Elsevier

Biomass and Bioenergy

Volume 72, January 2015, Pages 300-308
Biomass and Bioenergy

Gasification biochar as a valuable by-product for carbon sequestration and soil amendment

https://doi.org/10.1016/j.biombioe.2014.10.013Get rights and content

Highlights

  • Biomass gasification can combine efficient bioenergy production with valuable biochar residuals for soil improvements.

  • The two investigated gasification biochars are recalcitrant indicating soil carbon sequestration potential.

  • Gasification biochars are potential soil improvers due to high specific surface area, liming effect and low PAH content.

Abstract

Thermal gasification of various biomass residues is a promising technology for combining bioenergy production with soil fertility management through the application of the resulting biochar as soil amendment. In this study, we investigated gasification biochar (GB) materials originating from two major global biomass fuels: straw gasification biochar (SGB) and wood gasification biochar (WGB), produced by a Low Temperature Circulating Fluidized Bed gasifier (LT-CFB) and a TwoStage gasifier, respectively, optimized for energy conversion. Stability of carbon in GB against microbial degradation was assessed in a short-term soil incubation study and compared to the traditional practice of direct incorporation of cereal straw. The GBs were chemically and physically characterized to evaluate their potential to improve soil quality parameters. After 110 days of incubation, about 3% of the added GB carbon was respired as CO2, compared to 80% of the straw carbon added. The stability of GB was also confirmed by low H/C and O/C atomic ratios with lowest values for WGB (H/C 0.12 and O/C 0.10). The soil application of GBs exhibited a liming effect increasing the soil pH from ca 8 to 9. Results from scanning electron microscopy and BET analyses showed high porosity and specific surface area of both GBs, indicating a high potential to increase important soil quality parameters such as soil structure, nutrient and water retention, especially for WGB. These results seem promising regarding the possibility to combine an efficient bioenergy production with various soil aspects such as carbon sequestration and soil quality improvements.

Introduction

Biomass gasification for combined heat and power (CHP) production has the potential to become an efficient and flexible way to generate bioenergy, as a broad variety of biomass residues and other organic resources can be utilized [1], [2]. In Denmark, effective gasification platforms for the two major global biomass fuels, wood chips and cereal straw, are currently scaled up and close to commercial application: (1) Low Temperature Circulating Fluidized Bed gasifier (LT-CFB), specifically designed to produce energy from biomasses with high ash contents (such as straw) and (2) TwoStage gasifier, designed for converting woody biomass. The LT-CFB technology has been demonstrated in continuous operation, as a 6 MW demonstration plant, and the first 2 MW commercial plant for continues CHP production with the TwoStage process is about to produce power and district heating for a local community, Hilleroed Municipality, Denmark. This plant will produce approximately 64 tons of biochar residues annually, while the planned 60 MW full scale commercial LT-CFB plant is going to generate approximately 10 000 tons of carbon-rich residues per year. The potential further upscaling and expanding of those processes requires a strategy for the sustainable utilization of a growing amount of biochar residues produced. Recirculation and utilization of those residues to agricultural land, instead of costly disposing as a waste, would improve the sustainability and economy of the bioenergy production. Gasification biochar generally contains a considerable amount of minerals and recalcitrant carbon and is considered an attractive product for soil amendment due to its fertilizer and carbon sequestration potential [3], [4].

Carbon sequestration in soil mitigates the effect of climate change [5], and may furthermore help to maintain or even improve the soil fertility. This is of key importance to be able to fulfill the increasing global demand for producing crops for both food and energy [6]. Soil organic carbon (SOC) influences the physical, chemical and biological properties of the soil, and is essential for good soil quality [7]. Increasing SOC has been shown to improve soil aggregation, water infiltration, and water and nutrient retention [8], [9]. Traditional annual incorporation of crop residues such as cereal straw can increase soil organic matter content [10], therefore there is a concern that the removal of residues from the field for energy production may lead to soil degradation [11]. Gasification of biomass and returning the residual biochar-carbon to the field is regarded as a promising strategy combining effective bioenergy generation with the maintenance of soil carbon stocks [2]. Utilizing low quality wood and residues from timber harvesting for bioenergy production and subsequent addition of wood biochar to agricultural soils may be another strategy to increase SOC and improve arable soils' productivity, creating novel synergies between the agricultural and forestry sectors. Nevertheless, since there are qualitative differences in the molecular structure of pyrogenic carbon compared to the stable carbon derived from microbial/enzymatic soil processes [12], the impacts of substituting crop residue incorporation with the addition of gasification biochar (GB) on soil services are largely unknown and should be thoroughly investigated before implementing this into practice [8].

