJ/m3.
In the early 1980’s, rice husk-based gasification device was developed in China, using a down-draft fixed-bed gasifier of volume varying from 60 kW to 160 kW, which were applied in the local food industry and were also exported. The biomass power plant developed in China produces a valuable gas named producer gas - Syngas (Synthetic gas). The biomass (agricultural), used for this purpose is straw like materials such as: Rice stems, rice husks, cotton stalks, corn stalks, millet stalks, wood dust, cane trash, wheat straws, hemp palm husks and other forms of biomass. This system can produce electric power in the range of 200kW 6.0MW by using a few parallel modular units.
Since Biomass itself is almost sulfur free, the gasification process will not produce any sulfur oxide emissions (SOx). However, fuels such as diesel and HFO contain a considerable amount of sulfur. HFO could have as much as 4% sulfur and therefore requires special additional treatment to neutralize these emissions.
Similarly the gasification of biomass produces very low concentrations of nitrous oxide emissions (NOx) when manufacturing the Syngas and thereafter leads also to low emissions in the combustion engines.

In other EU countries, electricity from biomass is an option only lately starting to be considered by Greek companies. However, the currently used gasification technologies are still far from satisfactory. The main challenges faced is non-stable gas production process caused by local hot spots existing in the gasifier, non-flexibility to diverse biomass types, difficulty of scale-up, and low quality of product gas. Despite the great number of developments at different industries and the pilot plants available around the world, there are only a few that achieve a commercial operation.

Advanced technical level on the field of producer gas has been mastered by many countries such as Sweden, the United States, Italy, and Germany. In recent years, the United States had a breakthrough in biomass pyrolysis gasification, and researched and manufactured a set of biomass comprehensive biomass gasification set with gas turbine generation system for large-scale generation.

Country wise activities in brief
Australia
The Commonwealth Scientific and Industrial Research Organization (CSIRO), Australia's primary publicly funded research organization, is collaborating with the Cooperative Research Centre (CRC) for Coal in Sustainable Development, to operate an advanced coal gasification research facility. CSIRO has also established partnerships with other public entities and industry participants to tackle several gasification issues including researching technologies to measure coal conversion reaction processes, advancing coal and char reactivity knowledge, examining coal slag flow in gasifiers, and studying the water-gas-shift reaction in syngas processing.
Canada
The CANMET Energy Technology Centre-Ottawa (CETC-O) a R&D arm of Natural Resources Canada represents Canada's primary gasification R&D facility. CETC-O's research involves developing gasification, syngas treating, and H2 production technologies.
China
The Thermal Power Research Institute (TPRI) is a Chinese research organization devoted mainly to researching technologies and equipment of fossil-fired power plants, including gasification development.
Germany
The Federal Ministry of Economics and Labor (BMWi) in Germany initiated a R&D program called COORETEC (CO2 reduction technologies) leading to a wide variety of research projects in gasification with the goal of realizing zero-emission power plants.
India
The Indian Ministry of Power operates the Central Power Research Institute (CPRI) which engages in fossil fuel gasification research. The Ministry of New and Renewable Energy performs biomass gasification research for developing rural power generation.
Japan
In Japan, the New Energy and Industrial Technology Development Organization (NEDO) is coordinating the Multi-purpose Coal Gasification Technology Development (EAGLE). This program is aimed at developing the most advanced oxygen-blown, single-chamber, dual stage, spiral-flow gasifier that can efficiently produce syngas.
South Africa
Development of fluidized-bed gasification technology at the Council for Scientific and Industrial Research (CSIR) in South Africa has been supported by the recently-formed South African National Energy Research Institute (SANERI).
United Kingdom
The Department for Business Innovation & Skills (BIS), in the U.K. is working with the Engineering and Physical Sciences Research Council (EPSRC), the British Coal Utilization Research Association (BCURA), industry, and international partnerships to advance gasification technology.
Other Countries
Other countries are also involved in gasification R&D to varying degrees. The links subsection offers other resources on the world of gasification research.

GAS CLEANING IN GASIFIERS

Need for gas cleaning

If the gas is to be feed to the engine, it must be cleaned and conditioned. In case of down draft gasifier the gas which is usually hot and dusty may have up to 1% tar particulates. Proper clean up system increases reliability and avoids failure of gasifier system. The total combined tar and particulates allowable for engine is less than 5 mg/Nm3 which amounts 99% removal of dust particles .For effective removal a gas cleanup system must be designed based on magnitude, size and nature of containments.

Gas cleanup goals

To meet the required cleaning goals the first step is to select a gasifier design that minimize production tar and particulates and next step is to have clean up systems that removes articulates, tar and containments effectively. The gas cleanup goals are to remove solids which are abrasive, tar mist which can cause inlet valves, rings and other moving parts to stick and hence need thorough removal for reliable engine operation. Another drawback with the tar is that it usually contains the impurities nitrogen and sulphur in the structure, which during combustion of the fuel gas produce their corresponding oxides. Successful gasifier engine system requires gas cleanliness standards from 10 to even less than 1 mg/Nm3. Tar levels of different gasifier indicate that updraft gasifers generate 5- 20% tar and downdraft gasifers upto 1%.

Tar formation

Thermal decomposition of wood does not progress at even pace but results in the production of gas, char ( the main slow proylisis product) and tar by transferring heat to surface of the particle and subsequent heat penetration by conduction. Temperature development inside the particle, and corresponding intrinsic reaction kinetics dominate the decomposition rate and product distribution. The vapours formed inside the pores are subjected to further cracking leading to formation of addition gas and/or stabilized tars. The longer residence of vapour molecules inside larger particles (at lower temperature) can explain the increased formation of tar incase of slow pyrolysis. When the vapour products enter the surrounding gas phase, they can still decompose further if they are not condensed. The degradation products from the biomass constituents include organic acids like formic and acetic acid giving oil at low pH of about 2 to 4 which can attack mild steal. With high temperatures the tar components become heavier due to the greater extent of gas phase reactions, which convert the primary formed tar components to more stable and heavier non-substituted aromatic compounds.

Composition

The composition of wood tar is an awesome mixture of phenolic and non phenolic compounds that has taxed the capabilities of several analytical laboratories for over a century. Characterization continues even today. The major molecular species identified in hard wood and soft wood tars and pyrolignous acids are reported to be Pyridine (C5H5N) and Lutidine (C7H9N).The tar which may be mixture of higher hydrocarbon and water in mist form have wide range of boiling points ranging from 100-400 C. They contain fine char particles in the exit stream from 700-6000 mg/Nm3 acting as nucleation sites for tarry vapours. Tar is usually defined as the condensable organic compounds in the fuel gas, those compounds, which at ambient temperatures have a sufficiently low vapour pressure. By this definition tar contents may vary with the sampling and analytical method. Tar is a group of organic compounds including phenol, naphthalene and PAH (poly aromatic hydrocarbons) and their substituted derivatives.

Gas purification

For operating engine, pure gas is needed. The composition and quantity of dust drawn from a producer depends on the type of producer employed and the fuel used. The first step in processing the raw gas is to cool it at the gas producer exist so that tarry liquid may condense. A gas producer, which gives tar free gas, is supposed to contain less than 10 mg of total contaminants per Nm3 of producer gas. For normal type Imbert type downdraft gasifier dust loads are reported vary from 0.5 to 5 g/m3 gas when using wood blocks of 4XX4cm. Modern gas cleaning and purifying plants are varied in type but comprise of the following units:

• One or more cyclones or centrifugal separators
• Expansion box or dust separator
• Cooling tubes or boxes
• Baffle separators
• Filtering and purifying devices

cyclones or centrifugal separators may consist of single or multiples and built over dust collecting chamber. In cooling tubes gas is led through a set of tubes with header and collecting chambers having gills for air surfaces. Baffles separators are largely employed. In cylindrical or oblong containers plates are arranged within causing a change of flow path which results in deposition of large solid particles. Filtering and purifying devices may be of different types. (i) dry filtering in scrubbers packed with coke wood wool, wire wool at the bottom and fitted with cloth bags or trays at the top, (ii) washing by light petroleum by bubbling through oil or water or light petroleum, (iii) filtration in a scrubber moistened with water or oil filled with gravel, coke, wood wool, metal turnings or cork

Producer can employ can employ a variety of combinations mentioned above. Cloth filters are rarely used where water or steam is used in gasification as these filters get quickly clogged with a type of paste formed during operation. The practical option is the need for reduced time for cleaning the purification chains. In olden day gasifiers used for mobile uses built in cleaning devices such as brushes or scrappers were provided to reduce the time required for cleaning out these filters.

