Humidifying Desulfurization Of Circulating Fluidized Bed Boiler Desulfurization Fly Ash

Circulating fluidized bed combustion is an advanced technology of clean coal combustion technology. It has the advantages of adapting to a large variety of fuel, high combustion efficiency, lower pollutant emissions, and stable operation over a large range of load regulation. Due to these advantages, and with the higher requirements of environmental protection as well as the pressure of the energy shortage, circulating fluidized bed combustion technology has been developed rapidly in China and abroad.For the desulphurization of circulating fluidized bed boiler, when bed temperature is 850-900℃, Ca/S mol ratio is 1. 5-2. 5, the fineness of limestone particles is less than 2mm (often is 0.1-0.3) , SO₂ removal efficiency can reach 85-90% theoretically. But in real conditions, because of decreasing of circulating ratio, increasing of bed temperature, jamming of limestone particle's interspaces and so on, SO₂ removal efficiency is usually lower than that of theoretical. Furthermore, even if SO₂ removal efficiency in furnace is 90%, it is also impossible to meet the requirements of environmental protection because of the high sulfur content of fuel coal. So in this thesis a new desulphurization mode which is composed of furnace desulphurization and flue gas humidification desulphurization is put forward to increase the SO₂ removal efficiency of circulating fluidized bed boiler, and also the utilization efficiency of sorbents. Consequently, a modification technology that is high-performance, reliable and practical can be offered to the circulating fluidized bed boilers which have employed furnace desulphurization but still can't reach the requirements of environmental protection. In this thesis, a minitype test bed was built first, on which the desulphurization performance of the CFBC ash at low temperature was studied. And the influence of such factors as the relative humidity, reaction temperature, Ca/S and SO₂ concentration on the desulphurization efficiency were analyzed experimentally and theoretically.Experimental results show that, first, the relative humidity plays an important role in the desulphurization reaction of CFB boiler fly ash at low temperature. SO₂ removal efficiency will be higher with the increase of relative humidity at a fixed temperature. At the same time, the influence trend of relative humidity on the desulphurization efficiency is that, it has a less effect on the desulphurization efficiency when relative humidity is in the higher range, while has a greater effect when in the lower range. Under the condition of a certain fixed Ca/S mol ratio in this experiment, when the relative humidity increased from 25 % to 66. 67 %, the desulphurization efficiency increased from 19.65% to 37.20% when reaction temperature is 70℃, and increased from 3.67% to 16.80 % when reaction temperature is 50℃.Second, the desulphurization efficiency increases with the increasing of temperature when relative humidity is a constant. The reason is that the higher of the reaction temperature, the bigger of the absolute moisture content when relative humidity is a constant, so the more of the water on the surface of sorbents, and this factor is beneficial to ionic reaction of SO₂ removal. At the same time, the effect of temperature on desulphurization follows the chemical reaction rule, that is, the sorbents utilization ratio increases with the increasing of reaction temperature. At all conditions of relative humidity in this experiment, the increments of desulphurization efficiency at 70℃ are all more than 15%comparing with those at 50℃.Third, SO₂ concentration has a positive effect on the desulphurization efficiency when relative humidity is fixed. The desulphurization efficiency increases with the increase of SO₂ concentration, and the increase extent trails off. Besides, comparing with SO₂ concentration, Ca/S has a secondary effect on the desulphurization efficiency when sorbents surpassed the need greatly.The design of a pilot-scale test bed also was carried out. Through the study on the desulphurization performances and the sorbents utilization ratio of this complex desulphurization technology, it is hoped to find an optimal coordinated range of a lower Ca/S mol ratio, a higher bed temperature and a higher SO₂ removal efficiency as well as sorbents utilization ratio. The result will be a theoretical guidance for engineering application in a certain extent.The designed fume volumetric flow rate of the pilot-scale test bed is 1200 Nm³/h. The quantities of fuel, boiler material consumption, flow rate of spray water in desulphurization tower, as well as the measurements of boiler, cyclone segregator, heat exchanger, desulphurization tower, screw feeder and thickness of boiler and desulphurization tower insulating layers were designed and calculated.