Gasification of coal
Gasification of coal
Gasification of coal is a conversion technology which converts any carbon (C) containing material, such as coal, into synthesis gas (syngas). It is a high temperature process with temperature reaching typically 1,225 deg C. The temperature is optimized to produce a fuel gas with a minimum of liquid and solids. This process consists of heating the feed material coal in a vessel with or without the addition of oxygen (O2). Carbon reacts with water in the form of steam and O2 at relatively high pressure typically greater that 30 kilograms per square centimeter (kg/sq cm) and produce raw syngas, a mixture composed primarily of carbon monoxide (CO) and hydrogen (H2) and some minor byproducts. The byproducts are removed to produce a clean syngas which can be used (i) as a fuel to generate power or steam, (ii) as a basic chemical building block for a large number of uses in the petrochemical and refining industries, and (ii) for the production of H2. Gasification adds value to low- or negative-value feed stock by converting it to marketable fuels and products. The basics of the gasification process are given in Fig 1.
Fig 1 Basics of the gasification process
History and present development
Town gas, a gaseous product manufactured from coal, containing approximately 50 % H2, with the rest comprised of mostly methane (CH4) and carbon dioxide (CO2), with 3 % to 6 % CO, is a gaseous product manufactured from coal. It is being used since early 1800s.
The original process for Coal gasification was developed by the German researchers Franz Fischer and Hans Tropsch at the Kaiser Wilhelm institute in 1920s. Gasification was used extensively during World War II to convert coal into transportation fuels via the Fischer-Tropsch process. Sasol has built a plant at Sasolburg with the prime objective of converting low grade coal into petroleum products and the first liquid was produced from this plant in 1955. The coal gasification process has been used extensively in the last 50 to 60 years to convert coal and heavy oil into H2 for the production of ammonia / urea fertilizer. The chemical industry and the refinery industry applied gasification in the 1960s and 1980s, respectively, for feedstock preparation. In the past 10 to 15 years, the process is being used by the power industry in Integrated Gasification Combined Cycle (IGCC) plants.
Fischer- Tropsch process
The Fischer-Tropsch process is a catalyzed chemical reaction in which CO and H2 are converted into liquid hydrocarbons of various forms. Typical catalysts used are cobalt and iron. The main purpose of this process is to produce synthetic fuel. The utility of this process is mainly due to its ability to produce fluid hydro-carbons or H2 from a solid feed stock.
The original Fischer-Tropsch process is described by the chemical equation (2n+1)H2 + nCO = CnH(2n+2) + nH2O. Initial reactants in the above reaction (i.e., CO + H2) can be produced by other reactions such as the partial combustion of CH4 in the case of GTL (gas to liquid) applications as per the equation CH4 + 0.5O2 = 2H2 + CO or by the gasification of coal in the case of CTL (coal to liquid) as per the equation C + H2O = H2 + CO. The energy needed for the reaction of coal and steam is provided by adding air or O2. This leads to the reaction C + 0.5O2 = CO. The production and synthesis of syngas by Fischer- Tropsch process is shown in Fig 2.
Fig 2 Production and synthesis of syngas by Fischer- Tropsch process
Comparison of coal gasification with coal combustion
Gasification and combustion can essentially be considered as two ends of a range for reactions. The combustion is full oxidation while the gasification is partial oxidation. Also, combustion occurs in an oxidizing (excess O2) environment while gasification occurs in a reducing (O2 depleted) environment. Gasification is more efficient and has lower emissions. In case of gasification, water can be added as a reactant to increase the H2 content of the product. Tab 1 provides a list of the most significant reactions and the enthalpy change associated with each of these reactions. Looking at the first two reactions in the table, it is seen that coal denoted as C is reacted with one O2 atom (0.5 O2) to get CO and with two O2 atoms to get CO2. In reality, the second reaction is not a one step process as the solid phase C reacts with one O2 atom to produce CO which then reacts with the second O2 atom to form CO2. All of the reactions in the table are exothermic except the two reactions identified as gasification with steam and gasification with CO2. These two endothermic reactions are the reactions which are most often referred to as gasification, where the solid C is turned into a reactive gas through a reaction with a ‘non-reactive’ gas (H2O or CO2). In addition to that these two reactions being endothermic require high temperatures to proceed.