Electrostatic precipitators

These have been in long use in industry to clean the gas very effectively. In principle, the dirty gas is passed through a chamber, containing high voltage negative electrode, which makes the all particles and droplets negatively charged. These are then migrating to positive electrode, which are then washed by water stream. 20 cm dia and 1 m long precipitator at 10-30kV was found to give excellent performance (99.85% collecting efficiency) in a 75 hp gas generator at SERI but suffered from short-circuiting by soot, tar and water, which was later solved. The power consumption was low, 1.5 W/hp with very low pressure drop.

Wet scrubbers:

Small difficult to capture droplets are made to grow in size with time until they are large enough to be capture by providing adequate residence time in the scrubber volume. Particles grow in size by agglomeration and condensation by particle collision . Wetted scrubbers have been used widely for cleaning and cooling the gas by creating maximum contact between gas and liquid media. There are several types such as impingement plate, packed bed, sieve plate, spray tower and venturi scrubbers.

Spray towers are simplest type having a cylinder fitted with spray nozzles. Particle collection is accomplished by counter current impact of gas with liquid droplets spray towers are well suited for dust loads over 50 g /Nm3.Cyclones spray scrubbers make use of the principle of spray towers and dry cyclone separator which improves particle capture efficiency. These are self cleaning and collect particles regardless of size with low pressure drop and efficiency more that 97%.

Sieve plate scrubbers have liquid flowing downwards over a serious of horizontal perforated sieve plates. The contact may be improved with bubble caps, impingement plates or sieve plates while the gas pass upwards. A typical scrubber has 90% efficiency.

Impingement plate scrubber is similar to sieve plate scrubber but arranged with impingement target. Gas passing through orifice produces spray droplets resulting in large between dust particles. Five to ten liters/minute of water flow is required per 1000 cfm of gas flow.

Venturi scrubbers capture large particles by impaction & impingement and rinse away the deposits. The atomized droplets present considerable surface area for fine particles to be captured by diffusion.

In ejector scrubbers, the velocity of contacting liquid scrubs the entrained gas in an ejector venture scrubber by imparting axial and tangential velocity to the liquid jet. They are well suited for very dirty corrosive or abrasive materials.

Packed bed scrubbers use saddle spheres, rings as packing to enhance gas liquid contact area. They are free draining and pressure drop is 20cm water gage for 15 cm deep bed with 12 mm spheres.

Tar cracking:

Tar can be reduced by properly cracking it either internally or externally to the gasifier. Temperature above 800 C rapidly destroys tar and promotes reaction with char. A tar cracking chamber may be added to gasifier in which small amount of oxgen or air can be added to crack the final trace quantities to as low as 50 to 500 ppm. Tarry gas can be passed over a bed of hydrocarbon cracking catalyst at temperature between 950 and 1040 C resulting in gas containing 10 to 100 ppm of tar. Using dolomite lime up to 900 C tar can be cracked as is used in Swedish gasifiers.

WOOD CHARCOAL

Wood charcoal which is called simply charcoal is made by carbonization (destructive distillation, i.e. heating in absence of air) of wood at 600C.
Physico-chemical changes during wood carbonization
Following four stages are involved in the carbonization of wood:
- When the temperature reaches 100-120C, initial decomposition of wood takes place resulting in the formation of little distillate gas containing acetic acid and water.
- Active distillation of wood takes place upto 350C till the process is exothermic producing liquid products (like acetic acid, methyl alcohol, pyroligneous acid, tar etc.) and gaseous products containing Carbon monoxide, carbon dioxide, nitrogen, hydrogen, hydrocarbons etc.
- From 350 to 600C, slow evolution of residual volatile matters (i.e. gases) from the wood/charcoal left in 3rd stage.
Products in wood carbonization
Charcoal is the solid product left after the carbonization of wood. Hot gases are cooled to separate wood gas and liquid into two layers. Upper layer of the liquid is prodigious acid and the lower level is wood tar. Pyroligneous acid is an aqueous solution of acetone, methyl alcohol, and acetone and wood spirit mainly. Wood tar can be fractionated to separate many chemicals. Besides it is used as a supplementary plant fuel. Normally matured dense wood gives dense charcoal on carbonization.
Typical product yields in wood carbonization
Product Yield % air dried
Charcoal 30
Pyrolignious acid 38
- Acetic acid 8
- Wood spirit 15
- Water 15
Wood tar 10
Wood gas 22
Scheme of wood carbonization
Carbonization of wood is done in open pit (primitive method, now obsolete), kilns or metal retorts. Pits and kiln are located in forests and retorts in factories.
In open pit carbonization wood is burned in large heaps with restricted air. Yield of charcoal is 20%, which is of inferior quality. Besides, gas and liquid by-products are lost to the atmosphere, as they cannot be recovered. Even the charcoal that we get from domestic wood burning ovens also comes in this category.
In charcoal kiln also, charcoal (of better yield and quality) is the only product as gases and liquid by-products are not recovered. Kiln is parabolic in shape having typical dimension, radius=3 meters, height=2.5 meters and capacity=30m3 of stocked wood.
Wood is stacked on the ground with one verticals central passage acting s the chimney and a horizontal passage at the bottom for introducing fire to the center. The kiln is covered with thick layers of grass, leaves etc. and then plastered with a mixture of earth soil and charcoal dust. Initial firing is done with grass and twigs and then the wood is partly burned to supply heat for the process. Carbonization time is 7-10 days. After that fire is extinguished with water and the kiln is allowed to cool for a week are least before the charcoal is taken out.
Carbonization in metal retorts is done at a low temperature of 350C. Retorts may be of four types namely:
-Externally fired (heated) batch retorts
-Externally fire continuous vertical retorts
-Internally heated batch vertical r retorts
-Internally heated continuous vertical retorts.
Most recent design is internally heated retorts which use-forced recalculation of heated inert gases evolved during carbonization. Besides, it employs efficient mechanical handling of wood and charcoal and has high thermal efficiency.
Characteristics of products of low temperature carbonization of wood are:
- Charcoal yield is high (35%)
- Ash content of charcoal is low (below 2.5%)
- Volatile matter in charcoal is high (up to 15%); hence it can be easily ignited and burns at low rates.
- Heating value of charcoal is high (7500Kcal/kg)
- Charcoals produced have high vapor adsorption capacity.
High temperature carbonization of wood.
It is mainly carried out for the production of town gas and chemicals besides for charcoal. Carbonization temperature is 1000-1200C. Besides, town gas (which can be used for heating of domestic ovens) the valuable liquid chemicals like creosote (used as a wood preservative), turpentine, light & heavy oils are produced. Product characteristics are:
- Charcoal yield= 25%
- Heating value of charcoal = 8000 kcal/kg
- Gas yield=850 Nm3 gas/ton dry wood
- Gross heating value of gas = 3000kcal/Nm3
- Composition of gas: CO2 =13% CmHn (unsaturated hydrocarbons) =2%
CO=24%, CH4=15%, N2 =1%, H2=45%.
Uses of charcoal
- Because of its large specific surface area (150-450 m2 /gm) and light and porous nature, it is used for removal of obnoxious and coloring materials from solutions, gases, vapors, petroleum products etc. By adsorption on its surface.
- It can be used as a feedstock for gasification to make producer gas, which is used for domestic and industrial heating. During Second World War, this producer gas was used as a fuel in road vehicles in many countries.
- It is used as a clean and smooth burning fuel in domestic heating ovens but it is a costly fuel.
- Previously it was being used for metallurgical furnaces but now it has been replaced by coke. In blast furnace using charcoal instead of coke, the charcoal consumption can be upto 1-ton charcoal/ton pig iron for capacity of the blast furnaces of,000 tons pig iron per year.
- It is used very widely as fuel for blacksmith’s and metalworker forge furnaces/ovens.
- It is raw material for the manufacture of carbon disulphide.
- It is mainly used as a domestic fuel in developing countries.
Composition of charcoal.
A typical composition of charcoal is given below:
C=80% O2&N2=15%,H2=2% and Ash =3%
Merits of charcoal as a fuel
- It has a very high specific surface area compared to coal (20-200m2/gm coal)
- Its ash content is very low (below 3%)
- Its calorific value is high (6500-8000 kcal/kg).
Demerits of charcoal as a fuel
Its mechanical strength is very poor, hence it gets crushed to powder in operation, which is easily swept away in a current of gases, and also it may prevent the proper flow of gases in the furnace.

