Abstract
The European research project CLARA (chemical looping gasification for sustainable production of biofuels, G.A. 817841) investigated chemical looping gasification of wheat straw pellets. This work focuses on pretreatments for this residual biomass, i.e., torrefaction and torrefaction-washing. Devolatilizations of individual pellets were performed in a laboratory-scale fluidized bed made of sand, at 700, 800, and 900 °C, to quantify and analyze the syngas released from differently pretreated biomasses; experimental data were assessed by integral-average parameters: gas yield, H2/CO molar ratio, and carbon conversion. A new analysis of devolatilization data was performed, based on information from instantaneous peaks of released syngas, by simple regressions with straight lines. For all biomasses, the increase of devolatilization temperature between 700 and 900 °C enhanced the thermochemical conversion in terms of gas yield, carbon conversion, and H2/CO ratio in the syngas. Regarding pretreatments, the main evidence is the general improvement of syngas quality (i.e., composition) and quantity, compared to those of untreated pellets; only slighter differentiations were observed concerning different pretreatments, mainly thanks to peak quantities, which highlighted an improvement of the H2/CO molar ratio in correlation with increased torrefaction temperature from 250 to 270 °C. The proposed methods emerged as suitable straightforward tools to investigate the behavior of biomasses and the effects of process parameters and biomass nature.
Highlights
The EU’s Renewable Energy Directive (RED II) has set the goal of achieving a 14%renewable energy share in the transport sector by 2030 [1], and residual biomasses and agro-industrial waste can be exploited as sources to produce sustainable second generation biofuels [2], which are expected to significantly reduce greenhouse gas emissions [2,3].The gasification of residual biomass, to produce advanced biofuels, is a promising technology to achieve the goals of RED II
Pure oxygen ensures the production of a high heating value and nitrogen-free syngas, the latter feature being advantageous for the synthesis of biofuels; the provision of pure oxygen requires an air separation unit (ASU), which is usually associated with high capital and operational costs [5]
The biomass investigated in this work is wheat straw, one of the biogenic residues selected within the CLARA project [10,15,16,24]
Summary
Renewable energy share in the transport sector by 2030 [1], and residual biomasses and agro-industrial waste can be exploited as sources to produce sustainable second generation biofuels [2], which are expected to significantly reduce greenhouse gas emissions [2,3]. The gasification of residual biomass, to produce advanced biofuels, is a promising technology to achieve the goals of RED II. Gasification is a mature thermochemical conversion process suitable for biomasses, with syngas (mixture of H2 , CO, CO2 , CH4 , possibly diluted by steam and/or N2 [4]) as the main product; syngas is primarily used to generate heat and electricity, and is potentially exploitable to synthesize advanced biofuels (the latter option has not yet been fully implemented at the industrial scale) [5]. The chemical looping gasification (CLG) process is a new gasification concept, which avoids nitrogen dilution without requiring an ASU and allows for decent fuel conversions [5,7,8]
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