The general partial oxidation reaction is 2CHn + O2 = 2CO + nH2. The consumption of O2 in the process depends on the ash content and CV of the coal. Insufficient supply of O2 ensures partial oxidation of the coal. This reaction produces a mixture of gases namely H2, CO, CH4, and CO2. The end product is syngas. The mixture’s composition changes with the pressure.
|Tab 1 Gasification and combustion reactions|
|Sl. No.||Reaction process||Equation||Change in enthalpy|
|1||Gasification with O2||C + 0.5O2 = CO||-2180.3 kcal/kg C|
|2||Combustion with O2||C + O2 = CO2||-7844.7 kcal/kg C|
|3||Gasification with CO2||C + CO2 = 2CO||3484 kcal/kg C|
|4||Gasification with steam||C + H2O = CO + H2||2640.7 kcal/kg C|
|5||Gasification with H2||C + 2H2 = CH4||-1485.4 kcal/kg C|
|6||Water gas shift reaction||CO + H2O = CO2 + H2||-361.4 kcal/kg CO|
|7||Methanation||CO + 3H2 = CH4 + H2O||-1768.4 kcal/kg CO|
Coal gasification is carried out with limited amount of O2 which is around one-fifth to one-third of the theoretically O2 required for complete combustion. Only a fraction of C is burned for generation of heat. H2 and CO are the main products of gasification. CH4 and CO2 are the other two major products and their content goes up with increasing pressure and H2+CO content going down. In coal gasifiers, two physico-chemical processes take place. They are (i) pyrolysis or devolatilization process, and (ii) gasification process.
In the pyrolysis process, as the coal enters into the gasifier, it is first dried by the hot gases present in the gasifier. A series of complex physical and chemical process start slowly at temperature less than 350 deg C and accelerate as temperature exceeds 700 deg C. The composition of the released products of pyrolysis is dependent on the temperature, pressure and gas composition during pyrolysis. The pyrolysis process can be represented by the reaction Coal –> Heat –> Char –> Gases –> Vapours or liquid.
The three products which are produced by pyrolysis are (i) light gases such as CO, H2, CO2, CH4, and H2O (water vapour), (ii) tar which is a corrosive and viscous liquid composed of heavy inorganic and organic molecules, and (iii) char which is a solid residue mainly contains C.
The gasification process involves a series of endothermic reactions which are supported by the heat produced from the combustion reactions occurring inside the gasifier. These reactions are represented by the equations (i) C + O2 = CO2 with delta H = -94.05 kcal/mol, and (ii) H2 + 0.5O2 = H2O with delta H = – 68.3 kcal/mol. The major gasification reactions which take place are (i) water gas shift reaction, (ii) Boudouard reaction, (iii) shift conversion, and (iv) methanation.
In water gas shift reaction where the partial oxidation of C by steam occurs and is represented by the equation C + H2O = H2 + CO with delta H = 28/3 kcal/mol. During the Boudouard reaction, the char present in the gasifier reacts with the CO2 and produces CO. The reversible reaction is represented by the equation CO2 + C = 2CO with delta H = 38 kcal/mol. The shift conversion is an endothermic reaction and is known as water–gas shift reaction. Due to this reaction, there is an increase in the ratio H2 to CO in the gas. This reaction is used during the production of syngas. The reaction is CO + H2O = CO2 + H2 with delta H = – 10.1 kcal/mol. For methanation, nickel based catalyst is used. This catalyst at 1100 deg C and at a pressure of 6 kg/sq cm to 8 kg/sq cm accelerate the reaction of the formation of CH4 which is preferred in IGCC applications because of its high heating value. The reaction involved is given by the equation C + 2H2 = CH4 with delta H = 17.8 kcal/mol.
The complete gasification reactions are carried out in the gasifiers which are required to be operated at certain temperature in order to drive certain endothermic C – steam and C – CO2 reactions. The required temperature is maintained by heat evolved from exothermic reaction between O2 and coal.
Depending upon the medium of gasification, gasifiers are classified into two categories namely (i) air blown, and (ii) O2 blown. In air blown gasifiers, air is used as gasification medium while in O2 blown gasifiers pure O2 is used as gasification medium. When air is used as gasification medium, the N2 is simultaneously brought into the process which results in the product gas dilution. As a result product gas has a lower calorific value(CV).
Depending upon the contact between gas and fuel, there are four types of gasifiers (Fig 3). These are namely (i) moving or fixed bed gasifier, (ii) fluidized bed gasifier, (iii) entrained bed gasifier, and (iv) transport flow gasifier. All the four types of gasifiers are based on partial oxidation (gasification) of a carbonaceous (C containing) feed material (coal). While each of these can make an acceptable reducing gas for the production of DRI, the fixed bed and fluidized bed gasifiers are the preferred choice for high ash coals.
