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| Fig. 1: Block Diagram of IGCC Power Plant. (Source: Wikimedia Commons) |
Electricity is an indispensable amenity in present society. Among all those energy resources, coal is readily available all over the world and has risen only moderately in price compared with other fuel sources. As a result, coal-fired power plant remains to be a fundamental element of the world's energy supply. IGCC, abbreviation of Integrated Gasification Combined Cycle, is one of the primary designs for the power generation market from coal-gasification.
The main advantages that IGCC plants possess are inherited from gasification technology:
Feedstock Abundance and Flexibility - Coal is in abundance worldwide with price being comparably stable. Several gasifiers are designed to accommodate various grades of coal. Besides, gasifiers can handle pet coke and other refinery products.
Carbon Capture and Storage (CCS) - In IGCC process, carbon dioxide is removed and captured from syngas before entering the gas turbines. Hence the CO2 in the syngas is concentrated at high pressure, making the capture easier in a gasification plant as compared to a dilute exhaust stream. The captured CO2 can then be re-utilized or sequestered. In addition, this CCS process decreases the energy required for CO2 removal from the process streams as compared with a pulverized coal combustion plant, making an IGCC power plant more efficient. [1]
Environmental benefits - As gasification process requires higher operating condition, the syngas produced in the process is at higher temperature and pressure than the exhaust gases from combustion process, which allow for easier removal of sulfur and nitrous oxides, as well as trace contaminants such as mercury, arsenic, cadmium, etc. Gasification systems can lower emission levels of typical current U.S. permit levels by almost an order of magnitude and more than 95% mercury removal with minimal cost increase. [2] In addition, as gasification operates in low-oxygen environment, the sulfur in the fuel reacts to hydrogen sulfide instead of sulfur dioxide, which makes it much more easier to capture and remove. [3]
The reaction for coal gasification is: CxHy + (x/2) O2 → x CO +(y/2) H2 where x and y are from the actual composition of the coal. The reaction rate depends on the properties of the char, which is the solid product remaining after coal drying and devolatilization, and the conditions of the gasification vessel. Therefore, the type of the coal feedstock and the type of the gasifier should be selected accordingly.
To remove the most extent of carbon and increase the purity of H2 fuel, a water-gas-shift reactor can be used to process the syngas. The reaction is: CO + H2O → CO2 + H2. Two technologies have been developed for this reaction: sour shift and sweet shift. The former one involves the hydrolysis of CO in the presence of sulphur, while the latter method requires the hydrolysis of COS and removal of sulphur prior to the WGS reaction. The selection criteria include energy efficiency, steam requirement (in order to drive the the reaction equilibrium towards the products), and capital cost. [4] The choice of catalyst is another significant considerations for the reactor. Catalyst properties such as sulfur-sensitivity and high-temperature kinetics should be looked over.
Solvent selection is critical in this section. Two of the most widely used solvents in the industry are chemical- based solvent like methydiethanolamine (MDEA) and monoethanolamine (MEA), and physical-based solvent like Selexol and Rectisol. The selection criteria include partial pressure of acid gas in product and that in feed, energy requirement and capital cost.
The captured CO2 gas stream contains impurities including water, H2S, methane. The water present in the stream with CO2 to form carbonic acid that leads to equipment and pipeline corrosion. Hence, it's important to reduce the moisture level of the exhaust during the transportation of compressed CO2 for sequestration.
A combined cycle unit includes gas turbine, steam turbine, heat recovery steam generator (HRSG) and generator. The major contenders with extensive and tested knowledge in the market are General Electric (GE) and Siemens. The two major factors considered for the selection of combined cycle unit are required electricity generation capacity and the availability of manufacturer's data.
© Yisha Mao. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] "Cost and Performance Baseline for Fossil Energy Plants, Vol 1: Bituminous Coal and Natural Gas to Electricity", U.S. National Energy Technology Laboratory, DOE/NETL-2010/1397, November 2010.
[2] J. Katzer et al., "The Future of Coal," Massachusetts Institute of Technology, 2007.
[3] G. Booras and N. Holt, Pulverized Coal and IGCC Cost and Performance Estimates", Electric Power Research institute, October 2004.
[4] "Evaluation of Alternate Water Gas Shift Configurations for IGCC Systems", U.S. National Energy Technology Laboratory, DOE/NETL-401/080509, August 2009.
