Abstract
The pyrolysis behaviour of beech wood, two rice husk variants from Brazil (BRH) and Thailand (TRH) and a solid waste water treatment residue from textile manufacture (TIR) were investigated using a lab-scale, 2-stage fixed-bed reactor at 773 K. Char yields increased and volatile yields decreased with increasing ash content. The TRH released 40% less tar than the BRH which was attributed to the substantially higher potassium content of the Thai species. The combustion reactivity of the TRH char in air at 773 K was similar to the BW char and almost double the reactivity of the BRH and TIR chars. The BW and TRH chars had a greater volume of macropores indicating that char combustion occurs predominantly through the growth and extension of the macroporous pore network. A different trend was observed for the char gasification reactivity with CO2 at 1173 K. The Ca and Mg content of the chars were found to have a more important catalytic role in the char gasification reactions with CO2.The effect of exposing volatile products from beech wood pyrolysis to elevated temperatures (973–1173 K) and sand beds containing calcined limestone or dolomite in a simulated downdraft gasification environment was also investigated. Tar yields decreased after exposure to elevated temperature and calcined limestone or dolomite. Tar cracking favoured the production of CO. CO yields were between 22 and 23 wt% at 1173 K. Calcined dolomite was slightly more effective at cracking tar than calcined limestone, eliminating 98 wt% of the tar at 1173 K.
Highlights
Biomass utilisation has received renewed interest over recent years in response to growing concerns over volatile fossil fuel prices, energy security, and climate change [1]
Pyrolysis of the two rice husk species produced similar amounts of char and volatiles to the beech wood, the Thai rice husk (TRH) produced approximately 40% less tars than the Brazilian rice husk (BRH)
Beech wood (BW); BRH; TRH and via BET/BJH N2-adsorption analysis revealed that the textile manufacture (TIR) char had the highest surface area at 41 m2 g−1 (Table 6) and largest concentration of pores in the meso-porous region (2–50 nm) (Fig. 6a)
Summary
Biomass utilisation has received renewed interest over recent years in response to growing concerns over volatile fossil fuel prices, energy security, and climate change [1]. Downdraft gasifiers are a class of fixed bed reactor that are suited to applications of small scale, decentralised electricity and heat production [2,5]. We investigate the potential of calcined dolomite and limestone as (primary) tar cracking catalysts for downdraft gasification applications where the minerals could be added with the feed to reduce tar emissions and eliminate the need for secondary downstream tar removal. Calcined limestone is often discounted as a tar cracking catalyst for in-bed use in fluidised bed gasification processes due to concerns relating to particle sintering and agglomeration leading to more rapid catalyst deactivation and/or de-fluidisation [34] This behaviour presents less of a problem when considering the use of CaO as a single pass catalyst in a downdraft gasification process. The addition of lime has been found to prevent the formation of low melting point, alkali metal silicate eutectic salts that can lead to slagging and bed agglomeration issues, which can be problematic when handling biomass varieties containing large amounts of ash [37]
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