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Integrated CHP Using Ultra-Low-NOx Supplemental Firing

David Cygan and Derek Wissmiller, Gas Technology Institute Gearoid Foley, Integrated CHP Systems Corporation


The objective of the work presented is to deploy Gas Technology Institute’s (GTI’s) Flexible Combined Heat and Power (FlexCHP) system to deliver power and steam while holding Nitrogen Oxides (NOx), Carbon Monoxide (CO), and Total Hydrocarbon (THC) emissions below the California Air Resources Board (CARB) 2007 Fossil Fuel Emissions Standard for Distributed Generation (DG). This system, appropriately designated as FlexCHP-65, combines a Capstone C65 microturbine, a GTI-developed supplemental Ultra-Low-NOx (ULN) burner, and a 100 Horsepower (HP) heat recovery boiler by Johnston Boiler Company.

The supplemental ULN burner has demonstrated increased energy efficiency while meeting the 2007 Fossil Fuel Emissions Standard without the use of catalytic exhaust gas treatment. Preliminary laboratory testing with a 2.2 million Btu/h supplemental burner firing the exhaust from a 60-kW Capstone microturbine proved the capability of the system to deliver final stack NOx below 0.07 lb/MWh. Additional testing showed that the burner can be successfully scaled up to 7.5 million Btu/h. This also indicates the possibility of integration with megawatt- scale engines such as the Solar Mercury 50. Evaluation of a 3.5 million Btu/h burner firing with exhaust gas from a 65-kW Capstone microturbine is following the path to reduce NOx formed in the turbine and deliver final NOx emissions in the stack at levels which have not been achieved without Selective Catalytic Reduction (SCR). The resulting Combined Heat and Power (CHP) packages promise to make DG implementation more attractive, mitigate greenhouse gas emissions, improve the competitiveness of industry, and improve the reliability of electricity.

Market Barriers and Opportunities

Gas turbines have a number of beneficial features that have led to their widespread application for Combined Heat and Power (CHP), including their relatively simple design, low capital cost per kilowatt, low maintenance requirements, and lower emissions as compared to reciprocating engines. However, because of the need to operate at high excess air (225-550%), exhaust losses from gas turbine based CHP systems are relatively high and offer an opportunity for further cost savings.

A common approach to recoup some of the energy loss is through the use of supplemental burners (i.e. duct or parallel burners) to combust additional fuel in the oxygen-rich Turbine Exhaust Gas (TEG) and to raise the temperature for better downstream heat recovery in a boiler as shown in Figure 1. For example, with natural gas as the fuel and a final flue gas temperature of 275°F, reducing the excess air from 355% to 15% decreases the stack loss from 46% to 17% (higher heating value basis).

Even with low-NOx duct or parallel burner designs, however, CHP systems have encountered difficulty in satisfying stringent output-based emissions criteria, such as the 2007 California Air Resource Board (CARB) Fossil Fuel Emissions Standard. With these emissions criteria defined on a per unit energy output basis, distributed generation systems must achieve high efficiencies, in addition to low pollutant concentration levels in the stack for compliance.

©2013 ACEEE Summer Study on Energy Efficiency in Industry 2-1

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