【正文】 d to produce ammonium sulphate, which is a relatively highvalue product that can be used in fertilisers. The high value of this byproduct is the principal advantage of this process. With highsulphur fuels, the receipts from the sale of the sulphate can exceed the costs of operating the FGD plant. However, there could be mercial risks associated with this, because the price of ammonium sulphate and ammonia are both very volatile. A potential risk arises from the need to store ammonia on site, either in anhydrous form, or as a concentrated aqueous solution. This might cause serious difficulties in the planning stage, at certain sites. These plant are expensive to build, and require a large ‘ footprint’ similar to a limestone gypsum plant. The process has the advantage that there is no wastewater discharge, and there are unlikely to be problems of scaling and blockage. At certain sites, particularly those burning highsulphur fuels, or with the potential to do so, this process could be a very attractive one. However, it is unlikely to achieve very widespread use because very few plant are needed to 火電廠煙氣脫硝技術應用分析 36 satisfy the market for ammonium sulphate fertiliser in a particular country or region. Figure 1. The ammonia scrubbing process The WellmanLord Process The WellmanLord Process is regenerative, ie the active reagent used for removal of SO2 from the flue gas is regenerated in a second process stage, and returned to the first stage (absorber tower) for reuse. Consequently, the process does not involve the largescale consumption of lime or limestone, unlike other processes described here. The process involves the wet scrubbing of SO2 from the flue gas with aqueous sodium sulphite solution. It produces a saleable byproduct that, depending on the plant’s design, could be elemental sulphur, sulphuric acid or liquid SO2. The WellmanLord process has been installed on nearly 40 plant, in Japan, the USA and Germany. This includes over 3000MWe of electric utility boilers, and 火電廠煙氣脫硝技術應用分析 37 many industrial plant. However, there appears to have been no new plant built in recent years. Flue gas from the ESP and ID fan is passed through a booster fan before entering the gas/gas reheater (Figure 2). Here the gas is cooled as heat is extracted. The warm gas from the reheater enters the prescrubber/absorber and mixes with the process liquor. An equilibrium temperature is established, when the flue gas bees saturated with water vapour. In the prescrubbing stage, fly ash and HCL are removed. In the main absorber, the gas is scrubbed with the process liquor, to remove the required amount of SO2. Typically 9598% of the SO2 can be removed within the absorber. At the top of the absorber, the gas passes through demisters to remove suspended water droplets. After leaving the absorber, the gas is passed through the reheater again, to raise its temperature before being exhausted to the stack. A prescrubber is usually fitted upstream of the absorber, primarily to remove any HCL present in the flue gas. If HCL were to dissolve in the main absorber liquor, the concentration of sodium chloride in the liquor would progressively increase to levels where it would interfere with the chemistry governing the removal of SO2. The degree of desulphurization attained would hence progressively fall off. In the main absorber, the flue gas is scrubbed with aqueous sodium sulphite solution, forming sodium bisulphite. The sodium bisulphite is deposed by 火電廠煙氣脫硝技術應用分析 38 steam heating in an evaporative crystalliser to produce sodium sulphite and SO2. The sodium sulphite is returned to the flue gas absorber tower circuit for reuse, while the concentrated SO2 gas stream can then be treated as appropriate to produce a byproduct suitable for export. Whether this is concentrated SO2 liquid, sulphuric acid or elemental sulphur would depend on the local mercial environment. This process can achieve a SO2removal efficiency of well over 95% on highsulphur fuels. It is expensive to install but relatively cheap to operate and, as such, in relation to other processes, is best suited to high SO2removal requirements, highsulphur fuel, and plant with a long residual life. Comparative studies have suggested that the operating cost is very similar to that of the limestone gypsum process. The process also has the advantages that it does not require the consumption of large quantities of sorbent and does not produce large quantities of solid waste. Figure 2. The WellmanLord process Circulating Fluidised Bed (CFB) 火電廠煙氣脫硝技術應用分析 39 In the CFB process, the flue gas is passed through a dense mixture of lime (calcium hydroxide), reaction products and sometimes fly ash, which removes the SO2, SO3 and HCl. The final product is a dry powdered mixture of calcium pounds. The process has been mercially available for over 10 years, and is an expanding technology, particularly for retrofitting to small to mediumsized power plant. Because of its simplicity, higher performance, lower spatial requirement, and sometimes lower cost, it is nowadays being chosen instead of the more widely established spray dry process in certain applications. The process and variants on it are now supplied by several vendors, whose designs vary significantly, although the process chemistries are the same. The originator and most experienced vendor is LLB. Flue gas from the airheater (Figure 3) is carried through the inlet venture throat of the CFB reactor and passes upwards through a fluidised bed of lime, reaction products and fly ash particles contained within the vertical reactor tower. This removes up to 99% of the SO2 and all of the SO3 and HCl from the flue gas. Fro