Energy & Fuels 1998, 12, 443
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Book Reviews Developments in NOx Abatement and Control. By Hermine´ N. Soud and Kazunori Fukusawa. IEA Coal Research: London, 1996. This report is one in a series of reviews published by IEA on timely practical aspects of coal utilization technology. Nitrogen oxides (NO + NO2 designated as NOx) emissions from pulverized coal combustion contribute to the formations of acid rain, photochemical smog, and tropospheric ozone. NOx forms in combustion processes by the reaction of molecular nitrogen and atomic oxygen, and also by the oxidation of organically bound nitrogen compounds in the coal. The former reaction occurs at high temperatures (mainly at T > 1800 K) and has a rate of formation with high temperature dependence, while the latter, which dominates in pulverized coal combustion, forms also at moderate temperatures in oxidizing atmospheres. A noteworthy feature of the chemistry of nitrogen compound interconversion is the conversion of nitrogen species to molecular nitrogen in high-temperature reducing atmospheres. This provides the basis for combustion staging (pyrolysis followed by oxidation) as a method of reducing NOx emissions. Regulations in the industrialized countries are becoming tighter and the compliance is in part by the modification of the combustion process and increasingly also by postcombustion treatment of the flue gas. Developments in both the categories of abatement and control are discussed, and general comparative costs are given in the report. The Introduction (Chapter 1) provides a background and relates the report to previous IEA Reviews. The emphasis in the present contribution is on developments in NOx abatement and control technology in conventional pulverized coal boilers during the period of 1990-96. A comprehensive international view of NOx Emission Standards is provided in Chapter 2 with useful guidance through the maze of different units for emission standards such as mg/ m3, lb/million BTU, g/GJ, and ppm at 3% or 6% O2. The CEC directives, and the trend of national legislation in individual countries of Central Europe, South East Asia, and the USA, are also given. The effects of boiler type, size, configuration, and coal type upon NOx emission are discussed in Chapter 3. This is an important topic, and the report provides information from the technical literature but does not fully satisfy the reader who is seeking answers on causal, mechanistic relationships between boiler design parameters or coal type, and NOx emission. The gist of this report in Chapters 4, 5, and 6 includes the
discussions of the two main categories of methods for NOx emissions reduction: primary measures (combustion process modifications), postcombustion clean-up, and their combinations. Primary measures (Chapter 4), including burner optimization, air staging, flue gas recirculation, reburning, and low NOx burners, all supported by computer modeling, are the most cost effective means of pollutant emission control. The author’s data show that capacities of coal-fired units fitted with primary measures for NOx reduction worldwide amount to 189 GW. This makes NOx abatement by combustion modification an outstanding example of the impact of coal science on technology. As regulations are getting increasingly tighter, combustion process modifications may have to be supplemented by postcombustion flue gas treatment (Chapter 5). The topics include SCR, SNCR, Hybrid SCR/SNCR and Combined NOx and SOx removal processes. Discussions of SCR (selective catalytic reduction) design optimization, catalyst degradation, and experience with catalyst development, or those on the temperature window, reagent control, and utilization in the selective noncatalytic reduction process (SNCR), are useful to the reader looking for the comparative technical-economic merit of these NOx control methods. To reduce cost, especially of the additive ammonia, flue gas treatment can be usefully combined with primary measures (Chapter 6). NOx control costs get a detailed treatment including discussions of the methodology for retrofit with the various NOx reducing technologies (Chapter 7). Comparative costs of primary measures and postcombustion flue gas treatment are given for a wide range of boiler configurations and coal types. Chapter 8 summarizes the report’s most important findings. Chapter 9 contains a list of 221 references. The report is a well-written document, a welcome list of acronyms and abbreviations makes it even more reader friendly. It can assist the practising engineer in decision making about choices of retrofit options and academics with background information for their research. The cost of the report is £450 in IEA nonmember countries, £150 in member countries, and £75 for educational establishments in member countries. Ja´ nos M. Bee´ r, Massachusetts Institute of Technology
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