Agricultural bio-char production, renewable energy generation and farm carbon sequestration in Western Australia: Certainty, uncertainty and risk

Abstract

Reducing the vulnerability of agriculture to climate change while increasing primary productivity requires mitigation and adaptation activities to generate profitable co-benefits to farms. The conversion of woody-wastes by pyrolysis to produce bio-char (biologically derived charcoal) is one potential option that can enhance natural rates of carbon sequestration in soils, reduce farm waste, and substitute renewable energy sources for fossil-derived fuel inputs. Bio-char has the potential to increase conventional agricultural productivity and enhance the ability of farmers to participate in carbon markets beyond traditional approach by directly applying carbon into soil. This paper provides an overview of the pyrolysis process and products and quantifies the amount of renewable energy generation and net carbon sequestration possible when using farm bio-waste to produce bio-char as a primary product. While this research provides approximate bio-char and energy production yields, costs, uses and risks, there is a need for additional research on the value of bio-char in conventional crop yields and adaptation and mitigation options.

Introduction

Working Group III, in their contribution to the IPCC's Fourth Assessment Report (AR4) stated the high agreement and much evidence that soil restoration and land use change mitigation measures can be implemented immediately by using existing technologies. Working Group III also stated the high agreement and much evidence that soil carbon sequestration is the mechanism responsible for most climate change mitigation potential (Paustian et al., 1997). The IPCC's AR4 Synthesis Report confirmed that effective carbon-price signals can mobilise environmentally effective mitigation options in the agriculture and forestry sectors, including as improved land management practices that maintain soil carbon density and for soil carbon sequestration. However, to be able to successfully utilise soil carbon mitigation incentives, farmers will need to use iterative management processes that balance economic carbon sequestration benefits with conventional production co-benefits, and attitudes to risk (Intergovernmental Panel on Climate Change, 2000).

Decreasing the financial risks of farming in this period of relative climate policy uncertainty requires feasibility studies of synergies between conventional productivity and climate change mitigation and adaptation measures. Similarly, reducing farm investment risk in a changing climate will entail the greater use of monitoring to inform management practices that increase farm ecosystem stability and resilience to climate stress (Griffiths et al., 2000, Tobor-Kaplon et al., 2005, Harle et al., 2006, Brussaard et al., 2007). Therefore, sequestering carbon in agricultural soils is one such possible synergy that creates additional property rights for farmers, retains land values by soil conservation, and may improve conventional yields by modulating soil ecosystem variability (Klein et al., 2007, Milne et al., 2007).

There is considerable interest in finding reliable methods of sequestering carbon in agricultural soils to both reduce farm investment risk and cut atmospheric greenhouse gas concentrations, in a timeframe suitable to investors. Increasing the levels of soil organic carbon (SOC) by conventional agricultural management can take many years and involves significant uncertainty in regards to the resultant carbon fluxes (Denman et al., 2007). A report by the National Carbon Accounting System (NCAS) authored by Valzano et al. (2005), focussed on the impact of tillage on changes in SOC density in Australia. The report found that low tillage and stubble retention management practices only had an effect on SOC density up to depths of 30 cm in areas with mean annual temperatures between 12.8 and 17.4 °C and an average annual rainfall above 650 mm (Valzano et al., 2005). In Australian research plots that did show significant differences of SOC densities between using minimum disturbance methods and conventional tillage, the results have been modest. Farms using direct drill, retained stubble and moderate grazing production methods were found to have densities of around 57 t ha⁻¹ up to 30 cm of depth, while nearby heavily grazed farms using multiple crop tillage (with either tyned or disc implements), had SOC densities of 43 t ha⁻¹ up to 30 cm soil depths (Valzano et al., 2005). A study by Wright et al. (2007) on SOC and nitrogen levels over 20 years of various tillage regimes, found the no-tillage practices only increased SOC, dissolved organic carbon and total nitrogen by 28, 18 and 33% respectively, when compared to conventional tillage (Wright et al., 2007). While the benefit of using minimum tillage methods are clear for retaining natural SOC densities, sequestering sufficient volumes of SOC for carbon markets will likely require new approaches to purposefully add SOC to enhance existing carbon sinks.

The conversion of biomass to long-lived soil carbon species results in a long-term carbon sink, as the biomass removes atmospheric carbon dioxide through photosynthesis. Bio-char carbon species range in complexity from graphite-like carbon to high molecular weight aromatic rings, which are known to persist in soil for thousands to millions of years (Graetz and Skjemstad, 2003). Unlike fossil fuels, biomass is a renewable source of carbon and using it to produce bio-char can release energy with virtually no sulphur or mercury and very little nitrogen and ash waste (Antal and Gronli, 2003). Thus, producing bio-char from farm wood-waste appears to be one promising method of achieving greater levels of certainty and flexibility for integrating carbon sequestration accounting and renewable energy generation into conventional agricultural production (Lehmann, 2007). However, there remain large uncertainties of the effects of how bio-char applications to soil affect the surrounding ecology, and the productivity of particular crops in specific soil types and climates. This paper aims to reduce investment uncertainty for agriculturalists looking to diversify into converting biomass to bio-char and energy, with a special focus on experiences in Western Australia.