TFS 2000™ Low NOx Firing System

Presently 57% of U.S. electrical generation is coal based, and the Energy Information Agency projects that coal will maintain a lead in U.S. power generation over all other fuel sources for decades (EIA 1998 Energy Forecast). Yet, coal-based power is being strongly challenged by society’s ever-increasing desire for an improved environment and the resultant improvement in health and safety. The needs of the electric-utility industry are to improve environmental performance, while simultaneously improving overall plant economics. This means that emissions control technology is needed with very low capital and operating costs. 

This project has responded to the industry’s need for low NOx emissions by evaluating ideas that can be adapted to present pulverized coal fired systems, be they conventional or low NOx firing systems. The TFS 2000™ firing system has been the ALSTOM Power Inc. commercial offering producing the lowest NOx emission levels. In this project, the TFS 2000™ firing system served as a basis for comparison to other low NOx systems evaluated and was the foundation upon which refinements were made to further improve NOx emissions and related combustion performance. 

Three coals were evaluated during the bench-scale and large pilot-scale testing tasks. The three coals ranged from a very reactive Powder River Basin coal (PRB) to a moderately reactive Midwestern bituminous coal (HVB) to a less reactive medium volatile Eastern bituminous coal (MVB). Bench-scale testing was comprised of standard ASTM properties evaluation, plus more detailed characterization of fuel properties through drop tube furnace testing and thermogravimetric analysis. 

Pilot-scale testing in ALSTOM Power’s Boiler Simulation Facility (BSF) evaluated a number of low NOx subsystems under realistic boiler combustion system conditions at a large pilot-scale of 50-60 MMBtu/hr (15-18 MWt). Among the technologies evaluated in the BSF were finer coal grinding, oxidative pyrolysis burners, windbox auxiliary air optimization, and various burner zone firing arrangements in concert with strategic deployment of overfire air. 

Computational fluid dynamics (CFD) was used to evaluate the effectiveness of a number of overfire air schemes prior to testing in the BSF. Other technologies, such as an advanced boiler control system, coal and air flow balancing, and a Carbon Burn Out combustor, were also evaluated. An advanced boiler control system was conceptually developed to achieve optimal boiler performance, with regard to NOx and other control targets, under all boiler operating conditions. 

Bench-scale characterization of the three test coals showed that both NOx and combustion performance are a strong a function of coal properties. More reactive coals evolved more of their fuel bound nitrogen in the substoichiometric main burner zone than less reactive coals, resulting in lower NOx emissions. From a combustion point of view, the more reactive coal also showed lower carbon in ash and CO values than the less reactive coals at any given main burner zone stoichiometry. According to bench-scale results, the PRB coal was found to be the most amenable to both low NOx, and acceptably low combustibles in the flue gas, in an air staged low NOx system. The MVB coal, by contrast, was predicted to be the most challenging of the three coals, with the HVB coal predicted to fall in between the PRB and MVB coals. 

Pilot-scale test results fell largely in line with predictions from bench-scale testing as far as differences in coal properties were concerned. The most reactive coal (PRB) showed the lowest NOx, followed by the moderately reactive HVB and least reactive MVB coals. From the standpoint of combustibles in the flue gas, the PRB showed the lowest combustibles (carbon in ash and CO), followed by the HVB and MVB coals. Fifteen different variables were studied during pilot-scale testing, the results of which are contained in the report. 

The combination of firing system modifications resulting in the lowest NOx emissions is referred to as the Ultra Low NOx Integrated System. In general, firing system modifications, which reduce NOx emissions, also result in higher levels of carbon in the fly ash. When both NOx and combustion efficiency were equally weighed, the standard TFS 2000™ set of operating conditions/system components gave the best results for the HVB and MVB coals and the Ultra Low NOx Integrated System gave the best results on the PRB coal. Many of the firing system components developed in this project can also be applied to the TFS 2000™ firing system, resulting in improved NOx emissions without significantly impacting the carbon in fly ash levels. 

An engineering systems analysis and economic evaluation was performed to evaluate various NOx reduction options including the commercially available TFS 2000™ firing system, the Ultra Low NOx Integrated System developed in this project, and selective catalytic reduction (SCR). The various NOx reduction alternatives were evaluated as retrofit options for three tangential-fired utility boilers in the U.S.: (1) a 400 MW boiler on the East coast firing an Eastern bituminous compliance coal, (2) a 500 MW boiler in the Midwestern U.S. firing a Midwestern bituminous coal, and (3) a 330 MW boiler in the Western U.S. firing a subbituminous coal from the Power River Basin (PRB). The objective of the Engineering Systems Analysis and Economics Task was to evaluate the economics of various NOx reduction options to gain insight into the optimum NOx reduction strategy for different pulverized coal-fired units. 

Results from this economic analysis showed that switching to a PRB coal, in concert with installation of either a TFS 2000™ System or Ultra Low NOx Integrated System, was the most cost effective option (75-80% less than the cost of an SCR) if the cost of shipping the PRB coal to a particular site was not prohibitive. However, it was recognized that the optimum NOx reduction strategy is unit, site, and system specific.