Oxides of Nitrogen: Formation and Control in Resource Recovery Facilities

This paper presents an excellent review of the theory of solid fuel combustion and NOx formation and control in the context of solid waste combustion. After pointing out that most research on NOx formation from solid fuels has been done on coal, the authors correctly point out that the while the basic principles of fuel and thermal NOx formation are well understood and permit NOx reductions in the combustion of coal, the techniques used for coal may not apply to the combustion of MSW or RDF. NOx reduction by use of catalytic reduction and ammonia injection are clearly impractical for municipal waste, leaving control of combustion air supplied to the grate and to overfire air zones as the only available tech­ niques. The authors clearly point out the problem in siting new plants: California Air Resources Board has issued guide­ lines call for NOx emission limits as low as 140 ppm (dry volume, 12% carbon dioxide, 8 hr average), although manufacturers of solid waste combustion equipment have stated that consistently meeting such a low emission limit would be difficult if not impossible. California already has regulations calling for new facilities to demonstrate that a one hour ambient air quality standard for NOx of 470 p.g/m3 will not be exceeded at any point in the surround­ ing environment as a result of emissions from the proposed facility, whereas the rest of the country falls under the NAAQS of 100 p.g/m3• In the discussion which follows, however, the authors do not mention the levels of emissions which have been demonstrated, but choose to carry on the discussion sole­ ly in terms of "reduction" without elucidating to what levels NOx emissions can be reduced. This failure to cite real numbers makes the discussion barren and meaningless. Since no numbers are given, the reader is unable to deter­ mine whether or not there is a real problem in meeting the guidelines, or whether the reductions of 20-33% which were reported for various combustion improvements could be significant to pass or fail emission standards. There is clear evidence that starved air in the primary combustion (on the grate) and staged combustion over the fire can minimize NOx formation. Again, the authors do not cite any numbers to quantify how effective these methods are, nor whether or not they permit approach to the guidelines. This information would be of great value. It is certainly true that there is a dearth of information which is available to answer these questions, and that the authors did not promise to do so. On the other hand, there is published information which needs to be brought to light and a need for much more research in this area. The authors point out that continuous monitoring of NOx emissions, at least during efforts to test and optimize combustion, is the starting point. With monitors in place, optimum combustion conditions can be established. Major . modifications in existing and future units may be needed