Flue gas desulfurisation

Units called flue gas desulfurisation (FGD) units are being fitted to some power stations throughout the world to prevent the emission of sulfur dioxide gas. Here, the sulfur dioxide gas is removed from the waste gases by passing them through calcium hydroxide slurry. This not only removes the sulfur dioxide but also creates calcium sulfate, which can be sold to produce plasterboard (Figure 11.25). The FGD units are very expensive and therefore the sale of the calcium sulfate is an important economic part of the process. 

Flue gas desulfurisation (FGD) The process by which sulfur dioxide gas is removed from the waste gases of power stations by passing them through calcium hydroxide slurry. 

ABSTRACT Calcium-enriched bio-oil (CEB) can be used for flue gas desulfurisation in coal and waste combustion chambers. It is produced by mixing biomass derived fast pyrolysis oil with calcium oxide. The aim of the proposed project is to develop a technology i) to produce calcium-enriched bio-oil with a calcium content of 13 wt,%, and ii) to test the CEB in a combustion chamber by co-firing it with a sufur-containing fuel. In this paper the production method of CEB will be elucidated, and small-scale experiments related to CEB spraying will be presented. Finally, co-combustion experiments of a sulfur-containing fuel with CEB in a small flame tunnel (20 kW, ) will be reported. 

The sin >lcst flue gas desulfurisation technology is furnace injection, where a dry sorbent is injected into the upper part of the furnace to react with the SO2 in the flue gas. The finely grained sorbent is distributed quickly and evenly over the entire cross section in the upper part of the furnace in a location where the teiiq>erature is in the range of 750-12 50 C. Commercially available and cheap limestone (CaC03) or hydrated lime (Ca(OH)j) is used as sorbent. The sorbent reacts with SO2 and O2 to form CaS04. Below 750 C the reaction rate is too low. At temperatures over 1250 C the surface of the sorbent will be sintered, and the structure of the pores will be destructed, reducing the active surface area, The major part of S02-rcmoval takes place within 1 to 2 seconds. 

Some limestones contain over 95 % CaC03. Such chemical grade materials are particularly suitable for lime production, flue gas desulfurisation and a range of other processes. The quantities involved, however, amount at most to a few percent of the total extracted. 

Reaction with aqueous acids. In general, limestones react readily with adds and are used for acid neutralisation. High calcium limestones react readily with dilute hydrochloric and nitric acids at ambient temperatures, whereas dolomite and dolomitic limestones only react readily when the dilute acid is heated. The reaction of limestone with sulfurous acid (formed by the dissolution of sulfur dioxide in water) is the basis of a flue gas desulfurisation process (see section 12.5.2). The reactions with acids which form insoluble or sparingly soluble calcium salts (e.g., sulfurous, sulfuric, oxalic, hydrofluoric and phosphoric acids) are inhibited by the reaction product. 

The major use in environmental protection is flue gas desulfurisation. In 1994, the U.K. had not commissioned any major units. 

Commercially, the most important application is the use of wet scrubbing with limestone for flue gas desulfurisation (FGD) at electricity generating stations. For example, a 2000 MW station operating on a 2 % sulfur coal, at a 70 % load factor, requires about 300,000 tpa of limestone to remove 90 % of the oxides of sulfur. The majority of generating stations fitted with FGD use limestone, and about half of those oxidise the calcium sulfite produced to gypsum (CaS04 -2H20) [12.18]. Some stations use lime (see chapter 29), while others use a variety of other processes [12.3]. 

J.R. Cooper, I.A. Johnston, The Engineering Requirements of the CEGB s Programme for Flue Gas Desulfurisation . In IMechE Seminar on Fossil Fired Emissions , 6 Dec. 1988. 

L. Bier, H. Sauer, W. Bennewart, Construction and operation of a hydrated lime plant for flue gas desulfurisation if the Siersdorf power station , Zement Kalk Gips 6, 1992, 285-288 (English 8,1992 E207-E209). 

In some markets (eg the neutralisation of gaseous and aqueous acids) lime products compete with the still more expensive soda ash and caustic soda. In flue gas desulfurisation and soil stabilisation, lime products compete with limestone and other products. In building and construction the main competitive products are cement, fillers and air-entraining agents. 

The milk of lime-sulfite process was widely used for early power station flue gas desulfurisation projects. It had a lower capital cost than the limestone-sulfite process and gave high absorption and reagent efficiencies. Subsequently, the problem of disposal of the calcium sulfite sludge led to three variants based on lime to be adopted — the gypsum process, the dual alkali process and the maglime process. 

D.H. Stowe, Flue Gas Desulfurisation , Proc. 6th International Lime Association Congress, London, June 1986. 

C 1318-95 Test method for determination of total neutralizing capability and dissolved calcium and magnesium oxide in lime for flue gas desulfurisation (FGD) . 

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