Author(s)

M. P. Maraj

Abstract

Much of the literature surrounding the exergetic evaluation of boilers has focused primarily on large capacity and high-pressured systems, the majority of which are found in power plants. In Trinidad and Tobago however, approximately 76% of the boilers used are small low-pressured systems. This paper looks specifically at evaluating the exergetic performance of one such lowpressured boiler. The results of this evaluation confirm the large differences, described in the literature, that exist between boiler first and second law efficiencies and show that the combustion and heat transfer processes are responsible for these low exergetic efficiencies by consuming approximately 70% of the fuel’s entering exergy. The amount of excess air used for combustion significantly affects boiler performance. During this study, it was found that most of these smaller boilers were not equipped with excess air control devices and as such, excess air amounts varied significantly and was often much higher than the recommended 15%. It was also found that for every 20% decrease in excess air, combustion irreversibilities could be lowered by approximately 2–3%. Furthermore if the boiler was operated with the recommended 15% excess air, simulation shows that increasing the inlet combustion air temperatures using waste heat from the boiler’s stack could lead to a 7% reduction in combustion irreversibilities. While any improvement in boiler performance may have favourable consequences, these results indicate that dramatic decreases in boiler irreversibilities cannot be expected through air preheating and combustion control strategies and suggest that designs which minimize combustion in boiler systems should be investigated. Keywords: boiler, combustion, heat transfer, exergy analysis, irreversibilities.

Keywords

boiler, combustion, heat transfer, exergy analysis, irreversibilities