Several studies have shown positive impacts of pyrolysis biochar, produced at relatively low temperatures (400–600 °C), on soil properties [13], [14], which are, however, highly dependent on biochar feedstock and thermal processing conditions [15]. The physical properties of biochars, such as high porosity and specific surface area, may result in an increase of not only soil water retention [16], water infiltration, and cation exchange capacity [5], [13], but also soil microbial activity [14]. Chemical properties, such as low hydrogen-to-carbon (H/C) and oxygen-to-carbon (O/C) ratios, result in high stability of biochar against microbial degradation in soil [17]. Compared to pyrolysis biochar, GB is produced at higher temperatures (around 700–1100 °C), using low amounts of oxygen. Gasification results in higher energy yields compared to pyrolysis and leaves biochar with less, but more stable carbon, compared to pyrolysis biochar [15], [18]. Chemical characterization of GB, showing its stable structure, is well reported [4], [15], [17], [19], however studies on the effect of GB on soil and microbial processes are scarce. Concerns about the use of GB as a soil amendment include its possible content of Polycyclic Aromatic Hydrocarbons (PAH) [20], which proved to be highly variable, as e.g. in the studies of Wiedner [4] and Kloss [20], who measured values up to 15 and 33 mg kg−1, respectively. Especially the wood gasification biochars showed high PAH contents [4], [17].

The aim of this study was to evaluate the potential of the biochar residues from two gasification processes to exert a beneficial effect on soil carbon sequestration and soil quality. Through a short-term soil incubation study and physical and chemical analyses, the objectives were to investigate if the gasification biochars: (1) contain carbon recalcitrant to microbial degradation; (2) have a potential to improve soil physical and chemical properties; (3) have any negative effects on microbial biomass and (4) have a potential for higher carbon sequestration rates than those achieved with traditional direct soil incorporation of the feedstock (i.e. straw).

Section snippets

Biochar production

The two gasification biochars (GB) used for this study originated from continuously operated pre-commercial gasification demonstration plants. Straw gasification biochar (SGB) was produced in a Low Temperature Circulating Fluidized Bed gasifier (LT-CFB). The straw originated from winter wheat (Triticum aestivum L.) grown in Zealand, Denmark, but is of unknown provenance, date of harvest and chain of custody. Commercially produced wheat straw pellets were crushed prior to LT-CFB gasification for

Biochar characterization

Table 1 illustrates that 4 and 10% of the carbon in wood and straw feedstock, respectively, were retained in the biochar fraction. The chemical characterization of soil, feedstock and biochars is given in Table 2. Gasification of straw and wood chips led to mass loss of H and O, decrease of H/C and O/C atomic ratios and increase of ash percentage. The carbon content was higher, while H/C and O/C ratios were lower for WGB compared to SGB. The total content of 9 PAHs was 5 mg kg−1 in SGB and

Soil carbon sequestration potential

A markedly smaller proportion of added carbon was respired in the GB treatments compared to the straw treatments, which reflects the aromatic and recalcitrant structure of the residual carbon in these biochar materials [4] after energy production during the process of gasification (Fig. 7C). The addition of the high dosage of WGB resulted even in an initially negative CO2 flux, probably caused by binding CO2 through carbonation of soluble Ca and Mg contained in the biochar, forming CaCO3 and

Conclusion

In this study, we suggest that thermal gasification of biomass residues is able to combine the production of bioenergy and a biochar fraction that can exert a positive impact on soil quality. Our results showed that gasification biochar (GB) carbon is more resistant to microbial degradation compared to straw carbon and has a potential for soil carbon sequestration. Furthermore, the GBs in our study exhibited a potential as soil improving agents due to their high specific surface area, porosity

Acknowledgments

The financial support for this research was provided by the VILLUM Foundation VKR022521. We are grateful to DONG Energy for providing us with the biochar samples. We thank Henrik Spliid for help with statistical analysis, Mette Flodgaard and Anja Nielsen for excellent technical assistance, Jakob Munkholt Christensen for help with BET analysis, Rolf Jensen for help with SEM and Esben W. Bruun for practical advice concerning LICOR measurements.

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