Investigating the potential for a self-sustaining slow pyrolysis system under varying operating conditions.

Kyle Crombie,Ondřej Mašek

doi:10.1016/j.biortech.2014.03.134

Abstract

This work aimed to investigate the impact of highest treatment temperature (HTT), heating rate, carrier gas flow rate and feedstock on the composition and energy content of pyrolysis gas to assess whether a self-sustained system could be achieved through the combustion of the gas fraction alone, leaving other co-products available for alternative high-value uses. Calculations based on gas composition showed that the pyrolysis process could be sustained by the energy contained within the pyrolysis gases alone. The lower energy limit (6% biomass higher heating value (HHV)) was surpassed by pyrolysis at ⩾450°C while only a HTT of 650°C consistently met the upper energy limit (15% biomass HHV). These findings fill an important gap in literature related to the energy balance of the pyrolysis systems for biochar production, and show that, at least from an energy balance perspective; self-sustained slow pyrolysis for co-production of biochar and liquid products is feasible.

Highlights

Out of the three pyrolysis co-products, biochar (HHV $ 18 MJ kgÀ1) and bio-oil (HHV $ 17 MJ kgÀ1) can be regarded as medium to high-energy-density materials, while pyrolysis gas (HHV $ 6 MJ kgÀ1) (Bridgwater, 2006; Laird et al, 2009) is a low-energydensity product
As highest treatment temperature (HTT) increased for each heating rate, there was a decrease in the yield of char and a corresponding increase in the gas and liquid yields
No influence of heating rate was still seen for pine chips (PC) liquid and wheat straw pellets (WSP) chars yields due to a convergence towards similar values compared to 5 °C minÀ1 for these products at elevated HTT

Summary

Introduction

Out of the three pyrolysis co-products, biochar (HHV $ 18 MJ kgÀ1) and bio-oil (HHV $ 17 MJ kgÀ1) can be regarded as medium to high-energy-density materials, while pyrolysis gas (HHV $ 6 MJ kgÀ1) (Bridgwater, 2006; Laird et al, 2009) is a low-energydensity product. Extraction of high-value chemicals from pyrolysis liquids (bio-oil) and/or their upgrading to liquid biofuels is a promising route to efficient decarbonisation of transport and chemical industry (Bridgwater, 2012; Czernik and Bridgwater, 2004) Such chemical products can provide comparable revenue to fuel and energy products even with such relatively small amounts (around 5%) used for this purpose, making for an attractive alternative use for bio-oil (Bridgwater, 2012; Czernik and Bridgwater, 2004). This is achieved through the highly recalcitrant nature of biochar as well

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