In an IGCC, the coal gasifier integrates into the highly efficient and proven gas turbine combined cycle. This makes it possible to use coal also in the combined cycle mode.
What makes an IGCC efficient is the integration of energy balance of the gasifier with the combined cycle. Below are some of the points of integration.
How is it integrated?
Different combinations from manufacturers and process technologies are in the development and demonstration stages.
- Steam / or water for the gasifier is taken from the HRSG steam cycle.
- Air for the gasifier or the air for the Air separation unit is taken for the gas turbine compressor to save energy.
- The Nitrogen from the Air separation unit is send back into the gas turbine combustor for dilution. This dilution reduces NOx emissions from the Gas turbine.
- The hot gas from the gasifier is cooled in one or more heat exchangers to produce steam, which integrates with the HRSG steam cycle.
- Lower temperature heat exchangers are used to further heat the condensate in the steam cycle while cooling the gas.
- CO2 from the syngas generation is ready for capture.
Advantages and disadvantages.
The performance of an IGCC is compared to the highly efficient Natural gas fired combined cycle and the Ultra Supercritical pulverised coal fired plants.
- The overall thermal efficiency of the IGCC power plant is almost same with that of an Ultra supercritical coal fired power plant but still less than an Natural Gas fired CC plant. However, it is considerably higher than the conventional coal fired power plants.
- A part of the CO2 as output of the gasifier can be easily captured. CO2 generation per unit output will be around 20 % less than a Pulverized Coal fired unit. This could go a long way in meeting Carbon emissions and CO2 capture costs.
- The start up times of IGCC will be more than Pulverized Coal fired power plant due to the large number of sub systems. This makes the IGCC suitable only for base load operation.
- The biggest advantage of the IGCC is its capability to use a wide range of coals and other fuels. Gasification of a large number of fuels apart from coal like biomass or municipal has been demonstrated.
- Current cost of IGCC is higher than the Ultra supercritical pulverised coal fired plants without CO2 capture. However, as more and more units are in service, with the technology stabilising, and including CO2 capture costs the overall costs are expected to come down.
- More components, more heat exchangers increase maintenance costs and outage times.
- Cleaning of the Syngas from the gasifier removes most of the emissions like sulphur dioxide and PM10. In addition, dilution by using Nitrogen in the gas turbine combustor reduces the formation of NOx.The volume of gas to be handled is comparatively much less for emission control.
- Ash forms as slag which when quenched is a lot easier and less voluminous to handle and dispose than fly ash from coal fired power plants.
With control on emissions including Carbondioxide becoming stringent and dwindling Natural gas resources will pave the way for implementation of IGCC power plants.