Fig 3 Types of gasifiers
Moving bed gasification technology is the oldest technology and is being used widely. The gasifier is also known as fixed bed gasifier. Gasification medium slowly flows through a fixed bed of solid particles. The two possible configurations of this type of gasifiers are up-draft and down-draft depending upon the direction of flow of the gasification medium. The up-draft configuration is more commonly used since there is low tar content. The preferred feed coal size is 5 mm to 80 mm. The combustion zone attains a maximum temperature of around 1500 deg C to 1800 deg C and for the slagging and dry ash gasification zone a maximum temperature of around 1300 deg C. The temperature profile is formed over the bed, so that the feed coal is successively preheated, dried, pyrolyzed, gasified and combusted. Lurgi gasifier is the oldest moving bed gasifier technology.
The fluidized bed gasifier has the bed of solid particles which behaves as a fluid. In this type of gasifier, the particle size of the feedstock is less than 5 mm and the particles are suspended in the O2 rich gas. The rising gas reacts with the feedstock and maintains the fluidized state of the coal particles. A uniform temperature distribution is achieved in this type of gasifiers. Also, in this type of gasifier, the clinker formation and de-fluidization of the bed is avoided since the operating temperature is in the range of 800 deg C to 1050 deg C which is well below the ash fusion temperature. Ash discharge can be carried out in the form of either the dry or the agglomerated ash. Dry ash fluidized bed gasifier is traditionally being used for the low rank coals. The agglomerated ash fluidized bed gasifier is being used for any rank of coal.
Entrained flow gasifier uses pulverized coal particles of size less than 0.1 mm which are suspended in a stream of steam and O2 at high speed. Depending upon the method of coal feeding, dry (nitrogen being used as a transport gas) or wet (carried in water slurry), gasifiers are accepting almost any type of coal. Entrained flow gasifiers ensure high C conversion since they operate at a high temperature range of 1400 deg C to 1600 deg C (well above the ash slagging temperature). These gasifiers are of high capacity since the gas residence time is measured in seconds.
The transport gasifiers are dry fed non-slagging gasifiers. The transport gasifier is based upon the hydrodynamic flow field. It has excellent gas-solids contact and very low mass transfer resistance between gas and solids. It has a highly turbulent atmosphere which allows for high coal throughput and high heat release rates at a low temperature that avoids problems with slag handling and liner erosion.
In addition to the desired CO and H2, the syngas leaving a gasifier also contains other compounds. The product of gasification contains desirable components like CO, H2, CH4 and undesirable components like CO2, H2O, ash, entrained soot, tar, particulate matter, certain amount of H2S (hydrogen sulphide), and traces of ammonia, hydrochloric acid, hydrogen cyanide. Hence, cleaning of syngas is an important aspect of the coal gasification process. The undesirable components need to be removed from product gas. There are a number of techniques being used to remove the undesirable components.
Product gases (CO, H2, and CH4) of the coal gasification process have fuel value. If a fixed bed gasification technology is used, the syngas also contains aromatic organic compounds. Typically, 1 kg of bituminous coal can be converted into 1.5 cum to 1.7 cum of syngas.
In terms of feedstock flexibility, several gasification plant designs have been developed to utilize various grades of coal. Gasification results in very low gaseous emissions of conventional (non GHG) pollutants, due to the nature of the process operation. It also offers a potentially low marginal cost route for capturing the resulting CO2 by-product for either geological storage or enhanced oil recovery from the oil fields.
In addition, coal gasification processes require significant water use. They are also large emitters of CO2. For I ton of syngas, typical coal consumption is around 2.8 tons, water requirement is around 6.6 tons and CO2 generation is around 2.5 tons. The CO2 is released is as a byproduct and can be sold or compressed for conveying to the underground storage.
There are several gasifier concepts. A general description of the reactors of some of the major gasifier concepts is given below.
GE Energy gasifier – GE Energy acquired its gasification technology from Chevron in 2004. The GE coal gasifier comprises a single-stage, downward-feed, entrained-flow refractory lined gasifier to produce syngas. Coal/water slurry is pumped into the top of the gasifier, which together with O2 is introduced through a single burner (Fig 4). The coal reacts exothermically with the O2 at high temperature (1200 deg C to 1480 deg C) to form syngas. The syngas contains mostly H2 and CO, and slag.
Fig 4 Coal gasifier of GE energy
The slag which flows downwards is quenched and then removed from the bottom of the gasifier via a lock-hopper arrangement. The water leaving the lock-hopper is separated from the slag and sent to a scrubbing unit after which it can be recycled for slurry preparation. The raw syngas leaving the gasifier can be cooled by a radiant and/or convective heat exchanger and/or by a direct quench system, where water is injected into the hot raw syngas. The selection from these alternatives is a choice of cost and application.
