THE acronym stands for Integrated Gasification Combined Cycle (IGCC). It is a technology that aims to extract the maximum energy out of a fuel that is burnt. In the case of coal, the carbon conversion efficiency in an IGCC plant is significantly higher than that in a conventional pulverised coal (P.C.) fired power plant.
This is achieved by gasification, which converts coal into synthetic gas or syngas. Syngas is a mixture containing mainly carbon monoxide (CO) and hydrogen (H2) and some carbon dioxide as well.
Coal can be gasified in various ways by controlling the mix of coal, air or oxygen (O2)-enriched air or even pure O2 and steam within the gasifier. In an IGCC power generation plant, the gasifier is integrated with a combined cycle power plant (CCPP).
A CCPP employs more than one thermodynamic cycle. Heat engines are able to use only a portion of the energy that their fuel generates, which is usually less than 50 per cent. The remaining heat from combustion is generally wasted. But by combining two or more cycles, overall efficiency can be improved.
In an IGCC plant, high temperature syngas from the gasifier is used to run a gas turbine (G.T.) that generates power. This syngas may also be blended with natural gas, if required, to improve its calorific value. The waste heat in gasification and the hot gas coming out of the G.T. are further utilised to produce steam by heating water, which in turn runs a steam turbine to produce additional electricity.
Cleaning the gas coming out of the gasifier is a vital aspect of the system. The currently demonstrated gas clean-up systems require the gas to be cooled to less than 1000C with the help of heat exchangers and then reheated before combustion in a G.T. This obviously reduces overall efficiency but warm gas clean-up systems (which operate around 4000C) are under development the world over, including at Bharat Heavy Electricals Limited (BHEL). Of course, even this would require some cooling of the syngas.
The development of a warm gas clean-up system is also critical for overall economics to favour IGCC, even though the currently achievable efficiency of around 40 per cent is already higher than that of a P.C.-fired plant, around 30 per cent. IGCC efficiency is expected to go even up to 45 per cent, especially with the use of warm gas clean-up.
The gas is cleaned of dust particles and other matter that is not acceptable to a G.T., like alkaline substances, sulphur (SOx, H2S, etc.), and nitrogen oxides (NOx). In the BHEL technology, ammonia is removed using wet scrubbers. Particulate matter and dust particles are removed using barrier filters that are adapted to operate at around 4000C. Since Indian coal is low in sulphur content and also produces low nitrogen emissions, sulphur and NOx removal systems are not required both from the perspective of pollution norms and the requirements of the G.T.
Thus, an IGCC plant basically has three islands: a gasifier and associated coal/ash handling system, a gas clean-up system, and two turbines one gas turbine and one steam turbine (see the schematic flow diagram). None of the individual components of an IGCC plant is new; it is the integration that presents new engineering challenges.
There are several techniques of gasification that are commonly used fixed-bed, fluidised-bed and entrained-flow systems. A fluidised bed is made of solid (generally inert) particles. When air or other medium passes through these solids at certain range of velocities greater than the velocity at which the solids remain static and less than the velocity above which the solids are carried away or transported the solids agitate vigorously.
The process of combustion or gasification is achieved by initially heating the bed above the ignition temperature of the fuel (in our case coal) to be combusted or gasified and feeding the fuel. When ignited, the fuel energy is released and the combustion or gasification is sustained. The process is usually carried out under pressure (of over 10 bar or 10 kg/sq cm) and the reactor is, therefore, called Pressurised Fluidised Bed Gasifier (PFBG).
As G. Viswanathan of BHEL, Tiruchi, explained, why a fluidised bed is able to burn high-ash coal more efficiently than other combustors is because, as the fuel is burnt and gasified in the bed of particles, there is sufficient energy available for sustaining the fuel heat release even when the fuel is of inferior quality. According to him, fluidised bed combustion boilers developed by BHEL, for example, are able to use fuels such as washery rejects, which have more than 55 per cent ash.
To achieve higher carbon conversion, BHEL also improved upon the conventional design of the air distributor, a small component of the PFBG. The usual conical air distributor was changed to horizontal arrangement for improving the distribution of air.
This improved the uniformity of fluidisation and the bed temperature enabled the gasifier to operate at temperatures around 1,0000C, which resulted in improved overall gasifier performance, according to the report of the Principal Scientific Adviser.
The pressure achieved at the 6.2 MW Combined Cycle Demonstration Plant (CCDP) in Tiruchi is 8 bar. According to BHEL, the proposed 125 MW plant will use air in the gasifier, which will operate at temperatures above 9750C and at a pressure around 27-30 kg/sq cm. The syngas is expected to have a calorific value of 1,000-1,100 kcal/cu m and will be used in the G.T. without any blending. The power generated from the two cycles is expected to be in the ratio 60:40.
Because the gasification temperature is well below the ash fusion temperature of 1,5000C, the problem of slag formation a major problem in entrained flow gasifiers, which operate at higher than 1,6000C is avoided in BHELs PFBG.
Besides resulting in an overall reduction in greenhouse gas emissions, IGCC is also amenable to carbon capture and sequestration (CCS), technologies for which are under development, and hence is being promoted as capture ready. In principle, CCS is also possible in conventional coal-fired plants. The cost increase for CCS add-on is substantial because capture can be done only post-combustion. In IGCC, on the other hand, CO can be converted into CO2 and separated from syngas before combustion, captured and sequestered.
However, with CCS, both capital and power tariff costs are expected to increase by 25-30 per cent. Coal gasification also lends itself to creating the possibility of producing alternative transportation fuels.
R. Ramachandran
