Sorbent injection

Today s major emissions control methods are sorbent injection and flue gas desulfurization. Sorbent injection involves adding an alkali compound to the coal combustion gases for reaction with the sulfur dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based compounds are also used. Sorbent injection processes remove 30 to 60% of sulfur oxide emissions. 

Gaseous pollutants in combustion exhaust gases dry sorbent injection, semidry sorbent injection, wet sorbent injection, selective noncatalytic reduction of N()x (SNCR), selective catalytic reduction of NOx (SCR). 

Parametric tests conducted by US EPA on the Montgomery County South MSW incinerator have been reported.23 Three tests at each of six different test conditions were conducted to evaluate the effects on PCDD/F emissions of electrostatic precipitator (ESP) inlet temperature, sorbent injection into the furnace and into the duct leading to the ESP, and combustion temperature. The key findings were as follows ... 

An overview of these technologies can be found on the website of the lEA Coal Research reviews (1) on FGD technologies (including low cost retrofit options) and from the reviews provided by Soud (2, 3). In the present paper more details will be presented about sorbent injection techniques. 

For Ca(OH)2, removal efficiency of up to 50% are reported at a Ca S ratio of 2. For CaCOj, the removal efficiency will be even lower. Fine sorbent particle size (<5 pm) and an even distribution of the sorbent over the cross-section of a boiler significantly improves the process performance. Bjerle and others (1993), for example, reported results of laboratory scale experiments showing that S02-removal efficiencies > 95% can be achieved with furnace sorbent injection with particle size <3 pm. 

In medium-temperature desulfurization, the sorbent is injected into the economizer section of the boiler and reaction occurs at 500-600° C. This temperature is insufficient for CaC03 calcination to occur so that the feed is limited to Ca(OH)2 or CaO. Finally, the low-temperature process involves sorbent injection at some point in the system where the temperature is below about 350°C. At this low temperature, it is necessary to inject liquid water with the sorbent, either Ca(OH)2 or CaO, to promote the reaction. 

Dry sorbent injection is very similar to the use of spray-dryers, except that the sorbent is injected as a dry powder rather than as an atomized slurry. 

The most common sorbent is hydrated lime, but other sorbents can also be used. The sorbent is usually injected directly into existing ductwork, and so the amount of space required is negligible compared with that of other flue-gas desulfurization processes. This makes dry sorbent injection a very low-cost option. 

Unfortunately, the reactivity of dry absorbents is much lower than that of absorbent slurries or solutions, and so dry sorbent injection is only suitable for applications where less than 70% of the sulfur dioxide needs to be removed from the flue gases. ... 

It is also possible to use limestone in dry sorbent injection, as is done in the limestone injection multistage burner (LIMB) system (Fig. 10). In this system, pulverized limestone is injected into the boiler directly, where the temperature is high enough to flash-calcine... 

Fig. 10 The basic integrated LIMB dry sorbent injection system.

Spray-dryers or dry sorbent injection are a good choice if there is little space in the plant for installing new equipment, there is no market for by-products, and the plant emissions can be brought into regulatory compliance with relatively modest reductions in emissions. 

The tests were carried out in an experimental furnace designed to burn 500 lb/hr of pulverized coal. The sorbents were injected into the furnace flue gas as a dry powder, upstream of a baghouse filter. Operating variables considered included baghouse temperature and cleaning cycle time, sorbent particle size, sorbent/sulfur ratio, location of sorbent injection point, and sorbent injection schedule (continuous or intermittent). 

A considerable amount of interest has developed in utilizing dry-sorbent injection as a means of controlling SO2 emissions from coal-fired industrial and utility boilers.(l-7 ) A dry sorbent FGD system is one in which an alkaline material is injected into the boiler flue gas as a dry powder or aqueous slurry the sorbent reacts with SO2 to form a dry product containing sulfates and sulfites. The mixture of spent material and fly ash is removed from the gas stream by means of an electrostatic precipitator or baghouse filter. The spent sorbent is usually disposed of, but in some cases it may be regenerable or have commercial end uses.(2)... 

The dry-sorbent injection system is shown schematically in Figure 2. This rather simple system provides reliable and accurate feeding of solid sorbent. The entire feed mechanism, including sorbent storage, was placed on a weigh scale with digital readout. The sorbent feed rate was constantly monitored from the slope of weight loss vs time. 

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