BOILER DESIGN

Efficiency of a boiler is the result of its basic heat transfer design. Boiler designs that use all possible heat transfer surfaces to their fullest advantage consistently produce the most efficient source of steam or hot water with the lowest total lifetime costs.
SOFTWARE FOR FIRE-TUBE & WATER-TUBE BOILERS
Softwares are based on new theoretical findings on heat transfer in boilers. It delivers unprecedented proven +/-2% uncertainty. It can be used to carry out tasks like:
•    heat transfer analysis in boilers
•    boiler design optimization
•    retrofit of installed boilers.
Software calculates heat transfer in fossil fuel fired fire-tube and water-tube steam, hot water and waste heat boilers of arbitrary geometry and for arbitrary operating conditions (excess air, pressure up to 99bar/1435psi). Boiler can consist of multiple segments such as furnace, channels, chambers and tubes, with or without front or/and rear cooled door. The boiler segments can be combined, which makes calculation of next to every known boiler design possible.
Water-tube boilers are in essence of relatively simple basic design consisting of a large rectangular combustion chamber while fire-tube ones come in various designs. Following fire-boiler designs can be calculated with the software:
1. Tubular boiler
•    single pass or double pass
•    horizontal or vertical
•    tubes can have coiled-wire turbulators or dents.
2. Furnace (combustion chamber)
•    circular or rectangular
•    horizontal or vertical
•    with or without rectangular fins
•    wetback or dryback
•    with or without front cooled door.
3. Channel after furnace (if any)
•    one or more
•    circular or rectangular
•    horizontal or vertical
•    with or without rectangular fins
•    with or without baffles.
4. Boiler with 1, 2 or 3 tube assemblies after furnace (2-pass, 2-pass reverse flue, 3-pass, 4-pass)
•    horizontal or vertical
•    tubes can have coiled-wire turbulators or dents.
5. Redirection channel between two tube sets (one in 3-pass and two in 4-pass boilers)
•    circular or rectangular
•    horizontal or vertical.
6. Channel before boiler exit (if any)
•    one or more
•    circular or rectangular
•    horizontal or vertical.
7. Boiler with cooled rear door.
8. Waste heat boiler or exhaust gas boiler (such as hybrid biomass system) without or with supplemental firing .
9. Tubular economizer with or without fins (circular or rectangular). Can also be calculated separately as can stoichiometry of combustion.
Boiler can be made of following materials:
•    boiler steel
•    cast iron.
Economizer tubes can be made of following materials:
•    steel
•    copper.
Boiler liquid can be following:
•    water
•    Rankine cycle liquid
•    thermal oil.
Input and output values can be either in Metric or English units.
Order of calculations
1. A stoichiometry of the combustion, adiabatic combustion temperature, flue-gas enthalpy at that temperature, and a first estimation for the furnace exit temperature are calculated.
2. Convective and radiant parts of the heat transfer are considered to be simultaneously coexistent. For convection, the relevant temperature is mean logarithmic, for radiation the mean radiant temperature that is calculated by proprietary procedure.
3. The impact of the turbulators or tube dents is taken into account according to the proprietary developed procedure.
4. The convective and radiant parts of the heat transfer in the furnace are summed-up and deducted from the flue gas enthalpy at the adiabatic combustion temperature. The result is a temperature at the furnace exit.
5. The calculated flue-gas exit temperature is compared to the estimated one in step 1. If they do not meet the preset difference (0.1°C) a new estimation is calculated as the average of assumed one in step 1 and just calculated one. The procedure is then repeated.
6. This same procedure is applied to every boiler segment using the exit temperature of flue gas from one segment as being the initial temperature in the next section. Thus, the initial temperature in next boiler section is equivalent to the exit temperature from preceding section.
7. The sum of the heat transferred in all boiler sections represents its heat output which, when divided by heat released from fuel, yields the boiler efficiency.
Discrepancy sources between measured and calculated values
There are always differences between calculated and actual values. The discrepancies are of an operational (fouling, scaling on gas and water side) and of mathematical nature (heat transfer equations are always derived from experiments). In general, for new and well maintained boilers the discrepancy to actual values is as low as 2% (boiler output, steam capacity, efficiency).
Heat balance for a boiler
Fuel fed to a burner is converted to heat. The heat is absorbed by the water in the boiler, but not all the heat released from the fuel is used to heat the water. Some of the heat is wasted in the process. The heat balance of a boiler consists of accounting for all the heat units in the fuel used or wasted. It is a balance because it is the sum of all the heat consumed. Heat balance of a boiler is found by using the following equation:
A = B + C
A= heat energy available in the fuel
B= heat energy absorbed any the water in the boiler
C= heat losses
Therefore: Efficiency of the unit = B divided by A
Energy losses in a boiler
􀂃 Gases of combustion to atmosphere (about 9%)
􀂃 Incomplete combustion (about 1%)
􀂃 Moisture in fuel (less than 1%)
􀂃 Moisture in air used for combustion (less than 1%)
􀂃 Water vapor produced from the burning of hydrogen (about 2%)
􀂃 Unburned combustibles (about 2%)
􀂃 Radiation (about3%)
Total heat losses in this examples equal 18-19% While some of these losses are preventable losses over which the boiler operator has control, such as:
􀂃 Heat carried away in the dry chimney gases
􀂃 Incomplete combustion of the fuel
􀂃 Unburned combustibles (about 2%)

