Pyrolysis biochar systems, balance between bioenergy and carbon sequestration

Kyle Crombie,Ondřej Mašek

doi:10.1111/gcbb.12137

Abstract

AbstractThis study aimed to investigate the extent to which it is possible to marry the two seemingly opposing concepts of heat and/or power production from biomass with carbon sequestration in the form of biochar. To do this, we investigated the effects of feedstock, highest heating temperature (HTT), residence time at HTT and carrier gas flow rate on the distribution of pyrolysis co‐products and their energy content, as well as the carbon sequestration potential of biochar. Biochar was produced from wood pellets (WP) and straw pellets (SP) at two temperatures (350 and 650 °C), with three residence times (10, 20 and 40 min) and three carrier gas flow rates (0, 0.33 and 0.66 l min−1). The energy balance of the system was determined experimentally by quantifying the energy contained within pyrolysis co‐products. Biochar was also analysed for physicochemical and soil functional properties, namely environmentally stable‐C and labile‐C content. Residence time showed no considerable effect on any of the measured properties. Increased HTT resulted in higher concentrations of fixed C, total C and stable‐C in biochar, as well as higher heating value (HHV) due to the increased release of volatile compounds. Increased carrier gas flow rate resulted in decreased biochar yields and reduced biochar stable‐C and labile‐C content. Pyrolysis at 650 °C showed an increased stable‐C yield as well as a decreased proportion of energy stored in the biochar fraction but increased stored energy in the liquid and gas co‐products. Carrier gas flow rate was also seen to be influential in determining the proportion of energy stored in the gas phase. Understanding the influence of production conditions on long term biochar stability in addition to the energy content of the co‐products obtained from pyrolysis is critical for the development of specifically engineered biochar, be it for agricultural use, carbon storage, energy generation or combinations of the three.

Highlights

1.1 BackgroundGlobal climate change and the inevitable depletion of fossil fuel reserves are two major challenges facing the 21st century which have led to a research boom into new concepts and technologies for alternative energy sources and reducing greenhouse gases (GHG) emissions
At HTT < 450oC the slower heating rate produced higher stable-C concentrations compared to 100oC min-1 at higher HTT this trend disappeared as temperature played the dominant role, agreeing with similar trends seen in Chapter 3, Chapter 5 and Antal & Grønli (2003) work
Hemicellulose has been shown to decompose between 200oC and 375oC, leading mainly to the release of carbon monoxide (CO) and CO2, while cellulose decomposes at slightly higher temperatures between 250oC and 380oC, leading to additional release of CO, CO2 and small amounts of CH4

Summary

Introduction

1.1 BackgroundGlobal climate change and the inevitable depletion of fossil fuel reserves are two major challenges facing the 21st century which have led to a research boom into new concepts and technologies for alternative energy sources and reducing GHG emissions. In addition to bio-oils’ added benefits, biochar can offer numerous environmental and agricultural benefits such as improved soil fertility and long-term storage of C in the environment (Lehmann, 2007; Lehmann et al, 2009; Woolf et al, 2010) This is achieved through the highly recalcitrant nature of biochar as well as its ability to influence nutrient retention, water holding capacity, soil pH, CEC and reducing or supressing the emission of GHG such as CO2, N2O and CH4 (Lehmann, 2007; Chan & Xu, 2009; Manyà, 2012). Matovic (2011) proposed that if 10 % of the world biomass NPP was converted into 50 % charcoal and 30 % energy from volatiles, it would sequester approximately 4.8 GtC year-1 This would generate a C abatement value equivalent to almost 5 wedges in the carbon and climate stabilisation triangle (Pacala & Socolow, 2004)

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