A carbon arrestor process: a novel approach for the production of functionalised biochar

The growing global population has given rise to two key issues, management of greenhouse gas emissions and increased demand for food production. These two issues are linked through the global carbon cycle and aspects of these issues can be addressed through improvements to global agronomic practices. Current agronomic practices have resulted in a depletion of soil organic carbon and nutrients, with these components removed from soil during crop harvest. Reintroduction of these nutrients and carbon to the soil is imperative in improving soil structure and fertility and assisting in the improvement of global food production prospects. One means to reintroduce carbon and nutrients and increase their residence time in the soil is by utilising biochar.

Biochar, a carbon-rich and stable soil amendment, is a char produced from the pyrolysis of biomass for specific use as a soil amendment. It has been shown to possess carbon sequestration benefits and improve soil quality and structure upon application. Its physical and chemical properties both contribute to its effectiveness as a soil amendment. Biochar’s high level and potentially diverse porosity provide habitat for microbial and root development in soil while also improving soil aeration. The organic matter and minerals present in biochar can improve soil fertility. This mineral composition contributes to an alkaline nature and pH buffering capabilities, making biochar effective in treating acidic soils and acting as a liming agent. The basic cations in biochar (i.e. from magnesium, calcium, potassium) and their associated carbonate and oxide functional groups contribute to the characteristic negatively charged biochar surface. This charged or polarised surface enables ion exchange with minerals in the soil and water, resulting in potentially improved water and nutrient retention and reduced leaching capabilities, particularly when combined with the large surface areas brought about by the high level of porosity.

Despite these benefits of biochar, biochar production has not been widely accepted on a commercial level. Its lack of use in agricultural practice can primarily be attributed to an energy intensive and expensive production process, competing uses for the biomass used to produce the biochar and costs related to the application of biochar to soil. By addressing these barriers, processes and technologies for the commercial use of biochar may be developed. The development of a novel carbon arrestor process has been proposed for this purpose. A process based upon slow pyrolysis, utilising waste biomass and mineral additives, to produce a functionalised biochar.

A functionalised biochar is one in which the conventional attributes of biochar are enhanced or added to, producing an improved agronomic product. The production of a functionalised biochar through utilising mineral additives in the biochar production process is one potential pathway to producing an improved, commercial biochar product. Minerals rich in alkali and alkaline earth metals have been identified as potentially suitable additives as they may capture carbon dioxide during biochar production, have catalytic properties to produce cleaner gaseous products of higher calorific value, improve the energy efficiency of biochar production and offer both fertilising and soil acidity management properties into one soil amendment product.

A review of mineral additives used in biomass thermal conversion processes identified calcium rich minerals, specifically lime (CaO) and limestone (CaCO₃), as suitable for use in the proposed carbon arrestor process. Two waste biomass sources, sawdust and crop residue, were chosen as feedstocks for biochar production. Eucalyptus pilularis, or blackbutt, was used as the sawdust biomass, while the straw of Triticum aestivum, or wheat straw, was used as the crop residue. A slow pyrolysis process, utilising heating rates of 5, 10 and 20 °C/min and maximum heating temperatures of 400 – 500 °C, was investigated for the biochar production. Production and characterisation of biochar from biomass and CaO mixtures of 1:0, 4:1, 2:1 and 1:1 (w/w) respectively, was undertaken. As part of this characterisation, the gaseous products from the carbon arrestor process were also analysed. Specific characterisation methods employed included physical investigations of biochar via SEM-EDS, nitrogen adsorption for porosity and surface area as well as chemical investigations via solid-FTIR and reaction analysis through TGA-FTIR, micro gas chromatography and mass spectroscopy.

The yield of blackbutt derived biochar was lower than wheat derived biochar, at 18% and 30% respectively. The higher yield from wheat biochar was partly attributed to the higher ash content of the raw biomass at 10.9%, compared to 1.6% for blackbutt from proximate analysis.

With the introduction of CaO to pyrolysis, biochar yields for blackbutt fell to 14%, on a normalised basis, while wheat biochar yields increased to 36%, for CaO to biomass ratios of 1:1. TGA analysis demonstrated pyrolysis of blackbutt biomass was characterised by two main devolatilisation stages, being representative of hemicellulose and cellulose decomposition in the ranges of 200 to 288 °C and 288 to 338 °C respectively. Wheat straw pyrolysis presented only one major devolatilisation zone, representative of cellulose decomposition between 200 and 313 °C. The addition of CaO to pyrolysis resulted in a slight increase in peak devolatilisation temperatures, in addition to less mass loss being observed in the initial stage/s of devolatilisation as CaO concentration increased. With CaO present, an additional devolatilisation stage was apparent from 368 °C and 375 °C for wheat and blackbutt, respectively. This additional reaction stage was attributed to CaO catalysing secondary tar formation reactions to produce lighter gases, and decomposition of any Ca(OH)₂ formed during pyrolysis to CaO. Solid state FTIR confirmed that the presence of CaO in the pyrolysis process with wheat and at temperatures greater than 450 °C resulted in a reduction in oxygenated functional groups on the surface of biochar, which has the potential to improve char stability and longevity in the soil. Ultimate analysis of the biochars demonstrated a reduction in oxygenated compounds with the introduction of CaO in pyrolysis for blackbutt, while favouring higher carbon content in char. The addition of CaO in the pyrolysis process also improved product gas composition, favouring H₂ and CH₄ formation with a significant reduction in CO₂ production, as detected through micro GC analysis. The addition of CaO to the pyrolysis process resulted in chars of similar surface area even though biochar produced from the biomass alone had significantly different values at 201 m²/g and 6.7 m²/g for blackbutt and wheat respectively. Blackbutt biochar with 1:1 biomass to CaO had a significantly reduced surface area of 37 m²/g. This was the result of swelling and melting on the surface of the chars and re-solidification of tars blocking the pores. These results are indicative of a biochar structure with a variety of pore sizes, with potential to improve water and nutrient retention when applied to soil.

