Gasification of coal
Gasification of coal
Gasification of coal is a high temperature process which 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. Water may or may not be added. Due to decomposition reactions which take place in the vessel, a mixture of gases is produced where H2 and CO gases are prominent. Other gases in the mixture are water, methane and CO2. The mixture of CO and H2 is called synthesis gas or syngas. After the gasification of coal, petroleum liquid products are produced by Fischer Tropsch process. The basics of the gasification process is given in Fig.1
Fig 1 Basics of the gasification process
The gasification process is shown in Fig.2
Fig 2 Gasification process
History and present development
The original process was developed by the German researchers Franz Fischer and Hans Tropsch at the Kaiser Wilhelm institute in 1920s. The process was used by Germany and Japan during World War II to produce alternative fuels. Since then many adjustments and refinements have taken place and the term Fischer- Tropsch applies to a number of similar processes.
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. Lurgi and Sasol made a joint venture in 2001 and Sasol-Lurgi is presently the sole licensor for Sasol- Lurgi Fixed Bed Dry Bottom (FBDB) gasification.
According to a recent survey by the Gasification Technologies Council , there are 161 existing and planned commercial scale gasification plants worldwide. This represents a total of 414 gasifiers with a total synthesis gas production of 446 million N Cum per day of synthesis gas, which is equivalent to approximately 61 000 MW of thermal powerl or 800 000 barrels of oil per day. The Sasol plants in South Africa are the largest consumers of coal for gasification purposes (approximately 30 million tons per annum), the other coal-based gasification units being Dakota Gas Company in the U.S. and a few IGCC (Integrated Gasification Combined Cycle) demonstration plants.
Fischer- Tropsch process?
The Fischer-Tropsch process is a catalyzed chemical reaction in which carbon mono oxide and hydrogen 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 hydrogen from a solid feed stock.
The original Fischer-Tropsch process is described by the following 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 methane in the case of GTL (Gas To Liquid), gas to liquids applications:
OR by the gasification of coal in the case of CTL (Coal To Liquid):
The energy needed for the reaction of coal and steam is usually provided by adding air or oxygen. This leads to the following reaction:
C+ 0.5O2 =CO
The syngas conversion by Fischer- Tropsch synthesis is shown in Fig. 3
Fig 3 Syngas conversion by Fischer-Tropsch synthesis
Coal Gasification processes
Various coal gasification processes are based on fixed bed (Sasol Lurgi), Fluidized bed (Winkler) and Entrained bed (Shell and Texaco). The characteristics of these processes are compared in Tab 1. The most popular Fixed bed dry bottom type process has been described in detail.
|Tab 1 Important characteristics of important coal gasification processes|
|Subject||Unit||Fixed bed||Fluidized bed||Entrained bed|
|Pressure||Kg/Sq cm||Around 30||1||30-40||40-80|
|Gas outlet Temperature||Deg C||675||Around 850||1370||1320|
|Coal types (Feed Coal)||All ranks except coking coal||Low rank coals||All types||All types|
|Feed coal size||mm||Jun-50||0-9.5||Minus 200 mesh||0-0.5|
|Moisture in feed coal||Wt %||Up to 18||<5||No limit|
|Maximum ash content||Wt %||Up to 40||Up to 25||Up to 25|
|Ash withdrawl||Dry powder||Dry powder||Molten slag||Molten slag|
|Dry gas compositon|
|S compounds||Vol %||Around 0.5||Around 0.5||0.4||0.3|
|N2 and others||Vol %||Around 0.5||1||1||0.6|
|H2/CO ratio in gas||1.7 to 2||1.25||0.48||0.65|
|CV of gas||Kcal/N Cum||2600-2900||2640||2980||2700|
|Cold gas efficiency||%||>85||80-83||76-77|
Fixed bed dry bottom gasification process
Fixed bed dry bottom (FBDB) gasification process has been developed by Sasol-Lurgi. It is a medium temperature and pressure process. Coal is gasified typically at a pressure of around 30 bar (a) in the presence of high pressure steam and pure oxygen to produce a gas suitable for a variety of applications. This technology is a reliable, mature and robust gasification technology with proven commercial success. The Sasol-Lurgi gasifier technology contributes approximately 28% to the total global production of synthesis gas. Coals with high ash content and high ash melting point can be used in this process to produce a high H2/CO syngas which satisfies the high demand for hydrogen in Fischer-Tropsch synthesis.
The gasifier is referred as fixed bed gasifier, though it is more accurate to call this gasifier as moving bed since the coal bed moves downward under gravity.
Within the gasifier bed, different reaction zones (Fig 4) are distinguishable from top to bottom, namely the drying zone where moisture is released, the devolatilization zone where pyrolysis takes place, the reduction zone where mainly the endothermic reactions occur, the exothermic oxidation or combustion zone, and the ash bed at the bottom. Due to the counter-current mode of operation, hot ash exchanges heat with cold incoming agent (steam and oxygen or air), while at the same time hot raw gas exchanges heat with cold incoming coal. This result in the ash and raw gas leaving the gasifier at relatively low temperatures compared to other types of gasifiers, which improves the thermal efficiency and lowers the steam consumption.
Fig 4 Reaction zones in gasifier
Reactions in gasification process
The chemistry of gasification is extremely complex. The most important reactions relevant to the gasification process are similar to those of gas reforming, and the processes of gasification and reforming therefore have a lot in common. Both take place at relatively high temperatures (approximately 1200 deg C or more), which is a result of the exothermic combustion (oxidation) reactions which are required to drive the endothermic reduction reactions. The basic gasification reactions are the following:
C+0.5 O2+CO ?H = -123 kJ/mol
C+O2=CO2 ?H = -406 kJ/mol
H2+O2=H2O ?H = -248 kJ/mol
C+CO2=2 CO ?H = 160 kJ/mol
C+H2O=CO+H2 ?H = 119 kJ/mol
Water-gas shift reaction
CO+H2O=CO2+H2 ?H = -40 kJ/mol
Methane formation reaction
C+2H2=CH4 ?H = -87 kJ/mol
CO+3H2=CH4+H2O ?H = -206 kJ/mol
3C+2H2O=CH4+2CO ?H = 182 kJ/mol
CnHm=(m/4) CH4+(n-m/4) C
CnHm+(2n – m/2)H=nCH4
In addition to the reactions above, the coal goes through the stages of drying and pyrolysis as soon as it is exposed to heat. Pyrolysis reaction chemistry is complex, and involves free radical reaction mechanisms in addition to the cracking and hydrogenation reactions.
Characteristics of fixed bed dry bottom gasification process
The important characteristics of FBDB gasification process are given below.
- It is a viable process for the conversion of low grade, high ash content and high ash melting point coals to high value products.
- It can handle a variety of different feed stock
- It can feed ROM coals without external drying of the moisture
- It can be utilized for treatment of solid wastes
- It uses lump coal of 5-8mm size and hence does no need major grinding
- The “cold gas” thermal efficiency is high due to counter current operation. The temperature of gas and solid product is low at the exit.
- The requirement of oxidant is low because of high thermal efficiency
- Valuable by products such as tars, pitches, oils and chemicals are produced.
- A gas with a H2/CO ratio of 1.7 to 2.0 is produced. This gas is suitable for Fischer-Tropsch synthesis. Hence there is no need of additional water gas shift conversion to adjust the H2/CO ratio
- The process has limited ability to handle excessive fine coals. Broad particle size distribution can lead to coal segregation and channel burning
- Pressure drop can limit gas through put
- The process has relatively high steam consumption