The Effect of Borax Solution on the Reduction of Fine Particles in Flue Gas at a Commercial Circulating Fluidized-bed Boiler Firing Bituminous Coal

순환 유동층 보일러에서 석탄 연소 시 Borax Solution이 연소 배가스중 미세먼지 저감에 미치는 영향

Abstract

In this study, the effect of chemical additives on the reduction of fine particles was identified in $9.2MW_e$ commercial scale circulating fluidized bed boiler firing bituminous coal. Futhermore, a simple and effective method of fine particle collection has been developed to collect the fine particles of the boiler during fossil fuel combustion. Chemical additives were used to reduce particles below 10 PM in the flue gas, and borax solution was used as a chemical additive. In order to identify the particle behavior of PM 10 or less among the collected fine particles, it was analyzed through particle size analyzer and SEM analysis. The Borax solution tends to absorb molten mineral in the flue gas and make fine particles grow. As a result, it was analyzed that particles smaller than $10{\mu}m$ were reduced by using borax solution.

본 연구에서는 $9.2MW_e$의 상용규모 순환유동층 보일러에서 화학첨가제 사용에 따른 미세입자 저감에 대한 영향을 확인하였다. 또한 화력 발전소에서 화석연료의 연소 중 발생하는 미세입자를 포집하기 위하여 간단하고 효과적인 포집설비를 개발하였다. 연소 배가스 중 PM 10이하의 입자를 감소시키기 위하여 화학첨가제를 사용하였으며, 사용한 화학첨가제는 borax solution을 사용하였다. 포집된 미세입자 중 PM 10이하의 입자 거동을 확인하기 위하여 입도분석기와 SEM 분석을 통해 확인하였다. Borax solution은 배가스중에 용융된 미네랄을 흡수하여 미세입자를 성장시키는 경향이 있는 것을 알 수 있었다. 결과적으로 borax solution을 사용함으로써 $10{\mu}m$ 이하의 미세입자가 감소되었음을 확인할 수 있었다.

Fig. 1. Schematic diagram of the 9.2 MW_e CFBC boiler.

Fig. 2. Location of borax solution injection.

Fig. 3. Diagram of fine particle sampling apparatus.

Fig. 4. Size distribution of the collected fine particles according to the use of chemicals : (a) between cyclone outlet and convection pass inlet under 1,000 μm, (b) between cyclone outlet and convection pass inlet under 10 μm, (c) in the electric precipitator under 1,000 μm, and (d) in the electric precipitator under 10 μm.

Fig. 5. Result of the collected fine particle analysis according to the use of chemicals: (a) under 10 μm, (b) under 2.5 μm.

Fig. 6. Reduction ratio of the collected fine particles according to the use of chemicals: (a) under 10 μm, (b) under 2.5 μm.

Fig. 7. Na, K, S and Cl contents of fine particles collected in (a) between cyclone outlet and convection pass inlet, and (b) the electric precipitator.

Fig. 8. SEM (scanning electron microscopy) images of fine particle sampled: (a) at convection pass inlet in the absence of chemical addition, (b) at electric precipitator in the absence of chemical addition, (c) at convection pass inlet after borax solution was injected, and (d) at electric precipitator after borax solution was injected.

Table 1. CFBC operation condition

Table 2. Physical properties of fuel

Table 3. Borax solution compositions

Table 4. Result of fine particle analysis

Table 5. Portion of fine particle (under 2.5 μm and 10 μm) after borax solution (1200:1 and 800:1) used