Calorific value

Calorific value

The quantity of heat (Kcal) liberated by the combustion of unit quantity of fuel is called its calorific value. Unit of calorific value is Kcal/kg for solid and liquid fuels and Kcal/Nm3 for gaseous fuels. Nm3 means volume of gas in M3 at Normal Temperature and Pressure (NTP) which is zero deg0 C and 760 mmHg. Since the volume of gases varies sensitively with pressure (Boyle’s Law) and temperature (Charle’s Law) hence their volume is expressed at NTP in Nm3 to have a standard measurement.
Gross calorific value or higher heating value at constant volume is the quantity of heat liberated by combusting the fuel at constant volume in oxygen saturated with water vapour, the original material and final products of combustion being at a reference temperature (25degC) and the water obtained from the fuel being in the liquid state.
Gross calorific value at constant pressure implies that the combustion takes place at constant pressure and not at constant volume. In the laboratory determinations, solid and liquid fuels are burnt at constant volume and gaseous fuels are burnt at constant pressure. In the ovens and furnaces, however, the combustion takes place at constant pressure. The difference in the two corresponding values is small. For coal, the calorific value at constant pressure exceeds the calorific value at constant volume by about 5.5 Kcal/kg
Net calorific value or lower heating value at constant volume is the quantity of heat evolved when unit quantity of fuel is burnt at constant volume in oxygen saturated with water vapour, the originals material and final products of combustion being at a reference temperature (25degC) and the water obtained from the fuel being in the vapour state. The net calorific value is therefore less than the gross calorific value by the amount of the heat of condensation of water vapours, which at 25deg C is 583.5 kcal/kg of water. On the basis of hydrogen of water, this is equal to 5,252 kcal/kg 468.9 kcal/Nm3 of hydrogen. The following formula is used in calculating the net calorific value from gross calorific value of solid and liquid fuels approximately.
CN=CG-53 H
where CN and CG = net and gross calorific value in kcal/kg, respectively, and H= percentage of hydrogen of coal, including hydrogen of moisture and of water of hydration of minerals.
For a gaseous fuel, the formula is:
CN=CG-4.7V
where CN and CG = net and gross calorific values in kcal/Nm3, respectively, V= volume percentage (as H2) of total hydrogen of the gaseous fuel, including the hydrogen obtainable from other combustible components.
Net calorific value at constant pressure implies that the combustion takes place at constant pressure and not at constant volume.

Energy costing methods

There are two definite considerations to the study of energy costing:
a) cost of providing the energy service;
b) expenditure on energy by the user.
Both aspects must be considered since wastage can occur in either area. Furthermore, it must be decided if the user department is to be designated a portion of the fixed costs or if they are to be absorbed into the same manner, the method of allocation dose not matter. Controlling these costs should be undertaken using management accountancy techniques, and two methods useful for the control of future energy costs are budgeting and standard costing.
Budgeting
Budgeting is simply estimating future energy demand, in terms of steam, electricity, oil, gas, by the various departments. This may be done in terms of energy units, such as terms, kWh, Btu, or in monetary units. Controlling the system is exercised by comparing the budgeted unit costs with actual costs and accounting for any variation. The process of accounting for variation is called variance analysis and variations from the budget can be caused by:
a) different production volume (volume variance);
b) different energy consumption per unit (energy efficiency variance);
c) different cost of energy to firm (price variance).
The total variance is obtained from the sum of the three component variances. A sophisticated budget would project energy costs for various levels of output and is termed a flexible budget.
Standard costing
Standard costs are the expected costs of the various energy-related inputs into the plant (oil, gas, electricity, wages etc.). These standards costs divided by budgeted consumption by the user/departments gives the standard unit cost.

Fuel cell electrode fabrication

Fuel cell
There are several kinds of fuel cells, and each operates a bit differently. But in general terms, hydrogen atoms enter a fuel cell at the anode where a chemical reaction strips them of their electrons and electricity is produced through a chemical reaction with oxygen or another oxidizing agent.
Fuel cell consists of an anode (negative side), a cathode (positive side) and an electrolyte that allows charges to move between the two sides of the fuel cell. Electrons are drawn from the anode to the cathode through an external circuit, producing direct current electricity. The reactions that produce electricity take place at the electrodes. The first generation of polymer electrolyte membrane fuel cells (PEMFC) used PTFE-bound Pt black electro catalysts that exhibited excellent long-term performance.
There are two methods of preparing PTFE bonded fuel cell electrodes, namely dry and wet methods depending upon the form of PTFE used.
Wet Method
In this method, aqueous PTFE emulsion is used. The procedure used is as follows: The depyrophorised Raney-Ni powder is first mixed with promoters (e.g. Cu2O). This mixture is added to PTFE suspensions. Isopropanol is added to stabilize the rubber like resultant mixture. During this mixing the suspension of PTFE breaks and water is removed. This paste is heated in order to evaporate some water and isopropanol resulting into a plastisizable mass. Cold rolling of this mass is done to obtain a felt of 0.2 to 0.5 mm thickness. Finally this felt is rolled with a nickel net which also works as a current collector. The cold rolling results into more linkage of catalyst particles with PTFE strands.
D-1 PTFE suspension containing 60% of PTFE, mixed with the catalyst, is milled for one hour at 20±20C. The paste is calendar rolled into 0.1 to 0.2 mm thick sheet. The surfactants are removed by boiling the sheet in acetone. Thus obtained layer is used as catalyst layer. Gas side layers are prepared by using nickel black powder blended with PTFE dispersion. These two layers are rolled with a mesh of stainless steel to get the final electrodes. In later, use of dopants like chromium and titanium improves and stabilizes the polarization characteristics of Raney nickel electrodes.
Dry Method
Use of dry PTFE powder in making fuel cell electrodes is relatively new. Dry PTFE powder with Raney nickel catalyst is used to prepare fuel cell hydrogen electrodes. In this method 5-8 wt% of PTFE is added to catalyst. The blend is milled into a high speed machine with sharp blades. It formed a network of PTFE treads and lumps with catalyst grains in between. The high speed blade milling of the catalyst particles with PFE leads to PTFE coating on the catalyst grains. As a result of this process which is known as reactive mixing, a fluffy mass of the powder is produced. The next step is the rolling of this fluffy PTFE-catalyst mixture into a calendar to form a tape. This tape is further rolled on to a wire mesh of nickel.

Gas analysis

Gas analysis down to low concentrations

Enwave Optronics in its Product Announcement in May 2010 has introduced a new NOCH-2 GasRaman analyzers for multi gas analysis. The NOCH-2 GasRaman Analyzers can detect gases such as H2, N2, O2, CO2, NO2 and few more down to 0.025% at atmospheric pressure. The NOCH-2 GasRaman analyzers are suitable for laboratory and on-line applications requiring gas phase Raman analysis at an affordable price.

