United Technologies' IFPS design is a direct-fired slagging furnace that utilizes flame radiation to heat air flowing through alloy tubes.  The tubes are located within a refractory wall to protect them from attack by the coal combustion products.  The HITAF can currently be designed to heat air to 1700 to 1800ºF.  Combustion of natural gas, or another clean fuel, can be used to boost the temperature of the air to the gas turbine inlet temperature.

The United Technologies Research Center (UTRC) and the University of North Dakota’s Energy & Environmental Research Center (EERC) have designed, constructed, and operated a 3.0 million Btu/hour (3.2 x 10⁶ kJ/h) slagging furnace system (SFS).  Successful operation has demonstrated that the SFS meets design objectives and is well suited for testing of high-temperature heat exchanger concepts.  Test results have shown that a high-temperature radiant air heater (RAH) panel designed and constructed by UTRC and used in the SFS has the potential to produce a 2000ºF (1094ºC) working fluid temperature.  EERC has also constructed laboratory- and bench-scale test facilities to determine the corrosion resistance of refractory and structural materials and develop methods to improve this property.

The high-temperature air furnace (HITAF) concept potentially offers a higher-efficiency technology option for coal-fired power generation systems than conventional pulverized coal-fired boilers.  Concept analyses have indicated the potential to achieve the program targets for emissions, efficiency, and cost of electricity.  Higher-efficiency technology options for new plants as well as repowering are important to the power generation industry in order to conserve valuable fossil fuel resources, reduce the quantity of pollutants (air and water) and solid wastes generated per MW, and reduce the cost of power production in a deregulated industry.   Perhaps more important than the potential application to new, high-temperature power plants, the RAH panel and the convective air heater tube bank are potential retrofit technology options for existing coal-fired boilers to improve plant efficiencies, reduce CO₂ emissions, and extend their engineering lives.

The UTRC high-performance power system (HIPPS) plant arrangement is thus a combination of existing technologies (gas turbine, heat recovery boilers, conventional steam cycle) and new technologies (the HITAF including the air heaters, and especially the heater located in the radiant section).  A critical challenge is the compatibility of the slagging combustor with the high- temperature radiant air heater.  The key to the success of the concept is the development of integrated combustor/air heater that will fire a wide range of U.S. coals with minimal natural gas and with the reliability of current coal-fired plants.  Both of these key success criteria have been addressed.  The process air-based high-temperature heat exchanger is a potential enabling technology module that can form the building block of futuristic Vision 21 plants.

The HITAF is the key component of the UTRC HIPPS.   In one successful test, the HITAF achieved temperature capabilities up to 2000ºF, surpassing design expectations.  In this short test, the radiant heater panel of the RAH was successfully operated to heat the pressurized working fluid, air, to 2000ºF.   This success shows that an overall thermal efficiency nearing 55 percent (HHV) is achievable when HITAF is ready for commercial deployment.  The successful test has also opened the path to a nearly all-coal HIPPS plant with minimal need for a natural-gas-fired temperature boost.  The approximately 6 feet by 2 feet RAH panel forms a wall of the UTRC/EERC furnace nominally rated at 3 million Btu/hour.  The turbine air is heated by radiation to the panel wall and then to the three air transport tubes located behind the protective ceramic.  This success implies that an electricity production plant based on the IFC concept is a realistic possibility and supportive of a Vision 21 program objective that has identified high-temperature heat exchangers as a key enabling technology.

Bench-scale tests have successfully demonstrated the use of additives to modify slag properties.  Specifically, additives were successfully used to increase and decrease slag viscosity for acidic and basic slags.   Bench-scale tests with an alumina additive were successful in reducing refractory corrosion.  During pilot-scale tests, limestone added to the pulverized fuel was successfully used to prevent slag screen plugging and control differential pressure.   Bench-scale tests have been used to show that the products of coal combustion are not strongly corrosive toward the alloy MA-754 used to fabricate tubes for the RAH panel at temperatures below the slag melting point.  This information suggested the possibility of operating the RAH panel without fireside ceramic tile protection at furnace exit temperatures comparable to those found in conventional pulverized coal (PC) fired boilers.  This information would be very valuable in determining the potential application of the MA-754 alloy to repowering projects.  A gas-fired SFS test was therefore conducted to evaluate the performance of the RAH panel without ceramic tile protection.  The RAH heat recoveries without the ceramic protection ranged from 30 to 80 percent higher than with protection while firing with coal at a much higher furnace temperature (2800ºF).  There was no warping or damage to the RAH panel.