Doctoral Dissertations

Dujiu Yang

Date of Award

12-1993

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Engineering Science

Major Professor

L W. Crawford

Committee Members

B. Antar, M. H. MaCay

Abstract

In the open cycle coal-fired MHD system, ash is generated from burning coal in the combustor and downstream condensation of potassium seed species, and is a potential pollutant. A range of ash particle sizes is produced by the MHD system, including a high concentration of submicron particles. Research at the US.DOE coal fired flow facility (CFFF) has addressed ash removal with baghouse (BH) filters and an electrostatic precipitator (ESP). After filtration in the BH, most of the ash panicles are removed from the flow. However, some of the smaller particles pass through the filter into the exhaust air. At the ESP, submicron particles also pass into the exhaust air due to low collection efficiencies for this size particle. A particle agglomeration method presented in the dissertation can decrease the submicron portion of the ash and thus improve particle collection. When appropriate particles are injected into the flow, small particles will agglomerate around these particles so that more of the particle ash will be larger in size, and therefore be easily removed from the flow by the BH or ESP. The larger particles will also give higher overall energy conversion efficiency by raising superheater heat transfer efficiency and reducing energy consumption in the BH or the ESP. In this dissertation, a theoretical model is presented in two parts. The first part represents particle growth at the section where particles are injected. Because temperature at this section is very high, the flow is in the three phases and in the supersaturated state, and particles have thermophoretic coagulation, condensation growth and heterogeneous nucleation growth. A kinetic rate, modified coagulation equation, is given to describe these three processes associated with particle injection. This equation can be solved numerically for either each single phenomenon or as combination of these phenomena. The second part is the section after particle injection. This part considered what occurs in the 14 sections of the CFFF from cooling section 1 to the inlet of the BH and ESP. The primary process at this part is particle thermophoretic coagulation only. Discussions of thermophoretic coagulation, condensation, nucleation and particle injection are presented for the injection point and after injection point. The theoretical results show that particle size distribution is shifted to lower submicron particle densities with particle injection and the total number of particle is also even lower when additional particles are introduced into the flow after cooling section 1. Finally, measured particle size distributions at the inlet of the CFFF BH and ESP with and without particle injection are compared to the calculated distribution based on the theory developed in the dissertation. The experimental data support the theoretical prediction.

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