The world today is having some serious problems regarding to energy crisis. The energy demand is increasing in alarming rate with average 1.3 percent per year to 2030. The increase will be underpinned by economic and population growth^[9]. Power generation to meet electricity needs will be the biggest driver of higher energy demand representing more than 40 percent of the increase while fossil-base fuels will continue to provide the supplies for this demand with oil and gas close to 60 percent. Demand for coal will grow as the demand of electricity in developing country rise. Figure 1 shows the world energy demand by sector to 2030.

Figure 1. The world energy demand by sector ^[1]

The need for mitigating the effect of greenhouse gases emissions and the rapid increase of oil price make us to start thinking possible solutions to address the problem. One solution which have been the main long-term goal of The European Union is the conversion of the fossil-based fuel to sustainable energy, gaining higher energy efficiency and reducing its emissions. Even if we put higher energy efficiency into consideration, it is still not enough to fulfill the growing world’s energy demand. Thus, addressing the need of affordable and reliable energy supplies will not be easy. An effective combination of access, investment, technology, and trade is necessary to deliver reliable supplies^[1].

Nowadays, oil holds an important role in world’s energy system since it is used widely in industrial, residential, and transportation sectors. In future projection, the use of oil will be limited to transportation sector while natural gas and coal will be utilized in electricity generation. Although the efficiency of coal-fired unit with modern combustion technology is considerably high, this fact has caused some concerns in terms of conservation of resources and CO₂ emissions. Over the last 20 years, a great amount of work has been done to improve existing combustion technologies as well as investigating the alternatives. Of all the potential alternatives, coal gasification has come up with a very good chance to develop in the future with its Integrated Gasification Combined Cycle (IGCC)^[4].

IGCC produces electricity from solid or liquid fuels. The scientific community and major electricity corporation consider this technology as promising to produce cleaner electrical power in the future. First, fuel is converted to synthesis gas (syn-gas), a mixture of hydrogen and carbon monoxide, through gasification. Second, the synthesis gas is converted to electricity using a power generating unit that consist of gas and steam turbines which also include a heat recovery steam generator^[3].

Coal-based IGCC plants are not fully commercial. Although each major components of IGCC have been widely utilized in the industry or in power generation, the integration of a gasification and combined cycle power plant is considerably new. The objective of this technology is to achieve a better environmental performance at low marginal cost^[3]. Several IGCC pilot plants have been built recently to assess the possibility in commercializing the technology.

IGCC Process Description

IGCC technology is a power generation process that integrates gasification process with combined cycle power plant. The gasification system converts coal into synthesis gas which consists primarily by hydrogen (H₂) and carbon monoxide (CO). The synthesis gas is then used as fuel on a combined cycle power plant for electricity generation^[1]. Figure 2 shows the flow diagram of IGCC technology without CCS (carbon capture and storage).

IGCC system mainly consists of 4 major sections, air separation, gasification, cooling and clean up system, and combined cycle power plant. Air separation unit is responsible for separating air into its constituents and supplying pure oxygen into the gasifier. This process is held on a pressurized and cryogenic condition.

Figure 2. Integrated Gasification Combined Cycle technology^[1]

Coal gasification takes place in the presence of controlled air/oxygen and steam which maintain a reducing condition. Gasification is a partial oxidation of the feedstock which produces heat and series of chemical reactions. The process is carried out in an enclosed pressurized reactor. Most gasifiers have been oxygen blown because of the cost of handling large amounts of nitrogen and the effect it has in diluting the product. The air blown gasifier is less preferable since its product has low calorific value which is not desirable. But the oxygen blown itself also has some disadvantages, it requires higher degree of plant integration. This means that controlling and operating the plant is more like running the whole chemical complex plant than a traditional power station (UK Clean Coal Center).

In addition to its chemical energy (heating value), the hot raw synthesis gas contains sensible heat which may be recovered in heat exchangers to produce steam for the steam turbine. The use of synthesis gas coolers for this purpose increases efficiency, but adds capital costs. In theory, it would be desirable to clean the raw synthesis gas without cooling (as the sensible heat would be utilized most efficiently when delivered to the gas turbine), but the proven technologies for gas clean up operate at near ambient temperatures. In the gas clean up process, particles, sulfur and other impurities are removed. At this point, CO₂ may also be captured. Because of the high partial pressures of the species and the low volume flow of synthesis gas, the gas clean up process is very efficient and low cost compared to traditional flue gas cleaning^[3].

Recent studies have shown that an IGCC plant with CCS (carbon capture and storage) requires two additional pre-combustion stages than the conventional IGCC cycle plant (Figure 2) as illustrated in Figure 3. The two additional stages are the water gas shift reaction and the acid gas removal for the removal of CO₂ from the synthesis gas. In addition, a CO₂ compression stage is necessary to make transportation and storage of the sequestered quantity of CO₂ feasible. The addition of CCS technology decreases the overall process efficiency due to the power for compressing the CO₂. Another reason for the decrease efficiency is the installation of two additional stages, the amount of coal feed required needs to be increase. On the other hand, this can result in lower steam/carbon ratio in gasifier which will need additional supply of steam, thus lower the plant output power^[2].

Figure 3. Integrated Gasification Combined Cycle incorporating CCS^[1]

The clean gas is then fed to the combined cycle power plant. Combined cycle power plant consists of a combustion turbine/generator, a heat recovery steam generator, and a steam turbine/generator. The exhaust heat from the combustion turbine is recovered in the heat recovery steam generator to produce steam. This steam then passes through a steam turbine to power another generator, which produces more electricity. Combined cycle is more efficient than conventional power generating systems because it re-uses waste heat to produce more electricity (www.wci-coal.com).

(To be continued: Major IGCC Blocks)

References:
^[1] Christou C., Hadjipaschalis I., Poullikkas A, J. Rser. 2007 June.
^[2] Descamps C., Boualloua C., Kanniche M., J. Energy. 2007 July.
^[3] Maurstad O. An Overview of Coal Based IGCC Technology. 2005
^[4] Higman C. and van der Burgt M, Gasification. 2003.
^[5] Yong K.H., J. Hydrogen Energy. 2007 32 5088-5093.
^[6] http://www.aiche.org/uploadedFiles/Energy_Website/Publications/051206_IGCC.pdf
^[7] http://www.netequity.biz/docs/BioChip/SiemansGasification.pdf
^[8] http://www.bv.com/Downloads/Resources/energy_brochures/goc/rsrc_gasificationIGCC.pdf
^[9] http://www.exxonmobil.com/corporate
^[10] http://www.worldenergysource.com/articles/pdf/longwell_WE_v5n3.pdf