Biochars produced with CaO in situ to the pyrolysis process were impacted by carbonation and hydration reactions and the catalytic effects of CaO. This resulted in improved product gas composition as well as favourable properties for soil application, with lower levels of oxygenated compounds and improved porosity. It was determined that optimal biochar production occurred with a heating rate of 10 °C/min to a heating temperature of 500 °C, allowing for complete devolatilisation and secondary reactions and pore development.

Kinetic and thermodynamic evaluation based on the physical and chemical properties acquired during characterisation enabled the determination of a mass and energy balance on the pyrolysis process. It was concluded that the addition of CaO to biomass in pyrolysis acts as a catalyst, resulting in lower activation energies and enthalpies of reaction, translating into lower energy requirements for biochar production. The kinetic assessment of the pyrolysis process found that activation energy was reduced when CaO was introduced at a 1:1 w/w ratio. For wheat, this resulted in a 35% reduction in average activation energy from 226 kJ/mol to 146 kJ/mol while blackbutt reduced by 17% from 225 kJ/mol to 187 kJ/mol. The enthalpies of reaction determined for wheat and blackbutt were similar to those presented in the literature. The addition of CaO resulted in a reduction in the enthalpy of reaction for both wheat and blackbutt biochar to 141 kJ/mol and 181 kJ/mol respectively.

Drawing on the reaction enthalpies determined, a pyrolysis process was developed and a mass and energy balance performed, incorporating the drying and pre-processing of biomass, calcination of limestone and pyrolysis of biomass and CaO. The resulting biochar production process required a total energy input of 6.4 MJ/kg dry biomass to process 1 kg of dry wheat biomass with CaO on a mass ratio of 1:1. Comparatively, the heating requirement without CaO equated to less than a third of the energy needs, at 2.0 MJ/kg dry biomass. Mass balance data was used to perform an energy balance incorporating energy efficiencies where both syngas and bio-oil combustion were utilised to heat the calcination, drying and pyrolysis processes. This resulted in a net excess energy supply for wheat pyrolysis of 2.3 and 7.1 MJ/kg of biomass for a 1:1 CaO amended biochar and non-CaO amended biochar, respectively. By comparison, blackbutt pyrolysis would produce excess energy of 9.0 and 11.4 kJ/kg biomass, for 1:1 and 1:0 biochar respectively, reflecting the higher yield of syngas and bio-oil in blackbutt pyrolysis. Further energy efficiency could be achieved through utilising sensible heat from production through heat exchange, providing an estimated additional 3.6 and 1.4 MJ/kg biomass energy savings, for 1:1 and 1:0 respectively.

A short-term three-month incubation study was undertaken, utilising biochar made from wheat crop residue and CaO. This incubation study investigated the effect of biochar addition on soil mineralisation and priming. An estimation of the carbon residence time of biochar and other organic matter in the soil was also derived through a δ¹³C labelling method and exponential modelling approaches. It was concluded that the addition of CaO to wheat biomass in pyrolysis increased the carbon mineralisation rate of overall carbon in soil (with residue present in the soil) from 10% (± 0.0029) over 89 days for a non-CaO amended biochar, to 12.8% (± 0.0066) for a 4:1 CaO to biochar amended soil, and 13.8% (± 0.0077) for a 1:1 CaO to biochar amended soil. By comparison, the control soil (i.e. no biochar addition) mineralised at 12% (± 0.0069) and limestone applied to soil mineralised at 14.5 % (±0.0057) over the same period. When residue was not present in soil mineralisation rates reduced by 45 to 50%. Likewise, biochar carbon mineralisation increased with the addition of CaO in pyrolysis. Over the 89 day incubation, biochar mineralised between 0.9% (± 0.0006) and 21% (± 0.0199) of its total carbon pool, with CaO significantly diminishing the recalcitrance of biochar. Residue mineralisation was significantly higher at 56% (± 0.0172) to 68% (± 0.0257), also increasing with CaO addition. Limestone fully mineralised with residue present, and soil carbon was generally the most recalcitrant of all organic components in the soil, mineralising between 1.6% (± 0.0007) and 7% (± 0.0064). In terms of priming effects, wheat based biochar had a negative priming effect on soil and wheat residue. However, when adding CaO to the biochar, the biochar had a positive priming effect on soil and wheat residue over the first 60 days. This trended to a negative effect at day 89, in particular for the lower 4:1 CaO based biochar, suggesting a stabilising influence by the in situ CaO on soil organic matter over the long term.