See: http://www.enwaveopt.com/GasRaman.html

Gas Analysers

Witt gas analysers are fast, precise and multifunctional. The gas analysers are used as stationary or portable units for sample or continuous gas analysis for almost any gas and application, for example in food (MAP) or steel industry.
Fuel gas analysers
WITT gas analysers for hydrogen are used for permanent or sample analysis of gas mixtures and provide high quality and safety in production processes, for example in thermal treatment applications. It is an analyser, available for integration with gas mixers or as a stand alone unit, for continuous analysis (in-line) of the gas concentration for a variety of industrial applications. Measuring range O2: 0 to 100%, CO2: 0 to 30 or 100%, CH4: 0 to 10 or 100%, H2: 0 to 10, 30 or 100%, and He: 0 to 30 or 100%.
See: http://www.wittgas.com/EN/gas_analysers.html

FTIR gas analyzers
Horiba has introduced FTIR gas analyzers enables the detection and measurement of a wide variety of substances, such as PFCs, greenhouse gases and semiconductor / flat panel display (FPD) process gases. Legislation / guidelines encourages industry to act to reduce these substances due to their contribution to global warming. The use of a cell with a long optical path length enables measurement of low-concentrations down to the sub-ppm level. See http://www.horiba.com/scientific/

Throat less gasifier design

Gasification
Thermo chemical gasification is the conversion of carbonaceous feedstock such as biomass or coal by partial oxidation at elevated temperature a into a gaseous energy carrier. Gasification occurs in sequential steps: drying to evaporate moisture, pyrolysis to give gas, vaporized tars or oils and a solid char residue, followed by gasification or partial oxidation of the solid char, pyrolysis tars and pyrolysis gases.
The gas obtained on gasification contains carbon monoxide, carbon dioxide, hydrogen, methane, trace amounts of higher hydrocarbons such as ethane and ethene, water, nitrogen (if air is used as the oxidizing agent) and various contaminants such as small char particles, ash, tars and oils. The partial oxidation can be carried out using air, oxygen, steam or a mixture of these.
Air gasification produces a poor-quality, low energy density gas (4-7 MJ/ cu.m, higher heating value) which is suitable for boiler, engine and turbine operation, but not for pipeline transportation. Oxygen gasification produces a better-quality gas (10-18 MJ/ cu.m, higher heating value) which is suitable for limited pipeline distribution and for use as sythesis gas for conversion to methanol and gasoline.
Open-core downdraft gasifier
This type of gasifier was first devised by the Chinese for rice husk gasification and further developed by Syngas Inc. from work carried out at the Solar Energy Research Institute (now the National Renewable Energy Laboratory - NREL).
This type of gasifier is called as static bed or open core or throat less gasifier and is a simple reactor technology developed principally for small-scale or remote applications requiring fuel gas for heat or power. This type of gasifier has been developed with no throat and the bed is supported on a grate.
The reactor generally consists of two concentric cylinders (one may be sufficient), in which a stationary fuel bed is converted by a reaction front propagated through the fuel bed. When under suction created at the intake of an engine, the reactor top can be left open for refueling without venting producer gas.
Specifc gasifcation rate
Specifc gasfication rate (SGR) is an important parameter which expresses the rate of fuel consumption per unit cross-sectional reactor area. An optimum value of this parameter is used for designing different capacity range of throatless gasifiers.
Calculation of air to be supplied
The equivalence ratio (ER) is defned as the ratio of actual air used in a run to stoichiometric air requirement for the run where ER= (Amount of air used in a run)/(Amount of stoichiometric air). Knowing the elemental composition of raw material, the stoichiometric air requirement can be estimated (for dry rice husk the value is 3.35 cu.m / kg). Optimum value of equivalence ratio for gasification can be taken as 0.40. The air fuel ratio can be estimated using the expression, A/F=Amount of air used (kg)/Amount of dry material (kg). This calculation helps to find the amount of air to be supplied for gasification (through engine suction or an external blower).
Determination of gas flow rate
The gas flow rate can be determined by installing a calibrated orifice meter in the producer gas line. But due to the presence of tar in the gas the orifice meter tend to get foulded and soon will result faulty gas flow readings. It is therefore preferred to measure the air flow to the gasifier and make a nitrogen balance to estimate the gas flow rate. For this, install a pre-calibrated orifice meter in the upstream section of the gasifier (just before the gasifier) to measure the air flow rate. Knowing the producer gas composition, elemental analysis of biomass, feed rate and air flow rate, nitrogen balance over the gasifier may be carried out using the following procedure for gas flow rate determination. For this computation assumptions are made that air has a molar composition of 0.79 Nitrogen and 0.21 Oxygen and all the nitrogen entering the gasifier leaves it in producer gas.
Nitrogen input = Flow rate of air (Nm3 / h)* 0.79 + Fuel feed rate into the gasifier (kg/h)* Weight fraction of nitrogen in the biomass * (22.416 / 28)
Nitrogen output = Flow rate of producer gas (Nm3 / h)*mole / volume fraction of nitrogen in the producer gas
By equating the above two statements we can solve for flow rate of producer gas
Reactor sizing
Previous works report a specific gasification rate of approximating 170 kg/sq.m.h, which is an intermediate value between two levels and a maximum thermal efficiency exists at this value. Using an optimal value the size of the reactor can be readily computed from the energy demand on the system.
Theoretical modeling and experimental works have been done for moving bed open-core rice hull gasifier using a 45 cm diameter reactor. Results of their work showed an optimum gasification load or specific gasification rate of 125-175 kg/sq.m h, depending on bed height. According to another report the cold gas efficiency of 26 cm. reactor diameter gasifier reached a peak of between 50 and 60% at specific gasification rate of 200 kg/sq. m h.
Another report indicates an optimum value of specifc gasifcation rate for gasifcation of rice husk in throatless open core gasfier reactor as 192.5 kg/sq.m h. Optimum value of equivalence ratio was 0.40, the gas lower heating value of producer gas was about 4 MJ/N cu.m.and the cold gas efficiency was around 65%.
For determining the reactor diameter for a downdraft stratified gasifier , Reed has indicated an optimum value of specific heat rate as 390 kg/sq.m h for 8 to 75 cm diameter reactors. It is further reported that the maximum specific heat rate can go up to 580 kg/sq.m h with gasifier having mechanical ash removal unit. However it is reported that these figures were derived through experiments with a particular gasifier type and mode of operation. From the fore going it is clear that a preliminary value of 200 kg/sq.m h can be taken as a value for SGR for determining the reactor diameter.
Reactor construction
The reactor can be a batch fed type having constant diameter. It can be made from a minimum of 3 mm thick steel sheet. The gasifier consists of an inner reactor and a concentric containment tube. The containment tube and the reactor can be flanged together at the top. The top end of the reactor can remain open during the operation. Air entry into the reactor will be from the top and gas exit through preferably a stainless steel wire mesh grate, fitted at the bottom of the reactor. The bottom of the containment tube should be water sealed. The diameter of the containment tube can be selected in such a way that the producer gas velocity in the space between the reactor and the containment tube is around 0.6 m/s.
Starting a small gasifier
A small amount of char is placed over the grate followed by feedstock. The purpose of adding char over the grate was to protect the grate from high temperature damage. A suction blower can be connected in the down stream section of gasifier after the first filter to start the initial establishment of ignited charcoal. Then the feedstock is filled and the blower operated for some time to start the gasification. A flare burner may help find that combustible gas is generated from the gasifier.
Chinese gasifier design for rice husk
The gasifier consists of an inner tubular steel shell reactor of 25 cm diameter, open at the top and closed at the lower end by a stainless steel mesh screen. It was housed within a concentric 35 cm gas collector. The lengths of reactor and collector are 168 and 183 cm respectively for one hour continuous operation. Air enters the reactor at the open top and passes downward through the fuel column to the reaction zone when under suction from engine intake system. The gas from the reaction zone flows in the reverse direction to the hot outlet.
Raw gas is passed through a wet sieve plate (scrubber and particulates). The twin reactor design enables the engine to draw from one reactor while the other is being serviced.
The gasifier generates a nearly uniform reaction front propagating upwards at a velocity of 0.77-0.87m/h. With a temperature of the reaction front maintained at 950-1050°C.
Gas composition is as follows (Vol%): CO13.4%, H2 11.1%, CH422%, 0221.4%, N258.9%, H2O4.13%, lower heating value (LHV) 40223.8KJ/Nm3, Gas flow is 18.44 Nm³/h. Specific gas output is 2.39Nm³/kg rice husk. Specific gasification rate is 185 kg/m-h. Cold and raw gas efficiency was 52.4% and 72.2%.
Static bed 25 cm rice husk gasifier yielded an optimal value of specific gasification rate in the vicinity of 195 kg/m² -h. Cold gas efficiency (52.4%) and gas flow (18.44NM3/h) are favorable for selected duel-fuel engines.
Indian gasifier design for sugarcane leaf and bagasse
This is a low-density biomass gasification system for thermal applications. The gasifier can handle fuels like sugarcane leaves and bagasse, bajra stalks, sweet sorghum stalks and bagasse etc. The system delivered under laboratory conditions at 288-1080 MJ/h output levels. The HHV of the gas was 3.56-4.82 MJ/N cu.m. The system also produced char of about 24% by weight of the original fuel. It can be briquetted to form an excellent fuel for wood stoves or can be used as a soil conditioner. The system was retrofitted to a specialty ceramics baking LDO-fired furnace in a metallurgical company.

