By Douglas J. Smith,IEng,
BECAUSE OF THEIR low heating value and the presence of bound nitrogen, the firing of biomass, wood waste and sludges is limited, says Dr. Joseph Rabovitser of the Institute of Gas Technology (IGT). Although co-firing with natural gas does improve combustion and reduce NOx emissions, the benefits are limited by the amount of natural gas co-fired.
To reduce the emissions of NOx in stoker fired boilers burning biomas, wood waste and sludges, IGT and Detroit Stoker Com-pany have developed a process called “METHANE de-NOx.” According to Rabovitser, the technology has shown substantial economic, energy and environmental benefits relative to conventional natural gas co-firing.
The patented process uses natural gas injection above the stoker’s grate to create a uniform, oxygen deficient atmosphere. This promotes the decomposition of fixed nitrogen compounds and reduces the formation of nitrogen oxides. The added heat release at the stoker grate also stabilizes the difficult to burn waste fuels, says Rabovitser.
A METHANE de-NOx system has been installed on Unit 2, a natural gas co-fired, 300 MMBtu/hr wood waste and sludge-fired stoker boiler, at Boise Cascade Corporation’s paper mill in International Falls, Minn. The Gas Research Institute (GRI), IGT and the U.S. Department of Energy’s Office of Industrial Technologies funded the demonstration project.
During a two-week trial period, with the boiler under normal operating load, the system performed as designed. When compared to baseline co-firing operation, Figure 1, the system is able to reduce NOx emissions by more than 40 percent with 9.5 percent natural gas and up to 50 percent with 17 percent natural gas.
During the test period the sludge feed was increased to 4.2 ton/hr from the baseline level of 1.4 ton/hr. Based on the results obtained during the test, IGT estimates an annual savings of $400,000 in sludge handling and disposal costs. A saving of $270,000 in co-firing fuel costs was also achieved.
Since December 12, 1999 the METHANE de-NOx system has been in full-scale operation at the Boise plant handling 3 ton/hr of sludge while maintaining NOx levels significantly below those required by state regulations, says Rabovitser