The addition of lime to biochar highlighted a complex interaction of the lime with biochar and the soil. The carbonated CaCO₃ represents a labile component in the biochar, while the presence of CaO in pyrolysis encourages the removal of oxygenated compounds from the biochar, making the char more stable. The higher levels of labile matter decomposing over the short term resulted in much higher mineralisation rates, but over the longer term low-CaO dosed biochar, may be characterised with increased carbon stability. The short-term nature of the incubation study may bias residence time calculations to that of the labile matter, potentially resulting in an underestimation in residence time. These mineralisation rates converted to residence times, as MRT, for non-lime based biochar of 33 to 79 years for biochar carbon in residue-amended soil. The addition of CaO in pyrolysis, resulted in lower residence times of carbon in soil. For the 1:1 biochar, carbon was generally the lowest at 1.5 to 4.8 years, and an increase in MRT occurred for the lower dosed CaO, 4:1 biochar at 1.4 to 16.8 years. Using the MRT values and biochar yield data, the carbon sequestration benefit for non-CaO amended biochar in a residue-dosed soil was quantified as between 12.6% and 91% of carbon content in biochar remaining in the soil at the end of a 100-year life cycle using infinite Two Pool and Pool C methodologies respectively. For 4:1 CaO-dosed biochar the 100 year carbon levels were quantified between 0 and 20%. For higher 1:1 CaO-dosed biochars, carbon levels were lower again between 0 and 1% at 100 years.

A life cycle assessment was completed, where greenhouse gas (GHG) emissions and gross energy requirement were determined for the biochar lifecycle. The biochar lifecycle, from resource extraction to soil application, was assessed over a 100-year time-frame. Through substitution of fossils fuels with pyrolysis syngas and bio oil product, the GHG emissions of the biochar production process were minimised. This, in combination with the level of stable carbon in biochar, was sufficient to reduce net GHG emissions over the life cycle of the CaO amended biochar production and application to soil to below that of conventional lime treatment of agricultural soils for the 4:1 biomass to CaO based biochar. The overall global warming potential (GWP) of the 4:1 biochar was 741 and 729 kg CO₂-e/t dry biomass over the life cycle and the energy requirement was negative, representing a net excess energy of 7.0 and 10.9 GJ/t dry biomass for wheat and blackbutt sawdust respectively. The 4:1 CaO based biochar was determined to be a viable alternative to conventional liming practices used in agriculture for the application to soil with simultaneous tilling of crop residue, where the conventional base scenario produced a GWP of 845 kg CO₂-e/t dry biomass. Hence, the 4:1 biochar process produced a net GHG emission abatement of 107 kg CO₂-e/t dry biomass on average when compared to conventional practice.

Although the 4:1 biochar is a viable lime application alternative, the application of non-CaO amended 1:0 biochar with lime to soil was determined to have lower GHG emissions than the 4:1 biochar scenario at 594 kg CO₂-e/t dry biomass, producing the best outcome for GWP over its life cycle. This result was primarily attributed to the higher carbon sequestration benefit of the 1:0 biochar over the CaO amended biochar. However, on an energy demand basis, the 4:1 biochar presented on average 2.1% higher energy savings than the 1:0 biochar. On an overall life cycle assessment basis both the 4:1 biochar and 1:0 biochar with lime application to soil presented the best outcomes, outperforming the conventional agronomic practice of application of biomass and lime to soil. It is recommended that future investigation of the interaction of limestone with biochar in soil and assessing this against low CaO amended biochar be conducted over a longer term incubation study and field trials. This would refine and improve upon the findings of the current investigation and enable a further economic assessment, determining the cost on the CO₂ sequestered and an economic return based upon improved soil productivity.

The investigations presented in this thesis demonstrated that the carbonation and hydration reactions and the catalytic effects of CaO in pyrolysis impacted the quality of biochar, by improving product gas composition and forming a biochar with improved porosity and lower levels of oxygenated compounds. By producing biochar at 500 °C, under slow pyrolysis conditions, with low levels of CaO, sufficient carbon sequestration potential was retained in the biochar to validate its viability as an alternative to conventional agronomic lime treatment. These findings justify future investigation into the development of an integrated carbon arrestor process, producing low level CaO-amended biochar while utilising pyrolysis syngas and bio-oil products as a substitute to fossil-fuelled energy and for co-production of electricity. In addition to developing a product with carbon sequestration properties, the process benefits from utilising waste feedstock and producing excess energy while extending the functionality of biochar by adding liming attributes to its array of other benefits as a soil amendment.