The radiant cooling design uses a soot-tolerant radiant syngas cooler which generates high-pressure steam. Slag is quenched in a water pool located at the bottom of the reactor vessel, and removed through a lock-hopper. The syngas is further cooled after leaving the gasifier by a water scrubber to remove the fine particulate matter, before the gas is sent on to downstream processing. The direct quench system uses an exit gas water quench where hot gas leaving the gasifier is contacted directly with water via a quench ring. It is then immersed in water in the lower portion of the gasifier vessel. The cooled, saturated syngas is then sent to a scrubber for soot and particulate removal. The quench design is less efficient, but also less costly, and it is commonly used when a higher H2 to CO ratio syngas is needed.
Conoco Phillips E-Gas gasifier – The Conoco Philips E-gas gasifier was originally developed by DOW Chemicals and demonstrated at the Louisiana Gasification Technology Inc. (LGTI) from 1987 through 1995. It is an entrained flow gasifier and is shown in Fig 5. It is a two-stage gasifier with 80 % of feed to first stage (lower). The gasifier is coal-water slurry fed, O2-blown, refractory-lined gasifier with continuous slag removal system and dry particulate removal. The E-Gas process is good for a wide range of coals.
Fig 5 Conoco Philips E-gas gasifier
Shell gasifier – Gasification technology of Shell comprises a dry-feed, pressurized, entrained flow, slagging gasifier. The coal-based variant was developed in the 1970s. Coal is pulverized and fed to the gasifier through two sets of horizontally opposed burners using a transport gas (either syngas or nitrogen). Preheated O2 and steam (as a moderator) are mixed and fed to the injector, where they react with the coal to produce syngas consisting mainly of H2 and CO with only small amount of CO2 and no hydrocarbon liquids or gases. The hot product gases flow upward through a vertical membrane cylindrical wall, as shown in Fig 6.
Fig 6 Shell strained flow gasifier
Molten ash entrained with the upward-flowing syngas is deposited on the water walls and flows downwards. It is removed through the base of the gasifier where it is quenched in a water bath. The raw syngas leaves the gasifier in the temperature range 1370 deg C to 1480 deg C and is then treated with lower temperature recycled product gas to convert any entrained molten fly slag to a hardened solid material. It then enters the syngas cooler for heat recovery, generating high-pressure (HP) superheated steam. The bulk of the fly ash contained in the raw syngas leaving the syngas cooler is removed from the gas using either commercial filter equipment or cyclones. Any remaining fly ash is captured downstream with a wet scrubber.
Siemens gasifier – The Siemens gasifier is a dry-feed, pressurized, entrained-flow system, with a top-fired burner through which coal together with O2 and steam is introduced (Fig 7). It can be designed with either a refractory lining, for low ash feed stocks, or with a gas-tight membrane wall structure in the gasification section of the gasifier.
Fig 7 Siemens entrained flow gasifier
The molten slag formed in the gasifier flows down the reactor chamber into the quench section where it solidifies upon contact with water from a ring of quench nozzles and is removed through a lock-hopper arrangement. The gasifier can achieve C conversion rates higher than 99 % and the technology is well suited for all types of coals from anthracite to lignite.
KBR TRIG coal gasifier – The Transport Integrated Gasification (TRIG) technology was developed by the Southern company and KBR Inc. It is designed to process reactive low rank coals, including those with upto 50 % ash and high moisture content, and can be operated with steam and either air or O2 as the gasification medium. Air-blown operation is preferable for power generation, while O2 blown operation is better suited for syngas production. The simplified layout of the TRIG process is shown in Fig 8.
Fig 8 Simplified layout of the TRIG process
The system comprises a circulating gasifier, which consists of a mixing zone, riser, disengager, cyclone, standpipe, loop seal, and J-leg. This is designed to operate at high solids circulation rates and gas velocities, resulting in higher throughput, C conversion and efficiency. The raw syngas is formed in the riser portion of the unit, from which laden with unreacted solids it passes through a series of cyclones where the solids are removed. The ash material is recirculated through the riser to allow unconverted C to be utilized and to provide heat to the gasifier. As ash accumulates in the down comer, it is discharged from the unit. The gasifier operates at moderate temperatures and below the melting point of ash, which can increase the equipment reliability and availability. The latter is enhanced by the use of a downstream particulate filter, which eliminates water scrubbing and significantly reduces plant water consumption and effluent discharge.
