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Integrated gasification combined-cycle

Coal gasification technology, often referred to as Integrated Gasification Combined Cycle (IGCC), is the process of gasifying coal to produce electricity. The coal is gasified by burning finely-crushed coal in an environment with less than half the amount of oxygen needed to fully burn the coal. Essentially, the coal is not burned directly but undergoes a reaction with oxygen and steam. This produces what is known as synthetic gas or “syngas.” This gas is then combusted in a combined cycle generator to produce electricity. The technology integrates the production of purified gas and the production of electricity. In terms of environmental benefits, the technology reduces emissions of sulphur dioxide, particulates and mercury, as well as of carbon dioxide, in particular when combined with carbon capture and storage.

Introduction top

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Integrated Gasification Combined Cycle (IGCC) technology can reach a higher efficiency rate than typical coal combustion technologies, such as sub-, super- and even ultra-supercritical combustion. Where the latter can reach efficiencies of between 30 to 45%, IGCC plants could achieve an efficiency rate of higher than 45%. The latter is achieved by combining the two cycles of firing the coal gas and using the residual heat to produce electricity. The technology contains the following steps.

First, coal is gasified by creating a ‘shortage’ of air/oxygen in a closed pressurised reactor. The creates a chemical reaction of the coal with the oxygen. The product from this process is a mixture of carbon and hydrogen (CO + H2), which is also called synthesis gas or syngas or fuel gas. The syngas is subsequently cleaned and burned with either pure oxygen or air. This creates a superheated steam with which electricity is generated. The residual heat from this process is cooled down which creates another stream of steam to produce electricity. Efficiency of IGCC could be further increased if the process of purifying the syngas (removing of particulates and sulphur) could be done at higher temperatures. Currently, purification takes place at relatively low temperatures (around 50 oC), but techniques to clean at temperatures of around 500-600oC are tested. This could increase the overall efficiency of IGCC to over 60%. IGCC plants can also be configured to facilitate CO2 capture before the combustion of the syngas. In this process, the syngas is ‘shifted’ using steam to convert CO to CO2, which is then separated for possible long-term sequestration. This means that an IGCC power plant combined with carbon storage technologies can be completely carbon emission free. An example of a zero emission power and chemical plants which combines gasification with CO2 capture and storage can be found in Kedzierzyn in Poland.

 

igcc diagram

Figure 1: Overview of IGCC process: integrating gasification with combined cycle technology

http://thefraserdomain.typepad.com/energy/images/igcc_flow_diagram.gif

Feasibility of technology and operational necessities top

The operational process of electricity production in an IGCC plant contains of the following steps:

  1. Coal is transported to the installation where it is pulverized and dried.
  2. The pulverized coal is gasified and the ash in the feedstock is recovered as marketable slag.
  3. The gas is purified, among others by washing it with water. This water needs to be cleaned so that it can be re-used.
  4. Almost all of the poisonous hydrogen sulphide is removed from the syngas and purified to sulphur. A small part of the sulphur is emitted in to the air.
  5. The syngas is prepared (thinning and saturation) for combustion in the gasturbine, which starts rotating. The exhaust gases are cooled and the residual heat used for steam to drive the steam turbine.
  6. The electricity production is the output from the gas- and steamturbine.

Historically, most gasifiers have been oxygen-blown which reduces gasifier size, and hence cost, has a higher heating value of the purified syngas, so that smaller heat exchangers are required.

The disadvantage of oxygen blowing is that the required degree of plant integration is considerably increased. This means that controlling and operating the plant becomes more complex than in case of a traditional power station. Major challenges in this context are matching the requirements for availability, reliability and flexibility of operation at a competitive cost over a long period of time. In an air-blown systems, auxiliary power consumption is estimated to be less than 8%, compared with 10-15% for oxygen blown systems.

As the syngas is produced at temperatures up to 1700 oC (in entrained flow gasifiers), while the gas clean up systems operate at a maximum temperature of 600 oC, large heat exchangers are required. There is the possibility of solids deposition in these exchangers which reduces heat transfer.

Gasifiers may be able to use coal that would otherwise be difficult to use in pulverizing coal plants, such as those with a high sulphur content, or high ash content. The current demonstration units test various coals, and should resolve many of the technical issues. However, some demonstration plants have had problems with corrosion so that slurried coal was fed into the gasifier. In some case, the gasifiers had problems with handling different feedstocks, even in case where they had been designed to handle lower rank lignites.

