Scholarly article on topic 'Quantifying the Greenhouse Gas Reduction Benefits of Utilising Straw Biochar and Enriched Biochar'

Quantifying the Greenhouse Gas Reduction Benefits of Utilising Straw Biochar and Enriched Biochar Academic research paper on "Earth and related environmental sciences"

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Abstract of research paper on Earth and related environmental sciences, author of scientific article — Ali Mohammadi, Annette Cowie, Thi Lan Anh Mai, Ruy Anaya de la Rosa, Miguel Brandão, et al.

Abstract This study investigated the carbon footprint of two different biochar production systems for application to paddy fields. The impacts of using rice straw-derived biochar in raw form (System A) were compared with those arising from using rice straw biochar enriched with lime, clay, ash and manure (System B). The GHG abatement of the management of one Mg of rice straw in Systems A and B was estimated at 0.27 and 0.61 Mg CO2-eq, respectively, in spring season, and 0.30 and 1.22 Mg CO2-eq in summer. The difference is mainly due to greater reduction of soil CH4 emissions by enriched biochar.

Academic research paper on topic "Quantifying the Greenhouse Gas Reduction Benefits of Utilising Straw Biochar and Enriched Biochar"

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Energy Procedía 97 (2016) 254 - 261

European Geosciences Union General Assembly 2016, EGU Division Energy, Resources & Environment, ERE

Quantifying the greenhouse gas reduction benefits of utilising straw

biochar and enriched biochar

Ali Mohammade Annette Cowieab, Thi Lan Anh Maic, Ruy Anaya de la Rosad, Miguel Brandaoe, Paul Kristiansena, Stephen Josephafg

a School of Environmental and Rural Science, University of New England, Armidale, NSW 2351, Australia b NSW Department of Primary Industries, Beef Industry Centre, Trevenna Rd., Armidale, NSW 2351, Australia c Thai Nguyen University of Sciences, Thai Nguyen University, Thai Nguyen Province, Viet Nam d Starfish Initiatives, Armidale, NSW 2350, Australia e Department of Sustainable Development, Environmental Science and Engineering, KTH - Royal Institute of Technology, Stockholm, Sweden f Discipline of Chemistry, University of Newcastle, Callaghan, NSW 2308, Australia g School of Materials Science and Engineering, University of New South Wales, Sydney, NSW 2052, Australia

Abstract

This study investigated the carbon footprint of two different biochar production systems for application to paddy fields. The impacts of using rice straw-derived biochar in raw form (System A) were compared with those arising from using rice straw biochar enriched with lime, clay, ash and manure (System B). The GHG abatement of the management of one Mg of rice straw in Systems A and B was estimated at 0.27 and 0.61 Mg CÜ2-eq, respectively, in spring season, and 0.30 and 1.22 Mg CÜ2-eq in summer. The difference is mainly due to greater reduction of soil CH4 emissions by enriched biochar.

© 2016 The Authors.Publishedby ElsevierLtd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of the General Assembly of the European Geosciences Union (EGU) Keywords: Life Cycle Assessment; Biomass open burning; Enriched biochar; Methane emissions; Vietnam.

* Corresponding author. Tel.: +61 2 6773 2223; fax: +61 2 6773 3238. E-mail address: Ali.Mohammadi@une.edu.au, Mohammadia2011@gmail.com.

1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of the organizing committee of the General Assembly of the European Geosciences Union (EGU) doi: 10.1016/j.egypro.2016.10.069

1. Introduction

Unused agricultural residues (such as rice straw and rice husk) account for about 2.46 Gt C/yr, which represents about one fourth of the global CO2 emissions from fossil fuels [1]. Currently unused residues are either burnt in the fields or left in situ, returning most of the carbon to the atmosphere. Vietnam is one of the largest rice-exporting countries, and therefore a significant amount of rice straw is produced annually. In Vietnam, field burning of agriculture residues is a common way to manage residues after harvesting the crop. Biomass burning in the field releases a large amount of pollutants to the atmosphere which causes serious local and regional environmental impacts [2], and converts significant quantities of nutrients to gaseous form, which are then lost from the site [3].

