The influence of air staging in the reduction of SO₂ by limestone in a fluidized bed combustor

To meet the legislation on limiting NO_x and SO_x emission from large combustion plants, SO₂ emissions may need to be reduced by about 60-70%. The authors' technique for achieving significant reduction in SO₂ emissions from combustion systems burning large coals utilizes limestone for in situ reduction of SO₂ in fluidized bed combustors. A recent technique for reducing NO_x emissions combines staged combustion with ammonia injection. However, concern has arisen about the effect of low primary air:coal ratio (PACR), and hence low bed oxygen levels, on the rate of reaction between calcined limestone and SO₂. This paper reports on the significance of air-staging on the reduction of SO₂ emissions by limestone in a fluidized bed combustor. The experimental rig consisted of a stainless steel combustor 2 m high and 0.3 m² in section. Fluidizing air was supplied through a multihole distributor and secondary air was introduced into the freeboard via an injector whose height could be adjusted. The experiments were carried out at overall fluidizing velocities between 1 and 2 m s^-1, bed temperatures of 830-880°C, 5-40% excess air levels and bed particle size of 313 + 690 μm. Various levels of air-staging (10-50%) were investigated (equivalent to PACRs of between 0.84 and 1.4, respectively). An uncooled sampling probe located at the cyclone exit was used to determine SO₂. The results showed that SO₂ emissions could be reduced by over 50% using limestone under unstaged combustion conditions. There was little effect of staged combustion on SO_x emissions without limestone. When limestone was used, the effectiveness of sulfur capture decreased (by almost 50%) as the level of air-staging was increased. The bed temperature had the most significant effect on sulfur retention by limestone (10% at 880°C to almost 60% at 830°C). A decrease in overall fluidizing velocity resulted in an increase in SO_x reduction. The present work has provided significant data on the parameters that influence SO_x retention by limestone during staged combustion.