Large Utility Boilers Research Articles

Fluidized bed char combustion kinetic models

The effect of the kinetics of the heterogeneous reaction between the char and oxygen, and the importance of the subsequent homogeneous oxidation of carbon monoxide oil the performance of an atmospheric fluidized bed combustor have been examined by a comparative analysis of three models. Numerical calculations illustrating the effect of operating variables such as bed temperature, fuel and sorbent particle feed size and excess air were performed for values of the parameters which are pertinent to the design of large scale utility boilers. It is found that the kinetic models predict existence of multiple steady states over a wide range of operating conditions and values of model parameters. The model predictions of combustion efficiency, carbon loading and CO concentrations are in qualitative agreement with experimental data reported in the literature.

Objectives and status of fabric filter performance study

In October 1977, Southwestern Public Service Company executed a contract with the U.S. Environmental Protection Agency that called for a study to assess the performance of a fabric filter system installed on a large utility boiler that utilizes low sulfur Western coal. The project is now into its second year and the objectives of this paper are to describe the scope and intent of the study, as well as to report progress to date. In addition, some of the difficulties that we have encountered are discussed. Although some of these problems have resulted in procedural changes, the intent of the study has not been altered. This paper describes work being done in specific areas with which both the EPA and Southwestern are connected. These include fabric assessment, data collection, selection and installation of instrumentation, and overall fabric filter system performance. Results of the first performance test are also reviewed and the installation of a pilot baghouse is discussed.

The measurement and interpretation of radiation from fly ash particles in large pulverized coal flames

Narrow wave-band measurements have been used to determine the absorption coefficient of combustion chamber gases in large utility boilers with an accuracy of about ±25%. Effective absorption efficiencies and refractive indices for, the fly ash have been deduced where dust burden details could be measured. Modifications to instrument design would improve the accuracy. The absorption characteristics were shown to be influenced by the chemical composition of the ash. For a brown and a sub-bituminous coal of high alkali ash the measurements indicated a very high specific surface in the furnace, incompatible with sizing of collected fly ash samples, and a very high wavelength dependence of absorption. Small molten particles in the furnace gases could account for the high apparent surface area and the wavelength dependence. Results for two bituminous coals with high ash fusion temperatures indicate that an absorption coefficient K ¯ a of 0.4 to 0.5 would be adequate for eingineering purposes. However this value appears to be strongly influenced by chemical composition of ash; the cloud surface area may be more dependent on inherent ash and combustion characteristics than on the total ash or the coal grind.

Reducing inefficiency and emissions of large steam generators in the United States

Combustion process modifications to reduce inefficiency and emissions in contemporary large boilers are constrained by a number of important design and operating considerations such as steam temperature, ash fusion and material limitations. Plant efficiency gains, obtainable by further increasing combustion heat release efficiency, are in many cases negligible. However, depending upon standard operation, a large potential exists for increasing overall efficiency by lowering excess air levels, while at the same time decreasing pollutant emissions. Dramatic increases in fuel prices have now made investment in such improvements more attractive in the United States than it was in the past. Reduction of excess air level can in addition effect reductions in emissions of NO x and SO 3, and even of particulates, through equipment and operating improvements. High fuel prices have also made investment in air preheaters and economizers attractive for a larger number of boilers than ever before. Air pollutants can originate from components of the fuel, from incomplete combustion, or even through complete combustion processes. These various forms of pollution can be controlled in varying degrees during the combustion process by wet ash handling, SO 2 to SO 3 conversion minimization, or mixing and temperature control in the case of combustibles and nitric oxide. NO x reduction techniques, applied to a large gas-fired utility boiler, resulted in NO x emissions of 33 ppmV or less at power outputs up to 355 MW. For coal- and oil-fired boilers, NO x scavenging by ammonia injection, recently developed, can give dramatic reductions in NO x emissions without affecting the combustion process.

Combustion in large boilers: Design and operating effects on efficiency and emissions

Combustion process modifications in contemporary large boilers are constrained by a number of important design and operating considerations such as steam temperature,ash fusion, and material limitations. Plant efficiency gains obtainable by further increasing combustion efficiency are in most cases negligible. However, depending upon standard operation, a large potential exists for increasing overall efficiency by lowering excess air levels, while at the same time decreasing pollutant emissions. Dramatic increases-in fuel prices have now made investment in such improvements more attractive than it was in the past. Reduction of excess air level can also effect reductions in emssions of NOx and SO3, and even of particulates, through equipment and operating improvements. Pollution originates fromthe fuel, from incomplete combustion or through combustion processes. These various forms of pollution can be controlled in varying degrees during the combustion process by wet ash handling, SO3 conversion minimization or mixing and temperature control in the case of combustibles and nitric oxide. NOx reduction techniques, applied to a large gas-fired utility boiler, resulted in NOx emissions of 33 ppmV or less at power outputs up to 355 Mw.