Intermediate Temperature SOFC coupled gasifier

Fuel cell
Fuel cell is an energy conversion device (chemical energy is converted to electrical energy) that utilises a gaseous fuel and oxidising gas to produce electricity and heat, having less or no other emissions, when compared with other power generation technologies. It consists of three components (anode, cathode and electrolyte) and depending on type, can operate at a wide range of temperatures with relatively high electric efficiencies. Fuel cells are currently manufactured by a number of fabrication techniques, such as dry pressing, tape casting, screen printing, slurry coating, depending on the type of fuel cell.
Fuel cells are used also in many applications, either stationary (power generation) or traction. Currently, there are six major types of fuel cells that are developed. Among these, the Alkaline and Polymer Electrolyte fuel cells are operated at low temperature, the Phosphoric Acid at intermediate, while Molten Carbonate and Solid Oxide fuel cells are mainly high temperature fuel cells. Demonstration activities all over the world are trying to bring the manufacturing and commercialisation closer to reality.
Solid Oxide Fuel Cells
Interest in Solid Oxide Fuel Cells (SOFCs) capability to operate at intermediate temperature range had led scientists and engineers to focus the research and Development (R&D) efforts on the design and fabrication techniques. The capability to fabricate such fuel cells having thin electrode structures has been demonstrated by a number of groups worldwide. Additionally to this “Thin Film Technology”, material characteristics, especially solid-state ionic and proton conduction at low temperatures, has created a new research field that has attracted attention and interest in recent years.
Of the various types of FCs, the SOFC is the most demanding from a materials point of view. However, because it operates at relatively high temperature, it offers the significant advantage of simple fuel pre-treatment. This advantage creates opportunities for SOFCs where natural gas, biomass, diesel, military fuels, and gasoline are the abundant fuels. Applications where SOFCs may find dominant positions include distributed power, and, military transport applications, heat generation for the home and auxiliary power units. For various applications, the technology must reach a reliable level sufficient to allow the plants to operate unattended.
Operating constraints
SOFC’s must operate at high temperatures to enable diffusion of oxygen ions through the electrolyte made possible by reason of oxygen vacancies in the electrolyte crystalline structure. With conventional designs the anode is a composite of nickel and yttria-stabilised zirconia (YSZ). This composite is an electronic conductor (due to nickel) and also an ionic conductor (due to YSZ). Nickel, however, catalyses the formation of graphite from hydrocarbons, except for a narrow range of operating temperatures and only for methane, thus carbon formation with nickel based anodes is unavoidable for the wider range of hydrocarbon fuels available. Research reports suggest that anodes made from a composite of copper and ceria, or samaria-doped ceria, may remove this barrier in the future.
Cell geometry and construction
Currently, R&D is also focused on the fabrication of fuel cell units with different geometry, depending mainly on different specific requirements. Basically, three different designs are under development, which differ only in cell geometry.
• Tubular Design
• Planar Design
• Monolithic Design
Cell is the repetitive electrochemical building block that is connected either in series or in parallel, forming the “stack” or the “unit” of fabrication. The basic SOFC cell consists of the following common parts:
• The Anode
• The Electrolyte
• The Cathode
• The Interconnect (bipolar) plate
• The support tube (only in tubular design)
Intermediate Temperature SOFC
When the SOFC operates at intermediate temperature range (below 700 °C), some of the problems raised from high temperature operation can be overcome. Such problems include material high cost and efficiency losses. Additionally, several changes need to be made to cell and stack design, cell materials, reformer design and operation, and operating conditions in order to operate at intermediate temperatures. On the other hand, low temperature operation brings additional benefits, which include:
• Low cost metallic materials, such as ferritic stainless steels can be used as interconnect and construction materials. This makes both the stack and balance of plant cheaper and more robust
• More rapid start up and shut down procedures
• Corrosion rates are significantly reduced
Biomass Integrated Gasification Fuel Cell Systems
The combination of biomass gasification with a Fuel Cell Systems such as SOFCs is a highly promising approach to exploit the potential of biomass in combined heat and power generation.
In a first step the solid biomass is converted to a combustible gas mixture. The composition of the gas mixture depends on the employed reactor type, gasification agent, feedstock and operating conditions of the gasification process. It consists to a major extent of hydrogen and carbon monoxide, the rest being carbon dioxide, methane, other hydrocarbon species, water, diverse impurities (e.g. tars, alkali salts, sulfur, soot particles etc.) and nitrogen in case of air as gasification agent. The impurities are potentially performance degrading and have to be removed to some extent in order to meet the requirements of the employed fuel cell. The requirements depend on the specific fuel cell (FC) type and its design, catalyst materials and the operating conditions. The strong interactions between the composition of the gas mixture obtained from the gasification process and the fuel cell entail that optimal system integration is crucial for overall energy efficient and cost effective system.

Updraft gasifiers

Gasification
Gasification is a widely studied and applied technology to produce a mixture of combustible gases. It consists of several sequential processes which include: drying, pyrolysis to give gases, tars and char, cracking and oxidation of tars and, to a certain extent, oxidation of pyrolysis gases and gasification of char. Together with chemical processes and evaporation of moisture, transport phenomena also take place.
Applications
Typical applications might include using producer gas as a substitute for petroleum fuels in the standard gasoline or diesel engines that are commonly used in developing countries for electrical power production and water pumping or in local industries such as sawmills, rice mills and workshops. In addition, the gas can be used in standard heat appliances such as crop dryers and cement, lime, or brick kilns.
Fixed bed reactors are used in small-scale gasification while large biomass gasifiers are usually of the fluidized-bed or entrained –flow type. Fixed bed, updraft and downdraft reactors are, in general, of very simple construction and operation, and avoid the excessive costs of feedstock pulverization.
Principle
In the updraft gasifier the downward-moving biomass is first dried by the up flowing hot product gas. After drying, the solid fuel is pyrolysed, giving char which continues to move down to be gasified, and pyrolysis vapours which are carried upward by the up flowing hot product gas. The tars in the vapour either condense on the cool descending fuel or are carried out of the reactor with the product gas, contributing to its high tar content. The product gas from an updraft gasifier thus contains a significant proportion of tars and hydrocarbons, which contribute to its high heating value. Usually the gases are directly used in a closely coupled furnace or boiler.
Gas quality
The fuel gas requires substantial cleanup if further processing is to be performed. There is interest in the cleaning of the updraft gas for electricity production, as low temperature tars are more reactive and thus easier to be removed, than the high-temperature tars produced in much lower amounts by downdraft and fluidized bed gasifiers.
Advantages
The principal advantages of updraft gasifiers are their simple construction and high thermal efficiency. The sensible heat of the gas produced is recovered by direct heat exchange with entering feed, which thus is dried, preheated and pyrolysed before entering the gasification zone. Updraft gasifiers can be used in the sizes between 2 and 20 MWe.

