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.