Көмірді газдандыру-комбиналды цикл технологиясы және қоршаған ортаға тигізетін әсері

An integrated gasification combined cycle (IGCC) is a technology using a high pressure gasifier to turn coal and other carbon based fuels into pressurized gas—synthesis gas (syngas). It can then remove impurities from the syngas prior to the electricity generation cycle. Some of these pollutants, such as sulfur, can be turned into re usable byproducts through the Claus process. This results in lower emissions of sulfur dioxide, particulates, mercury, and in some cases carbon dioxide. With additional process equipment, a water gas shift reaction can increase gasification efficiency and reduce carbon monoxide emissions by converting it to carbon dioxide. The resulting carbon dioxide from the shift reaction can be separated, compressed, and stored through sequestration. Excess heat from the primary combustion and syngas fired generation is then passed to a steam cycle, similar to a combined cycle gas turbine. This process results in improved thermodynamic efficiency, compared to conventional pulverized coal combustion.

Coal can be found in abundance in the USA and many other countries and its price has remained relatively constant in recent years. Of the traditional hydrocarbon fuels oil, coal, and natural gas coal is used as a feedstock for 40% of global electricity generation. Fossil fuel consumption and its contribution to large scale emissions is becoming a pressing issue because of the adverse effects of climate change. In particular, coal contains more CO2 per BTU than oil or natural gas and is responsible for 43% of CO2 emissions from fuel combustion. Thus, the lower emissions that IGCC technology allows through gasification and pre combustion carbon capture is discussed as a way to addressing aforementioned concerns.

The solid coal is gasified to produce syngas, or synthetic gas. Syngas is synthesized by gasifying coal in a closed pressurized reactor with a shortage of oxygen. The shortage of oxygen ensures that coal is broken down by the heat and pressure as opposed to burning completely. The chemical reaction between coal and oxygen produces a product that is a mixture of carbon and hydrogen, or syngas. CxHy + (x/2)O2 → (x)CO + (y/2)H2
The heat from the production of syngas is used to produce steam from cooling water which is then used for steam turbine electricity production. The syngas must go through a pre combustion separation process to remove CO2 and other impurities to produce a more purified fuel. Three steps are necessary for the separation of impurities:
Water gas shift reaction. The reaction that occurs in a water gas shift reactor is CO + H2O CO2 + H2. This produces a syngas with a higher composition of hydrogen fuel which is more efficient for burning later in combustion. Physical separation process. This can be done through various mechanisms such as absorption, adsorption or membrane separation. Drying, compression and storage/shipping. The resulting syngas fuels a combustion turbine that produces electricity. At this stage the syngas is fairly pure H2.

A major drawback of using coal as a fuel source is the emission of carbon dioxide and pollutants, including sulfur dioxide, nitrogen oxide, mercury, and particulates. Almost all coal fired power plants use pulverized coal combustion, which grinds the coal to increase the surface area, burns it to make steam, and runs the steam through a turbine to generate electricity. Pulverized coal plants can only capture carbon dioxide after combustion when it is diluted and harder to separate. In comparison, gasification in IGCC allows for separation and capture of the concentrated and pressurized carbon dioxide before combustion. Syngas cleanup includes filters to remove bulk particulates, scrubbing to remove fine particulates, and solid adsorbents for mercury removal. Additionally, hydrogen gas is used as fuel, which produces no pollutants under combustion. IGCC also consumes less water than traditional pulverized coal plants. In a pulverized coal plant, coal is burned to produce steam, which is then used to create electricity using a steam turbine. Then steam exhaust must then be condensed with cooling water, and water is lost by evaporation. In IGCC, water consumption is reduced by combustion in a gas turbine, which uses the generated heat to expand air and drive the turbine. Steam is only used to capture the heat from the combustion turbine exhaust for use in a secondary steam turbine. Currently, the major drawback is the high capital cost compared to other forms of power production.

Pre combustion CO2 removal is much easier than CO2 removal from flue gas in post combustion capture due to the high concentration of CO2 after the water gas shift reaction and the high pressure of the syngas. During pre combustion in IGCC, the partial pressure of CO2 is nearly 1000 times higher than in post combustion flue gas. Due to the high concentration of CO2 pre combustion, physical solvents, such as Selexol and Rectisol, are preferred for the removal of CO2 vs that of chemical solvents. Physical solvents work by absorbing the acid gases without the need of a chemical reaction as in traditional amine based solvents. The solvent can then be regenerated, and the CO2 desorbed, by reducing the pressure. The biggest obstacle with physical solvents is the need for the syngas to be cooled before separation and reheated afterwards for combustion. This requires energy and decreases overall plant efficiency.

In 2007, the New York State Attorney General's office demanded full disclosure of "financial risks from greenhouse gases" to the shareholders of electric power companies proposing the development of IGCC coal fired power plants. "Any one of the several new or likely regulatory initiatives for CO2 emissions from power plants including state carbon controls, EPA's regulations under the Clean Air Act, or the enactment of federal global warming legislation would add a significant cost to carbon intensive coal generation"; U. S. Senator Hillary Clinton from New York has proposed that this full risk disclosure be required of all publicly traded power companies nationwide. This honest disclosure has begun to reduce investor interest in all types of existing technology coal fired power plant development, including IGCC. Senator Harry Reid (Majority Leader of the 2007/2008 U. S. Senate) told the 2007 Clean Energy Summit that he will do everything he can to stop construction of proposed new IGCC coal fired electric power plants in Nevada. Reid wants Nevada utility companies to invest in solar energy, wind energy and geothermal energy instead of coal technologies. Reid stated that global warming is a reality, and just one proposed coal fired plant would contribute to it by burning seven million tons of coal a year. The long term healthcare costs would be far too high, he claimed (no source attributed). "I'm going to do everything I can to stop these plants. ", he said. "There is no clean coal technology. There is cleaner coal technology, but there is no clean coal technology." One of the most efficient ways to treat the H2S gas from an IGCC plant is by converting it into sulphuric acid in a wet gas sulphuric acid process WSA process. However, the majority of the H2S treating plants utilize the modified Claus process, as the sulphur market infrastructure and the transportation costs of sulphuric acid versus sulphur are in favour of sulphur production.