Dry gas cleaning

As gas cleaning options, a choice can be made for either low-temperature "wet" or high-temperature "dry" gas cleaning. The latter type of gas cleaning is advantageous for the overall energy balance but many process elements are not yet fully proven technology. 

Table I Efficiencies and operating conditions comparison between six filter technologies for hot and dry gas cleaning. ... 

Table 2 General characteristics of existing hot and dry gas cleaning technologies...

Economically, the redox process only makes sense if the expenses for the redox converter can be balanced by savings in the gasifier and primary gas clean-up system, i.e. if the system as a whole does not cost more than a conventional fixed bed down-draft gasifier with wet or dry gas clean-up (benchmark). It is expected that the redox... 

Suitable for gas drying, gas cleaning and gas mixture separation, as well as catalysis purposes. 

During the semi-dry gas cleaning process, a liquid, usually a lime slurry, is injected into the flue gas by means of one or more spray nozzles at the top of an absorption tower. The slurry then reacts with the pollutants. Some of the liquid will evaporate into the gas stream, and it is possible with carefnl flow control to arrange matters so that the effluent at the bottom of the absorber is effectively dry. 

The dried charge is pneumatically fed to a burner mounted on the roof of the reaction shaft. Normally oxygen is used to get an autogenous smelt. The offgases analy2e around 70% SO2 and can be used for the manufacture of sulfuric acid after conventional gas cleaning. 

Optimized modern dry scrubbing systems for incinerator gas cleaning are much more effective (and expensive) than their counterparts used so far for utility boiler flue gas cleaning. Brinckman and Maresca [ASME Med. Waste Symp. (1992)] describe the use of dry hydrated lime or sodium bicarbonate injection followed by membrane filtration as preferred treatment technology for control of acid gas and particulate matter emissions from modular medical waste incinerators, which have especially high dioxin emissions. 

As stated, the dry gas seal does come with its own set of disadvantages. The biggest of these is that the buffer gas must be reliable. Loss oi buffer gas in some cases will reverse the differentia] pressure across (iie seal faces, which will damage the seal in short order. The seals will opei ate at a zero differential pressure level, but when possible, even a small differential in the proper direction is recommended by the manufacturers. Another disadvantage is the requirement for clean and dry gas at the seal... 

One of the main functions of the seal gas system is to supply clean (filtered) dry gas to the primary seal. Figure 8-15 shows a typical sea gas... 

Gas clean up methods can be classified into two distinct routes wet low temperature cleaning and dry high temperature cleaning. 

With good dry scrubbing sorbents, the controlling resistance for gas cleaning is external turbulent diffusion, which also depends on energy dissipated by viscous and by inertial mechanisms. It turns out to be possible to correlate mass-transfer rate as a function of the friction factor [Ergun, CEP 48(2) 89 and 48(5) 227(1952)]. 

The latest installations incorporate a waste heat boiler in the off-gas cleaning system to recover sensible heat from the rotary kiln off-gas. There is sufficient sensible heat in the off-gas from the SL/RN process to generate 500 to 700 kWh/t of DRI, depending on the type of reductant used. 

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