Torrefaction of biomass

Fuel wood is often difficult to use because of its poor combustion characteristics such as low heating value, variable moisture content which is often high, hydroscopic nature, smoking during combustion, etc. For a number of other applications, wood is often upgraded to charcoal. The charcoal-making process is inefficient with the product containing only about 55% of the energy of the original raw material in well-managed, commercial operations and as little as 20% in traditional processes.
Torrefaction
Torrefaction is a thermo chemical treatment of biomass at 200 to 320 °C. It is carried out under atmospheric conditions and in the absence of oxygen. During the process, the water contained in the biomass as well as superfluous volatiles are removed, and the biopolymers like cellulose, hemicellulose and lignin partly decompose giving off various types of volatiles. The final product is the remaining solid, dry, blackened material which is referred to as “torrefied biomass” or “bio-coal”. After the biomass is torrefied it can be densified, usually into briquettes or pellets using conventional densification equipment, to further increase the density of the material and to improve its hydrophobic properties.
Benefits
Torrefaction appears to be an attractive option for upgrading wood to a product which retains about 90% of its energy and can be substituted for charcoal in a variety of applications. Biomass which is typically thermally unstable usually leads to formation of those condensable tars in gasifiers, making problems in down-stream equipment such as choking and blockage of piping. Torrefaction eliminates this problem.
The important advantages of torrefied wood include high energy yield and hydrophobicity so that it does not regain moisture during storage. Torrefaction achieves a stable low moisture content of 3%, reduction of mass by 30%, retention of 90% of original energy content and removal of smoke producing agents. All biological activity is eliminated reducing the risk of fire and stopping biological decomposition. Torrefied wood has a heating value of approximately 22,500 kJ/kg and highly friable and can be easily crumbled or pulverized. Torrefied biomass has excellent combustion properties; the fuel can be readily co-fired with coal, further gasified or fed to pyrolysis units.

Gasifier in textile sector

Fossil fuel has been the primary source in the last two decades to meet the thermal energy demand of small as well as large industries. The numbers of small-scale industries that use fossil fuels to meet the heat requirements are quite large. In such industries constant efforts are made to increase the efficiency of combustion devices to meet both efficiency and emissions standards. Over the years, many of the solid fuel based devices have been converted to petroleum-based fuels due to the availability and the compactness of the combustion system without serious concern on the economics of operation. With the present escalation of cost of petroleum fuels, the overall economics has been affected. Economics along with the environmental considerations has resulted in looking at alternate sources of energy. Industries have adopted the use of petroleum-based fuels for various applications, apart from generating electricity using internal combustion engines. Some of the applications are the low temperature requirements like, drying of various food and non-food items, hot air for specific process requirements, etc. high temperature requirements are in boilers for steam generation, thermic fluid heaters, furnaces in heat treatment industries, steel processing, ceramic sector, etc. This has led to the use of petroleum fuels for stationary applications, which other wise could address the transport sector. A significant part of fossil fuel is used for industrial applications to meet the thermal energy requirements. Further, the petroleum fuel usage has an impact on the Greenhouse gas emissions. Combustion of petroleum fuels degrades air quality with adverse impacts. It leads to emission of pollutant such as particulates, SO2, NOx, CO and GHG (largely CO2).
Biomass as an energy source
Even though many of the above mentioned applications could be addressed using biomass, the industrial sector has not taken note. With the adaptation of gasification technology, nearly all the advantages of using petroleum fuel as a thermal source of energy can be availed. Further, the use of biomass has an economic advantage along with environmental benefits.Biomass has been one of the main energy sources ever since the dawn of civilization although its importance is dwindling after the expansion in use of oil and coal in the late 19th century. The recent resurgence of interest in biomass energy in important countries of the world is not surprising considering the benefit it offers in the current context. It is renewable, widely available, carbon neutral and has the potential to provide significant productive employment. Power and liquid fuels could also be produced by the gasification route using energy plantations grown on non-agricultural lands. The present methods for utilization of these resources are highly inefficient. On the other hand, utilization of the residues through gasification route becomes economical and promising for thermal and power needs of rural areas and small-scale industries. This will also reduce the pressure on the worsening fuel wood situation. Biomass is the most convenient form of renewable energy because the built-in storage technologies based on stored energy are most suitable for energizing irrigation pump-set and industrial thermal applications.
Biomass gasification
Biomass gasification is basically conversion of solid biomass such as wood waste, agricultural residues etc., into a combustible gas mixture normally called “Producer gas” (or Low Btu gas). The solid biomass is burnt in the presence of limited air or oxygen to produce a low or medium calorific value gas. Partial combustion process occurs when air supply is less than adequate for combustion of biomass that contains carbon, hydrogen and oxygen and oxygen molecules and complete combustion would produce carbon monoxide as well as hydrogen also, which are both combustible gases. Solid biomass fuels are usually inconvenient, have low efficiency of utilization and can only be used for certain limited applications. Combustion is the normal conversion process for direct thermal use in cooking, heating space and water, or generation of steam with low efficiency. Conversion of the same biomass to a combustible gas mixture (producer gas) removes most of these problems associated with the use of solid biomass fuels. While conversion to gas results in loss of energy of up to 25 percent, use of gas can be highly efficient and hence overall efficiency could be very high. Also it can be employed at any scale and hence is ideally suited for decentralized application whether for shaft power, electricity or thermal energy applications.Number of thermal applications of gasifier systems has shown adequate and immediate promise. These applications involve diverse situation: where biomass might already be in use with traditional technologies; situations where biomass may not be currently in use but is available as a by-product; and situations where biomass may need to be procured for a switch over from fossil fuel. However even in situations where thermal energy is currently being provided by bio-resources, careful study of the application and effective development of application packages becomes necessary.
Use of boilers in textile industries
Textile industry can be classified according to the fiber being processed or the processing operations. In most of the knitted garment industries, thermal processing is one of the steps involved in the production chain. For thermal processing such as heating, boiling, hot air production, boiler operation etc. direct burning of wood is employed. Locally made boilers are used for the production of steam. Steam is used for thermal processing of clothes. They use various types of boilers ranging from steam production of 100 kg/h to 1000 kg/hr at moderate promises.
Retrofitting gasifier with boiler
Most common fuels used in the boilers are wood, diesel or electricity. Direct burning of wood involves less efficient processes and other fuels like oil, diesel involves higher cost of operation, which increase production cost. Burning of firewood by conventional method is very inconvenient and inefficient. Hence there exists scope to introduce gasification process having a fairly high efficiency. In thermal gasifiers, biomass like wood waste is burnt in controlled conditions and fuel gas is produced. This wood gas can be burnt in specially designed burners to get intense flame. Adopting gasifier of fairly high efficiency can reduce the cost of fuel. These gasifiers can be retrofitted with industrial boilers for thermal requirements.

Gasification of Bagasse

For countries growing sugar cane there is a large potential of sugar cane bagasse for use in energy production. Bagasse is burned in the sugar mills to produce heat for the drying process in the sugar production. But the utilisation efficiency of the bagasse is very low in the conventional processes of both combustion and heat exchange which needs improvement so that surplus of bagasse could be used for electricity production.

Gasification

Gasification is a widely studied and applied technology to produce a mixture of combustible gases consisting of several sequential processes which include drying, pyrolysis to give gases, tars and char, cracking and oxidation of tars and gasification. The solid fuels can be effectively harnessed by converting them into a gaseous combustible producer gas which has a gross calorific value of 3.5-5 MJ N/m3 comprises mainly of carbon monoxide (25% v/v) and hydrogen (15- 20% v/v). It can be combusted in suitable burners with flame temperatures exceeding 1000degC and can be used for industrial thermal applications.

Reactors

Two main classes of chemical reactors have been employed for bagasse gasification: fixed bed and fluidised bed reactors. Fixed bed, updraft, downdraft and open core reactors are, in general, of very simple construction and operation, and avoid the excessive costs of feedstock pulverisation. These reactors can operate at high carbon conversion; long solid residence times and low ash carry over. The quality of the produced gas is better for the downdraft configuration. But scaling up of this system cannot be done to large capacities. On the other hand the complex and expensive technology of fluidised bed reactors allows very huge capacities, very good solid-gas contact and easy scalability, but with pulverized feed stock.

