Gas Cleanup

Tar formation is one of the major problems to deal with during biomass gasification [2]. Tar condenses at reduced temperature, thus blocking and fouling process equipments such as engines and turbines. Tar-removal technologies can be divided into gas cleaning after the gasifier (secondary methods) and treatments inside the gasifier (primary methods). Although secondary methods are proven to be effective, treatments inside the gasifier are gaining much attention, as these can reduce the downstream cleanup.

In primary treatment, the gasifier is optimized to produce a fuel gas with minimum tar concentration. The different approaches of primary treatment are (a) proper selection of operating parameters, (b) use of bed additive/catalyst, and (c) gasifier modifications. The operating parameters such as temperature, gasifying agent, equivalence ratio, residence time, etc., play an important role in formation and decomposition of tar. There is a potential of using some active bed additives such as dolomite, olivine, char, etc., inside the gasifier. Ni-based catalysts are very effective not only for tar reduction, but also for decreasing the amount of nitrogenous compounds such as ammonia. Also, reactor modification can improve the quality of the product gas. The concepts of two-stage gasification and secondary air injection in the gasifier are of prime importance. However, primary measures cannot solve the purpose of tar reduction without affecting the useful gas composition and heating value. A combination of proper primary measures with downstream methods is observed to be very effective in all respects (Figure 9.4).

Secondary methods are conventionally used as treatments to the hot product gas from the gasifier [2]. These methods can be classified into two distinct routes: "wet" low-temperature cleaning and "dry" high-temperature cleaning. Conventional wet

Tar free gas

Clean Gas

Tar free gas

Primary methods:

-gasification conditions -gasifier design -use of additives/ catalysts

Secondary methods:

-mechanical separation -thermal cracking -catalytic cracking

Removal of:

-particles

-N, S and halogen compounds

Primary methods:

-gasification conditions -gasifier design -use of additives/ catalysts

Secondary methods:

-mechanical separation -thermal cracking -catalytic cracking

Removal of:

-particles

-N, S and halogen compounds

FIGURE 9.4 Tar removal approach in biomass gasification.

Bag filters Ceramic or " metal candles

COS hydrolization

Amine treating

Scrubber (water+NaOH)

Scrubber (with H2SO4)

b ZnO guard bed

FIGURE 9.5 Schematic view of "wet" low-temperature cleaning.

low-temperature syngas cleaning (Figure 9.5) is the preferred technology in the short term. This technology has some efficiency penalties and requires additional wastewater treatment, but it is well established. Hot gas cleaning consists of several filters and separation units in which the high temperature of the syngas can partly be maintained, achieving efficiency benefits and lower operational costs. Hot gas cleaning is specifically advantageous when preceding a reformer or shift reactor, because these process steps have high inlet temperatures. Hot gas cleaning after atmospheric gasification does not improve efficiency because the subsequent essential compression requires syngas cooling anyway. Hot gas cleaning is not a commercial process yet, since some unit operations are still in the experimental phase. However, within some years, hot gas cleaning will become established and commercially available.

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