New batteries

The new batteries can withstand extremely high rates of charge and discharge which will cause electrodes used in conventional Li-ion batteries to rapidly deteriorate and fail. The greatest advantage of the new batteries is that charging laptop or cell phone in a few minutes, rather than an hour.
Construction
The new batteries are made from a carbon (C) nanorod base topped with a thin layer of nanoscale aluminum (Al) and a "scoop" of nanoscale silicon (Si), the structures are flexible and able to quickly accept and discharge Li ions at extremely fast rates without sustaining significant damage. The segmented structure of the nanoscoop allows the strain to be gradually transferred from the C base to the Al layer, and finally to the Si scoop. This natural strain gradation provides for a less abrupt transition in stress across the material interfaces, leading to improved structural integrity of the electrode.
Charging cycle
The anode structure of a Li-ion battery physically grows and shrinks as the battery charges or discharges. When charging, the addition of Li ions increases the volume of the anode, while discharging has the opposite effect. These volume changes result in a buildup of stress in the anode. Too great a stress that builds up too quickly, as in the case of a battery charging or discharging at high speeds, can cause the battery to fail prematurely. This is why most batteries in today's portable electronic devices like cell phones and laptops charge very slowly, the slow charge rate is intentional and designed to protect the battery from stress-induced damage. Due to their nanoscale size, nanoscoops can soak and release Li at high rates far more effectively than the macroscale anodes used in today's Li-ion batteries.
Limitation
A limitation of the nanoscoop architecture is the relatively low total mass of the electrode, according to the researchers and to solve this, the researchers are to trying to grow longer scoops with greater mass, or develop a method for stacking layers of nanoscoops on top of each other or to grow the nanoscoops on large flexible substrates that can be rolled or shaped to fit along the contours or chassis or chassis of the device.
The nanomaterial for a new breed of high-power rechargeable lithium (Li)-ion batteries has been  developed at Rensselaer Polytechnic Institute.