Design

The fixed bed reactor can gasify sugar cane bagasse and wood chips for production of gas that can be used in an internal combustion engine to produce electricity for a rural community. The design of gasification units is often based on feedstock reactivity and gasification characteristics. The general system comprises of a reactor, a gas conditioning system, a bagasse feeding system and the instrumentation and controls. For example, a downdraft, throat less and open-top reactor with an internal diameter of 75 cm and an active bed height of 1.25 m. can be used for a thermal output of 1080 MJ/ h. High temperature resisting firebricks can be used for the hot face followed by cold face insulation. A gas conditioning system consisting of a dry dust collection system can be used to eliminate the problem of wastewater. A high temperature char/ash coarse settler and a high efficiency cyclone separator can be used along with a high temperature resisting induced-draft fan.

Gasification characteristics

Gasification characteristics can be grouped into: thermo chemical (ash content, volatile products, reactivity of volatile products, etc.), intraparticle rate (thermal properties, moisture content, size, kinetics and energetic of chemical processes, etc.) and extra-particle rate (heat transfer from reactor to particle, residence time and mass transfer conditions depend, in their turn, on the type of gasification unit). This point is very important because the knowledge of gasification characteristics of bagasse is a crucial factor in making the technology attractive for wide use in industrial and commercial use.In general, for the sake of economy, a gasification plant should be able to operate with different, locally available feed stocks, whose nature and condition, presumably, change in the course of the year.

Activated carbon from Coconut shell

Coconut shell

Activated carbon is a non-graphite form of carbon which could be produced from any carbonaceous material such as coal, lignite, wood, paddy husk, coir pith, coconut shell, etc. Coconut shell is used for manufacturing activated carbon. Coconut Shell is carbonized by using pit, drum, destructive distillation method etc.

Uses

Shell based activated carbon is extensively used in the process of refining and bleaching of vegetable oils and chemical solutions, water purification, recovery of solvents and other vapours, recovery of gold etc. It is used in gas masks and a wide range of filters for war gases and nuclear fall outs and for the removal of colour and odour of compounds for protection against toxic gases.

Activation process

Coconut shell based activated carbon units adopt steam activation process to produce good quality activated carbon. Activated carbon manufactured from coconut shell is considered superior to those obtained from other sources mainly because of small macropores structure which renders it more effective for the adsorption of gas/vapour. Steam activation and chemical activation are the two commonly used processes for the manufacture of activated carbon. However coconut shell based activated carbon units are adopting the steam activation process to produce good quality activated carbon.

Steam Activation

The process of steam activation is carried out in two stages. First, the coconut shell is converted into shell charcoal by carbonization process which is usually carried out in mud-pits, brick kilns and metallic portable kilns. The coconut shell charcoal is activated by reaction with steam at a temperature of 900-1100 degC under controlled atmosphere in a rotary kiln. The reaction between steam and charcoal takes place at the internal surface area, creating more sites for adsorption. The temperature factor, in the process of activation is very important. Below 900degC the reaction becomes too slow and is very uneconomical. Above 1100degC the reaction becomes diffusion controlled and therefore takes place on the outer surface of the charcoal resulting in loss of charcoal.

Properties

pH Value 6.5 - 7.5, Methylene value adsorption mgm / gm 190 – 350, Adsorption capacity at % by mass (min) 45, Moisture (max.) 5%, Ash (max) 5%, Hardness 90.

Reactivation

Activated carbon is extensively used for the process of refining and bleaching, but after utilization, the "spent" carbon, as it is called, can be removed and re-activated for further use. This is done primarily with granular activated carbon because particles will be too small to be effectively re-activated. This process allows for recovery of approximately 70% of the original carbon. This number also allows for any physically lost in the shipment process. The re-activated carbon is then mixed with a portion of new carbon for higher effectiveness and is then used in the process.

Activated carbon applications

Numerous activated carbon products of different selected grades and base materials like bituminous coal and coconut are available in the market. There are three main physical carbon types such as granular, powder and extruded (pellet). Each type of activated carbon product has a different surface area, pore size distribution, size and shape and they can have properties tailored to the application.

• Air Treatment – Control of potentially harmful or environmentally damaging substances to the atmosphere.

• trapping impurities - Trapping carbon-based impurities ("organic" chemicals), as well as things like chlorine

• Drinking Water Treatment – Purification of water for human consumption at treatment works or in home filters.

• Effluent Water Treatment - Control of potentially harmful or environmentally damaging substances to water courses.

• Food and Beverage – Essential processing stage in the production of a variety of food products. • Industrial Processes – Purification and/or catalysis stage for a huge range of industrial applications.

• Medical – Activated Charcoal Cloth used in wound dressings, odour control filters and masks, and for ingested poison treatment.

• Personal and Collective Protection – Carbon for military and industrial respirators and building containment filters.

Activated carbon

Activated carbon, also called activated charcoal or activated coal is a form of carbon that has been processed to make it extremely porous and thus to have a very large surface area available for adsorption or chemical reactions. It has a high degree of microporosity, just one gram of activated carbon has a surface area in excess of 500 m2.

Suitable raw materials

Activated carbon can be produced from carbonaceous source materials such as agricultural by-products like nutshells, peat, wood, coir, almond shells, apricot stones, cherry stones and grape seeds and lignite, coal, petroleum pitch etc.

Uses

The carbon adsorbents can be applied for the removal of tri halomethanes and metal ions from water, purification of waste solutions, water treatment, separation and concentration of trace elements and radioactive isotopes, production and analysis of high purity substances, fuel gas treatment and hazardous waste remediation etc. It is also used for the removal of undesired colour, smell and other impurities in many types of industries including basic drugs, fine chemicals, glucose, sugar, electroplating plasticizers and so on.

Production processes

Activated carbon can be produced by one of the following processes:

Physical reactivation: The precursor is developed into activated carbons using gases. This is generally done by using one or a combination of the following processes:

Carbonization: Material with carbon content is pyrolyzed at temperatures in the range 600–900 °C, in absence of oxygen (usually in inert atmosphere with gases like argon or nitrogen). Activation/Oxidation: Raw material or carbonized material is exposed to oxidizing atmospheres (carbon monoxide, oxygen, or steam) at temperatures above 250 °C, usually in the temperature range of 600–1200 °C.

Chemical activation: Prior to carbonization, the raw material is impregnated with certain chemicals. The chemical is typically an acid, strong base, or a salt (phosphoric acid, potassium hydroxide, sodium hydroxide, zinc chloride, respectively). Then, the raw material is carbonized at lower temperatures (450–900 °C). It is believed that the carbonization / activation step proceeds simultaneously with the chemical activation. Chemical activation is preferred over physical activation owing to the lower temperatures and shorter time needed for activating material.

Simple one-step method for agricultural by-products

Agricultural by-products have also proved to be promising raw materials for the production of activated carbons because of their abundant availability at a low price at the source of production. Examples are almond shells, nut shells, apricot stones, cherry stones and grape seeds. A simple one-step method for the production of activated carbons is a feasible alternative to the traditional two-stage process for the production of activated carbons by consecutive carbonization of the raw material and high temperature activation (900–1000°C) of the solid product from carbonization.

Agricultural by-products can be subjected to steam pyrolysis-activation at treatment at temperatures of 600 –700°C. The presence of water vapour during pyrolysis leads to a considerable increase in the liquid and gas product yields in addition to reducing the sulphur content in liquid and solid products. The properties of carbon adsorbents obtained by steam pyrolysis of depend on the treatment conditions such as the temperature and duration of treatment and the choice of raw materials.

Preparation of activated carbon from biomass

The activated carbons can be prepared by one-step pyrolysis of almond shells, nut shells, apricot stones, cherry stones and grape seeds, in the presence of steam.
The raw material is heated in an atmosphere of pure water vapour at a heating rate of 15°C per min to a final carbonization temperature of 800°C for duration of one hour. After the treatment it is left to cool. In order to obtain the carbons with steady content of surface oxides the steam flow is interrupted at a temperature of 300°C. The carbon particles preserve the shapes of the raw materials particles of regular shape.

Preparation of carbon adsorbents

Tar from steam pyrolysis and furfural are treated with 5% H2SO4 and the mixture is heated to 160°C with continuous stirring. The solid products obtained are heated to 600◦C in a covered container at a heating rate of 10°C per min under nitrogen. Oxidation of carbonizates with air at 400°C is used to obtain activated carbon with acidic surface properties. Steam activation with water vapour at 800◦C for one hour is used to obtain activated carbon with alkaline surface.

