The Department of Energy National Energy Technology Laboratory (NETL) continually promotes the advancement of technologies to generate electricity effectively. One such technology involves developing indirectly fired power systems (IFPS) to make coal plants more effective.

Currently, NETL supports the development of two types of IFPS called high performance power systems or HIPPS. The goal of the HIPPS is to develop a power plant that is 47-50% efficient, has emissions that are 1/10 (or less) of the federal New Source Performance Standards (NSPS), and produces electricity that is 10% less costly to make than in current coal-fired power plants.

The IFPS technology uses an indirectly fired gas turbine combined cycle in which hot gases that drive the gas turbine are heated by high temperature heat exchangers. The hot gas leaving the gas turbine is then used to create steam that drives a steam turbine. The system is similar to Integrated Gasification Combined Cycle (IGCC) and Pressurized Fluidized Bed Combustion (PFBC) systems. However, using the IFPS, the actual combustion gases from burning coal do not come in contact with the turbine. This difference is important because it means that the hot gas entering the turbine does not have to be cleaned—cleaning high-temperature gases is extremely expensive.

Using current technology and gas turbines, the IFPS concept can achieve about 47% efficiency. When the technology is ready for commercial deployment with advanced turbines, it is expected to approach 55% efficiency.

	Click on diagram below to see larger view.
	Simplified view of the HITAF Process.

First type of IFPS:
United Technologies Research Center
The United Technologies Research Center (UTRC) is developing the first IFPS configuration. In the UTRC process, coal is burned in a coal-fired high-temperature air furnace (HITAF), which acts like a boiler but without water in a conventional coal plant. Inside the HITAF, compressed air is heated in a heat exchanger to a temperature that is close to the gas turbine (GT) inlet temperature. If necessary, natural gas or a clean coal-derived fuel combustor could be fired into the hot air exiting the furnace to raise the temperature. The hot air drives the gas turbine, which provides over half the combined cycle’s power output from one generator (GEN). The diagram to the right shows a simplified view of the process.

The exhaust from the gas turbine is still hot at this point and so is split into two streams. One goes to the HITAF and the other to a heat recovery steam generator (HRSG) that produces steam to drive the steam turbine (ST). The flue gas from the HITAF also goes to the HRSG to help produce steam. The steam turbine drives the other generator to produce the rest of the plant’s electricity. The efficiency of this indirectly fired combined cycle plant will be around 47.4%. To control emissions, the system will use control technologies including low-NOx burners.

Second type of IFPS: Foster Wheeler Development Company
Foster Wheeler Development Company (FWDC) is working with the Department of Energy on a second type of IFPS, which is entirely a coal-fired process. The process works like this:

1. 	Click on diagram below to see larger view.
   	Foster Wheeler process.

   Coal is fed into a pressurized fluidized bed pyrolyzer [which is part of FWDC's HIPPS] that converts the coal to syngas and char, similar to APFBC systems.
   
2. The char and syngas are separated once they leave the pyrolyzer.
   The char is used as fuel in the HITAF that creates steam, superheats it, and also heats compressed air for the gas turbine to 1400ºF.
   The syngas stream from the pyrolyzer goes to a gas turbine combustor that further heats the air stream for the gas turbine generator to 2350ºF.
   
3. The hot air exhaust from the turbine goes through a recuperating heat exchanger to preheat the turbine air before it enters the HITAF.
   
4. A portion of the exhaust air goes to the HITAF as combustion air. The unused portion goes to a heat recovery system (HRSG) to help create additional steam.
5. A wet flue gas desulfurization (FGD) scrubber controls SOx emissions from the HITAF while gas combustion turbine exhausts, low-NOx burners, and a selective catalytic reduction system (SCR) control NOx emissions. The diagram to the right shows the Foster Wheeler process.

	Click on diagram below to see larger view.
	Repowering Diagram

The Foster Wheeler design could also be used to repower an existing pulverized coal power plant. Repowering is when older plants using outdated technology or aging equipment are upgraded with new technology and/or equipment to increase output or efficiency or meet new emission reduction standards. In this case, to repower a plant, the pyrolyzer and gas turbine system from the HIPPS design are added to the existing system to process the coal before it is fed to the boiler. The fuel gas from the pyrolyzer is fired in the gas combustion turbine to create electricity. The char from the pyrolyzer and the exhaust from the gas turbine go to the existing boiler. The char, along with coal, would provide fuel for the furnace, and the turbine exhaust would provide the combustion air. Repowering in this way could increase plant electrical output by up to 50% and increase plant efficiency by 4 to 6%.