About biomass conversion technologies

Biomass composition
Biomass fuels consist of three main segments: wood, waste, and alcohol fuels. Wood energy is derived from the following sources: round wood, used primarily in the industrial and electric utility sectors; wood fuel, used predominantly in the residential and commercial sectors; and wood byproducts and wood waste, which are usually used in the industrial sector. The chemical composition of plant biomass varies among species and has approximately 25% lignin and 75% carbohydrates or sugars linked together in long chains or polymers as cellulose and hemi-cellulose. The lignin fraction consists of non-sugar type molecules that act as a glue holding together the cellulose fibers.
Biomass categories
• Solid Biomass, which includes tree, crop residues like rice husk, bagasse, coconut shells, jute waste, etc. and animal and human waste.
• Biogas which is obtained by anaerobically digesting organic material to produce combustible gas methane.
• Liquid biofuels which are obtained by subjecting organic materials to one of various chemical or physical processes to produce usable combustible liquid fuels.
Conversion methods
A number of technological options are available to make use of a wide variety of biomass types as a renewable energy source. Conversion may release energy directly in the form of heat/electricity or, may convert it into another form such as liquid biofuels or combustible biogas. There are basically three types of conversions:
• Thermal Conversion- A process in which heat is used to convert biomass into another chemical form.
• Chemical Conversion – A range of chemical processes may be used to convert biomass into other forms so that fuel may be used more conveniently, transported or stored.
• Biochemical Conversion- It involves anaerobic digestion fermentation and composting.
Gasification technology
Gasification is one of the most promising technologies in biomass applications particularly because of higher efficiency compared to boiler power systems, amenability to fuel synthesis and environmental friendly features. Biomass gasification has evolved over a long period, but it has not reached a solid commercial stage, except during periods of crises and only for some specific applications while, other gasification technologies, fed by fossil fuels, are currently widely used on industrial scales.
Historical evolution gas plants
The first producer gas plant was designed around 1850. After 1880 the technology found wider application. Producer gas plants used during the First World War were very well described in the literature. The application of small gasification systems for traction assumed enormous proportions by the end of Second World War. Approximately 1 million vehicles were powered by wood blocks, peat, charcoal or anthracite. When oil and gas took over the predominant part of coal and wood, the development of producer gas plants was limited to countries with exceptional local circumstances, such as South Africa where large pressurized Lurgi plants were in operation. It was only around 1970 that attention was paid again to small scale gasification of biomass especially in remote areas of developing countries. Some year’s later research and development programmes were initiated as a result of the energy crisis and concern for the environment. Many conference papers and workshop document these activities and present a good survey on recent research and development on biomass gasification.
Combustion in a controlled atmosphere is the conversion of solid or liquid to a gas. If the oxygen supply is restricted, incomplete combustion of fuel occurs releasing combustible gases such as CO,H2 and CH4. Investigations on Gas producer-Engine systems, a century old technology and forgotten art, have experienced renewed interest in recent years. More than 12000 gas producer plants were in operation in the United States and Canada during the 1920s and 1930's.
Austria, Finland, Germany and Sweden have increasing number of ongoing projects and initiatives since 1970. The development of this technology has been encouraged by the price of fossil fuels and an enhanced interest for biomass gasification as a future alternative. But this technology has not sufficiently achieved a sustainable commercial status. Experimentation activities, demonstration projects and pilot plants, have proved the future potential of the technology.
Small and large scale projects have followed different development routes. In the case of large scale, interest has shifted from electricity generation to biofuel production, primarily due to the failed demonstration projects of the technology coupled with combined cycle for electricity generation. On the other hand, in small scale projects, cogeneration applications have gained interest over heat production. Also in small scale experimentation the causes which have hindered the technology to reach the expected commercial stage has been the lack of resources to demonstrate its competitiveness. Objectives of achieving the success of gasification technology will depend on incentives created by all administrative levels.
Power generation modes
Biomass is a renewable source of energy that can be made use of effectively for thermal as well as electrical energy generation. The biomass based power plants are of smaller capacity, usually situated either at agro-based industries, where the biomass is generated or at a place where crop residue or other type of biomass is located. This decentralized power plant will help in supplying quality power and reduce the transmission and distribution losses. Biomass based power plants can either be as cogeneration or as independent power plant.
Cogeneration
Cogeneration is defined as generation of process heat and motive power by the sequential use of energy from a common fuel source. This is an energy efficient technology as it utilizes the low grade exhaust heat from the steam turbine for process heating. This enhances the efficiency of energy utilization from 35% in the conventional power generating system to 70-90% in the co generating system.
Combustion
Combustion route is an established and reliable system to generate power from biomass. This system consists of boiler, turbine, power evacuation scheme and interfacing with the grid. This system requires water treatment plant, deaerator, DM water plant and condensate polishing unit, condenser and cooling tower.

Tape Calandering of Solid oxide fuel cell

Solid oxide fuel cell
A solid oxide fuel cell (SOFC) produces electricity directly from oxidizing a fuel and has a solid oxide or ceramic, electrolyte. Advantages of solid oxide fuel cells include high efficiency, long-term stability, fuel flexibility, low emissions, and relatively low cost. The largest disadvantage is the high operating temperature which results in longer start-up times and mechanical and chemical compatibility issues.
Methods of fabrication
Currently, all R&D efforts are focused on fabrication methods of ceramic SOFC components.
SOFC components manufacturing processes.
Stiff structural ceramic parts can be manufactured by:
Extrusion
Dry pressing
Tape casting and
Calandering
Functional ceramic parts can be manufactured by:
Screen printing
Slurry coating
EVD, PVD and
Plasma and flame spraying
Tape Calandering
In Tape Calandering , a continuous thin sheet or tape of controlled thickness is produced. A high intensity mixer is used to mix the ceramic powder, binder and plasticiser. The mixing results in heating-up the batch and softening the binder to form a plastic mass. Then, the mass is rolled to form a thin, flat tape using a two-roll mill, and adjusting the spacing between the two mills controls the tape thickness.
This technique is used to fabricate monolithic FCs via a co-sintering process, where the individual tapes are laminated in a second rolling operation, and FCs with very thin electrolyte layer, which enables to operate at low temperatures (600 ºC) is produced.
The fabrication details of typical 2 cm2 area fuel cell components which includes preparation of anode, electrolyte, cathode cermet pastes, nickel net (60 mesh and grip on anode side) are given below.
• Anode: NiO,
• Cathode: perovskite Ca0,9La0,1MnO3 (CLM),
• Electrolyte (SDCm-1)
Cermet proportions
• Anode: Electrolyte cermet, at: 1:1, % by volume
• Cathode/electrolyte cermet, at 1:1 % by volume
• Binder : 3% Teflon by mass to every component
• Nickel net dimensions: Anode: 22x22 mm,
• Anode/Cathode thickness: 0.5 mm each, including nickel net
• Electrolyte thickness: 0.5 mm
• Sintering of samples: 600 °C, 2 h under nitrogen atmosphere

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.