Electricity from waste water

Electricity can be produced from waste water using microbial Fuel Cell (MFC). It can turn the organic wastes into a source of electricity.
Fuel cell

A fuel cell is an electrochemical energy conversion device that converts the chemical energy from fuel (on the anode side) and oxidant (on the cathode side) directly into electricity. There are many types of fuel cells, depending on what kind of fuel, electrolyte and oxidant they employ. Fuel cells use hydrogen gas which is produced from fossil fuels.
Microbial Fuel Cell

Microbial Fuel Cell makes the treatment of organic pollutants a direct producer of electricity, not a consumer. Further it expands fuel-cell technology to use renewable organic materials as a fuel; the MFC can use organic fuels that are wet, the usual form for wastes and fuel crops. The MFC, by operating at ambient temperature, can double to triple the electricity-capture efficiency over combustion, while eliminating all the air pollution that comes from combustion.

Mechanism

MFCs function on different carbohydrates but also on complex substrates present in wastewaters. As yet there is limited information available about the energy metabolism and nature of the bacteria using the anode as electron acceptor; few electron transfer mechanisms have been established unequivocally. Nonetheless, the efficient electron transfer between the microorganism and the anode (e.g., microorganisms forming a biofilm on graphite fibers) seems to play a major role in the performance of the fuel cell.

Designing considerations for 1MW DOWNDRAFT GASIFIER WITH TAR CRACKING

This write up gives complete considerations for designing and setting up a biomass gasifier based power plant of 1 MW to meet the energy needs of an industry.
1. Need for fixed bed gasifiers
As gasifiers up to 1MW capacity may be of the fixed bed type. Due to its simplicity, this type is recommended. Fluidized bed gasifiers can be built and operated only at higher capacities. Two down draft gasifiers of Imbert type are recommended. Two gasifiers each of 500 kW capacities will give the following advantages.
i. If one unit is shut down for maintenance, the other will deliver power without interrupting the operation of the FC power plant.
ii.Both 500 kW units can work near full load taking advantage of the higher efficiency of the gasifiers at full load.
iii. Flexibility in meeting the fuel wood inventory even in lean periods of supply.
2. Components of the gasifier plant
The gasifier plant is proposed to have the following components in order: a) Fixed bed gasifier (down/up draft), b) Cyclone, c) Hot gas cleaning unit, d) Tar cracking unit , e) High temp. tar trap (filter), f) Secondary gas cleaning unit, g) Shift converter, h) CO2 Scrubber (if needed), I) Boiler, j) Combustor and k) Heat exchangers.
The fuel handling, communition and drying equipments will include: a) Chain conveyors for handling fuel logs to crushers, b) Two stage toothed roll crushers to produce chips of required sizes, c) Chain conveyor for taking chips form the roll crushers to the storage bin, which is assisted by a hot air blower for drying chips, d) Storage bin, e) Screw conveyors with lock hoppers for handling the chips and feeding to the gasifiers, f) Gas sampling ports, g) Temperature sensors, h) CO alarm and oxygen masks and I) Flame arrestors.
3. The gasifier
3.1 Dimensions
The key dimensions of a typical down draft gasifier can be worked out based on a hearth load of 1 Nm3 /cm2. h as follows: Throat diameter: 450mm; No of nozzles: 10; Air nozzle diameter: 18mm ; Fuel container diameter: 1.5 m, Height: 2.5m; Overall dimensions: 2.5 m; Height : 3.0m.
3.2 Insulation of the gasifier
The thickness of insulation with kaolin is 500 mm for the assumed heat load.
3.3 Biomass storage
The requirement of biomass is about 18 tonnes per day. The storage space is required for an inventory of 600 tonnes to meet the demand for one month.
The wood logs can be stored as a stack facilitating natural drying. An area of 40 m by 30m is needed for the installation of two stage roll crushers and chain conveyors to feed the logs into the crushers. An additional area of a minimum of 2m by 7m is needed for the trucks to unload the logs into the storage space.
3.4 Chips handling
In order to feed the gasifiers with chips of say 30 by 40 by 70 mm size, roll crushers are needed. In the first stage logs are broken down to small pieces. In the second stage chips of desired size are prepared. The power consumption of the crushers and chain conveyors will be 30 kW with 225 mm serrated rolls to handle logs of up to 150 mm diameter.
The chips shall be dried by hot air generated by the waste heat available from the combustor exhaust gas through a heat exchanger or by dilution with air.
It is proposed that the dry chips shall be fed by screw conveyors to the gasifiers from the top. The chip storage bunker may contain a scraper for feeding the horizontal screws fitted at the end of the storage space.
The screw conveyor may consist of a horizontal section to collect chips from the bunker, an upright section to the top of the gasifier and a short section to feed the gasifiers. Lock hoppers may be provided at appropriate locations to prevent gas leakage during feeding. The diameter of the screw is 225 mm in this case.
3.5 Ash handling
A short screw section can be used followed by pneumatic handling after collecting from the bottom of the gasifiers. The grate needs to be kept cool in order to avoid warping. For collecting ash from the bottom a rotating grate with a scraper can be used.
3.6 Air handling
A continuous supply of air at about 30 % of the stoichiometric requirement is needed. This shall be supplied by a blower of 600 Nm3 per hour capacity. The hot air blower for chip drying shall have a capacity of 200 Nm3 per hour.
4. Tar cracking
Particulates, volatiles and tar have to be removed from the producer gas prior to the reformer in case power is to be produced by internal combustion engine. Particulates are removed in a cyclone and hot gas filters. For tar cracking a temperature of 800 to 900 deg C is necessary. Several studies indicate that dolomite is an efficient catalyst. Glanshammar dolomite has been shown to yield 2 g tar per kg DS. The hydrogen content of the gas is increased at the same time. It is possible to have the tar cracking unit either integrated with the gasifier taking advantage of the heat loss at high temperature zone or to set up the unit outside the gasifier. The dust load and fluctuation in temperature coupled with gas leakage problems favour a separate tar cracker .
4.1 T.C. Unit specifications
The amount of tar contained in the producer gas ranges as follows. Downdraft: 0.001 to 0.01 kg per kg dry feed.
Studies conducted on tar cracking using pyrolysis gas indicate that a temperature of 800 to 900 deg.C is required to minimize the tar content in the exit gas. A catalyst load of 0.25 to 0.30 kg per kg DS may be adopted. The detailed dimensions is to be worked out based on the component fractions, amount of tar, regeneration possibilities of catalyst, catalyst granulometry, temperature profile, etc.
4.2 Combustor
A combustor is needed to produce hot gas to heat the reforming catalyst bed to around 900 deg. C. A swirl gas combustor of 300 mm diameter and 280 mm shroud length can be tentatively suggested. The fuel gas can be taken from the producer gas plant. The requirement shall be calculated based on the differential temperature, catalyst bed temperature, catalyst bed temperature and heat capacity of the bed.
4.3 Drying of chips
It is wise to dry chips rather than logs. Hot air at 60 to 70 deg C is recommended. Hot air can be obtained by mixing about 10 times air with the combustor exhaust gas.
5. Steam boiler
Steam has to be supplied to the reformer and the shift converter to get hydrogen rich gas in case of running a fuel cell. The hot water from the fuel cell generator and waste heat from the gasifier can be used for steam generation. The steam capacity of the boiler is estimated to be 800 kg per hour at a pressure of 20 bar giving super heated steam at 850 deg.C.
6. Housing and instrumentation
The housing requirement is estimated as follows. Biomass storage, crushers and dryer: 50 m by 40 m. Gasifiers, tar cracking units and reformer: 10 m by 4.5m, Steam boiler, combustor and heat exchanger: 2.5 m by 3 m. Safety instruments such as CO monitor & alarm (150ppm), oxygen masks, fire protection machineries will also be housed. Flame arresters are to be provided in the gasifiers and combustors. Data monitoring and acquisition systems for chips level in bunker and gasifier, temperature and gas flow need to be incorporated.