Given the relatively long start up time needed for integrated systems (compared to pulverising coal combustion units) IGCC plants may only be suitable for base-load operation.

Present demonstration plants have made clear that another operational necessity of IGCC plants is that the technology will not work at altitudes of above 90 meters (300 feet).

IGCC plants use about 30% less water for cooling purposes than a conventional coal-fired power plant. However, using water for cleaning the gas creates water contamination problems. For example, the US Wabash River Generating Station (see below) emitted arsenic, selenium and cyanide and therefore did not comply with the water permits (Wabash River Energy Ltd., August 2000).

IGCC plants are smaller in physical size than a conventional coal-fired power plant and the outside facilities consist mainly of vessels and pipes. The gas turbine exhaust stack is the tallest element of the plant with a length of, depending on the local terrain and dispersion modelling, typically 80 to 90 meter. This is about half of the length of the stack in case of conventional coal-fired power plant.

In terms of waste management, the ash from gasifying the feedstock is recovered as marketable slag for, e.g., a local cement market. The sulphur is captured as elemental sulphur and sold to the fertiliser industry for agricultural use. The mercury is captured, encased in steel drums and sent to regulated hazardous material processing facilities for permanent disposal. No lime scrubbers are required.

Status of the technology and its future market potential top

The competitiveness of IGCC depends on a number of aspects, such as cost of other (fossil fuel-based) technologies, cost of alternative energy sources, share of coal in the global electricity market (see below). For example, in several rapidly growing markets in developing countries (China, India and South Africa) coal is likely to play an important role.

Presently, there are a number of IGCC power plants operational, whereas others are in the stage of preperation.

In the USA, IGCC (demonstration) plants have been built as part of the Clean Coal Technologies programme launched by the Department of Energy, such as:

In Europe, the following IGCC plants are operational:

illustration © climatetechwiki.org

Figure 3: IGCC Puertollano, Spain, 315 MW capacity, built in 1995

Clara Verhelst, Op hete kolen..., KU Leuven, Belgium

Finally, in Japan the Nippon Oil Corp. Refinery in Negishi (Yokohama, Japan) is associated to an IGCC power plant, although it does not use coal, but solid residues from the refinery (oil tars, bitumen).

In addition to these plants, there are more IGCC plants operational worldwide which use the technology for gasification of biomass or oil (IEA, 2008). The diagram below shows an overview of plants in development, operational or cancelled/ delayed.

IGCC plants, development deployment diffusion

Figure 2: Status of Global IGCC Project Pipeline

Emerging Energy Research, 2007. Global IGCC Power Markets and Strategies, 2007–2030 December 2007

The stimulation of IGCC plants in developing countries could partly be supported through the Clean Development Mechanism (CDM). In September 2007, the CDM Executive Board decided to make coal-fired power plants eligible under the CDM. When the business-as-usual practice in a country is subcritical coal-based power plants, the introduction of IGCC power plants would ceteris paribus reduce GHG emissions.

How the technology could contribute to socio-economic development and environmental protection top

The main environmental benefits of IGCC technology plants stem from the capability to cleanse as much as 99% of the pollutant-forming impurities from coal-derived gases. Sulphur in coal, for example, emerges as hydrogen sulphide and can be captured by processes used today in the chemical industry. In some methods, the sulphur can be extracted in a form that can be sold commercially. Likewise, nitrogen typically exits as ammonia and can be scrubbed from the coal gas by processes that produce fertilisers or other ammonia-based chemicals.

The efficiency gains mentioned above are another benefit of coal gasification. Future concepts of IGCC incorporate a fuel cell or fuel cell-gas turbine system that could achieve efficiencies in the range of 60%.

Climate top

IGCC plants have the lowest CO2 emissions among coal power plants (Ordorica-Garcia, 2006). An IGCC plant emits around a quarter less CO2 than a pulverizing coal power plant (appr. 750 g CO2/kWh vs appr. 1 kg). To compare: A natural gas combined cycle plant emits approximately 400 gCO2/kWh.

Coal gasification offers the possibility, when using oxygen in the gasifier, of preparing CO2 as a concentrated gas stream. In this form, it can be captured relatively easily. In a conventional (pulverised) coal or gas fired power plant, CO2 can only be removed after combustion which with current technologies is economically less attractive. In the case of IGCC, CO2 can be removed before the syngas is fed into the gas turbines. According to Ordorica-Garcia (2006), capturing 80% of the CO2 would reduce emission to less than 200 g/kWh, which would involve an energy penalty due to CO2 capture of around 25% of the total auxiliary power output.