Lurgi dry bottom pressurized coal gasifier – The Sasol Lurgi gasification process comprises the reaction of steam and O2 with lump sized, low or medium caking coals on a rotating grate at pressures of 20 kg/sq cm to 30 kg/sq cm. The gasifier for the dry bottom pressurized coal gasification process is shown in Fig 9.
Fig 9 Lurgi dry bottom pressurized coal gasifier
In the bottom combustion zone at the grate, the coal char is burned with O2 to provide energy for the gasification reactions. As the coal moves down the gasifier, it is heated by the upward-flowing syngas which leaves the gasifier. The heat causes the coal to dry followed by devolatilization. Some of the devolatilized products escape before reacting and leave the gasifier with the raw syngas. As the devolatilized coal moves down, it is gasified with combustion products from the combustion zone below. In the dry ash mode of operation, excess steam is injected with O2 to keep the temperature below the ash fusion temperature. A motor driven rotating ash grate is used to remove ash in a ‘dry’ state and also to support the coal bed.
The counter-current flow of gasification agent and fuel results in a high thermal efficiency of the gasifier to produce a raw gas with heating values of around 2650 kcal/cum to 2850 kcal/cum. Depending on the characteristics of the feed coal, the product gas contains by volume 25 % to 33 % CO2, 15 % to 21 % CO, 35 % to 41 % H2 and 10 % to 13 % CH4. For use as syngas, CH4 and CO2 are required to be removed.
Since the 1960s, the Lurgi process has been improved through increases in reactor size and components, extension of the feed coal slate to include low rank coals, and the use of air instead of O2 as the gasification agent. In addition, the design has been demonstrated for operation at upto 100 kg/sq cm pressure in order to increase the gasifier throughput while at the same time increasing the CH4 content of the raw gas.
The British Gas Corporation, in co-operation with Lurgi, developed a new design of the gasifier bottom in order to avoid the problems associated with rotating equipment in the fuel/ash bed, while simultaneously overcoming the limitation set by the ash softening temperature in the gasification zone. This resulted in the BGL slagging gasifier. The gasifier differs from the standard Lurgi reactor through (i) the replacement of the grate and ash lock by a hearth for liquid slag tapping, (ii) the introduction of the gasification agent O2 and steam by means of tuyeres instead of through the grate, and (iii) the use of refractory lining in the lower part of the reactor body to reduce heat loss.
BGL slagging gasifier also operates at higher gasification temperatures than the standard Lurgi gasifier and, hence, the CO/CO2 ratio in the product gas is higher and the CH4 content correspondingly lower. Typical gas compositions by volume are 2 % to 3 % CO2, 55 % to 60 % CO, 25 % to 28 % H2 and 6 % to 9 % CH4. The high temperature provides for a better steam utilization and, therefore, the amount of water which is needed to be cleaned and processed, is much reduced. Coal ash is converted into slag which forms a non-leachable glass on removal. This requires a low slag viscosity, which is obtained by adding fluxing agents, usually limestone or basic blast furnace (BF) slag
MHI gasifier – The Mitsubishi Heavy Industries (MHI) gasifier is based upon the Combustion Engineering air-blown slagging gasifier and co-developed between Combustion Engineering (and its subsequent owners) and MHI. It has a dry feed system, suitable for low rank coals having high moisture contents. It is an air blown two-stage entrained bed slagging gasifier utilizing membrane water-wall construction. The gasifier is shown in Fig 10.
Fig 10 MHI gasifier
Synthesis Energy Systems gasification technology
Synthesis Energy Systems (SES) has a worldwide exclusive license for the U-Gas gasification technology, which is a single stage fluidized bed system and which can provide a low-to-medium heating value syngas. The flowsheet of SES gasification technology is given in Fig 11. SES gasification technology is particularly suitable for gasifying low quality fuels, including all ranks of coal.
Dried and ground coal is fed via a lock-hopper into the gasifier, which is fluidized by a mixture of steam and O2. These reactant gases are introduced at the bottom of the gasifier through a distribution grid, and at the ash discharge port in the centre of the distribution grid. The bed is maintained at temperatures ranging from 840 deg C to 1100 deg C depending on the softening temperature of the ash within the fuel. At such conditions, the concentration of fuel ash (mineral content) particles within the gasifier increases such that they begin to agglomerate and form larger particles, which are selectively removed from the fluidized bed by gravity. This design allows for 95 % or more of the fuel’s C getting gasified.
Fig 11 Flowsheet of SES gasification technology