For the calculation of CO2-eq. emission reduction of IGCC plants basically two methodologies approved by the CDM Executive Board can be applied. First,methodology for conversion from single cycle to combined cycle power generation --- Version 3 - ACM0007 can be used when an existing plants is upgraded to a combined cycle power generation plant. Second, approved consolidated methodology 13 – ACM0013 can be used for greenfield fossil fuel plants (e.g. new plants, no retrofits of existing plants). However, in order to limit the applicability of the methodology and the scope for these projects, it was decided that the methodology can only be applied in those countries which generate more than half of the electricity using coal or natural gas. In practice, this limits this type of projects to China, India, and South Africa.

Moreover, within these countries the number of projects is also limited since the baseline for the GHG emissions in the absence of a CDM coal-fired plant (or gas) project must be determined using the data of the 15% most efficient coal-based (or gas-based) power plants. Therefore, if 15% of the most efficient coal-based power plants are CDM projects, then a new CDM coal-based power plant can only generate credits if it increases its efficiency even further so that it can reduce GHG emissions below the benchmark or baseline.

A first example of a CDM IGCC project in preparation is China's first self-developed IGCC power station which is scheduled to go online in Tianjin in 2011. This would be the first IGCC plant outside the industrialized world (Wuck and Michaelowa, 2010). The plant will have a capacity of 250 MWh. The Tianjin IGCC project is a major part of the Green Coal Power Program initiated by Huaneng Group in 2004 and supported by seven large State-owned enterprises involved in power generation, coal resources and investment, jointly founded a company to implement the program (China Daily, 2010).

Financial requirements and costs top

Ordorica-Garcia et al. (2006) have estimated the costs of power production with IGCC technology at USD 2176 per kWe net output production. Other cost projections range from € 1800-2300/kWe net, whereas IEA (2008) estimate that the cost target to commercialisation is USD 1400/kWh. According to Energy Justice.net, IGCC plants are estimated to be 20 to 47% more expensive than traditional coal plants, whereas costs per unit of output for a natural gas combined cycle plant would amount to USD 634/kWh (Ordorica-Garcia et al., 2006).

Rosenberg et al. (2005) have argued that general construction costs have risen by 100 to 300%. Next to the cost increases of the materials used, another important reasons is that the technology needed to be improved to increase IGCC plant reliability. According to Hamilton (2004), IGCC in the USA is relatively risky for private investors and requires government subsidies.

When adding CO2 capture technology to the IGCC technology (80% capture as in Ordorica-Garcia et al., 2006) cost would increase by over USD 700/kWhe, so that total costs would amount to USD 2900/kWhe.

References top

China Daily, 2010, Clean' coal power to go online in Tianjin. Available at: http://www.chinadaily.com.cn/m/tianjin/e/2010-02/24/content_9495753.htm

Hamilton, B.A., 2004. Coal-Based Integrated Gasification Combined Cycle (IGCC): Market Penetration Recommendations and Strategies, study for the Department of Energy’s National Energy Technology Laboratory, p. 52.

Ordorica-Garcia, G., Douglas, P., Croiset, E. and Zheng, L., 2006. Technoeconomic evaluation of IGCC power plants for CO2 avoidance, Energy Conversion and Management 47 (2006) 2250–2259.

Rosenberg, W.G, Alpern, D.C. and Walker, M.R., 2005. Deploying IGCC In This Decade with 3Party Covenant Financing, Vol. I, Revision, John F. Kennedy School of Government, p. 2.

Scherb, J-S., 2009. Integrated Gasification Combined Cycle (IGCC), A clean, efficient way to generate power with coal. Available at: http://knol.google.com/k/jean-samuel-scherb/integrated-gasification-combined-cycle/3opar7xno0682/12#

Verhelst, C., 2006. Op hete kolen..., Katholic University of Leuven, Belgium. Available at: http://wordingenieur.asro.kuleuven.be/uploads/docs/wordir/Clara_Verhelst_Op%20hete%20kolen%20KVIV.pdf

Wabash River Energy Ltd., 2000. Wabash River Coal Gasification Repowering Project Final Technical Report (PDF). Work performed under Cooperative Agreement DE-FC21-92MC29310. The U.S. Department of Energy / Office of Fossil Energy / National Energy Technology Laboratory / Morgantown, West Virginia.

Wucke, A. and Michaelowa, A. 2010. GTZ CDM Highlights Newsletter, Issue 81, p. 4.