Over the past few years, application of biochar, a solid product of thermal decomposition of biomass, has been recommended as a soil amendment in agriculture [4]. Biochar can enhance soil carbon stock and nitrogen retention as well as improving soil functions [5]. Numerous field studies with biochar amendment have shown high potential in decreasing GHG emissions and improving crop yield in paddy fields [6, 7]. Sui et al. [8] reported a significant decrease in CH4 emissions up to 87% following rice straw-derived biochar amendment in Chinese paddy soils.

Most studies have used high rates (5-48 Mg ha-1) [7, 9] straw biochar in rice cropping systems, and Mohammadi et al. [10] considered that the maximum agronomic benefits occur at rates over 18 Mg ha-1. However, some research has suggested that biochar can be beneficial at lower rates if treated with minerals [11]. The high application rates of biochar increase farm expenses. To overcome this constraint, the development of enriched biochar, having higher mineral content, surface functionality, exchangeable cations, and higher water-extractable organic compounds has been proposed [11-13]. Chia et al. [13] characterized a woody biochar enriched with manures, minerals and clays. Their chemical analyses of the enriched biochar revealed that it has high concentrations of exchangeable cations, available phosphorus and high acid neutralizing capacity. The clay, minerals and manure were incorporated into the biochar structure and a higher concentration of dissolved organic carbon was obtained in soil amended by enriched biochar relative to unamended soil [14]. Joseph et al. [15] found that an acacia wood-derived biochar mixed with clay, chicken litter, and minerals improved growth of wheat at a low application rate (100 kg ha-1). Sarkhot et al. [12] reported that enrichment of woody biochar with dairy manure effluent can promote carbon and nitrogen storage in soil and mitigate soil GHG emissions.

Life Cycle Assessment (LCA) has been applied to investigate the carbon footprint (CF; net GHG emissions) of biochar production systems from a perspective of various feedstocks [16-18], crop production [10] or land treated [19] as functional unit. These studies are limited to use of raw biochar and did not assess application of enhanced biochar where this amendment may have different impacts on paddy soils.

This study performed LCA to evaluate the CF of two different biochar production systems, using rice straw biochar and enriched rice straw biochar produced by a slow pyrolysis technology for application to paddy soils. Open burning of rice straw was assumed as the baseline scenario.

2. Materials and methods

2.1. Scope and the functional unit of the assessment

This study calculated the CF of rice straw management where biochar made from straw is converted into biochar and applied to soil ("straw biochar", System A) and where biochar is made from straw mixed with lime, clay, ash, manure and wood and applied to soil ("enriched biochar", System B). The functional unit (FU) is the management of 1 Mg of dry rice straw. The climate change impacts of System A and System B were investigated in spring and summer rice cropping seasons, and the greenhouse gas mitigation potential through use of straw biochar and enriched biochar is discussed.

2.2. System boundary

The system boundary of Systems A and B are shown in Fig. 1. Biomass open burning, the most common practice for disposal of straw in North Vietnam, was considered as the baseline scenario for the two systems and included as an avoided process in each biochar system.

System A: Straw biochar

In this system at the time of rice harvest, straw is removed, cut in small pieces, and pyrolysed with some wood in a top-lit updraft (TLUD) drum oven for biochar production. The straw biochar produced was assumed to be returned into the rice paddy fields where the biomass originated (Fig. 1A).

System B: Enriched biochar

In this system rice straw is collected from fields, cut in small pieces and mixed with clay, lime, wood, manure and ash in the TLUD drum oven to produce an enriched biochar (Fig. 1B). Around 30% of rice straw was open burnt in order to generate the ash, and clay was assumed to be collected from the paddy soils where the enriched biochar is returned. Lime in the enriched biochar was considered to behave in the same way as lime application alone. It is common practice that households in the research area use wood, in traditional three-stone cook-stoves (3S-stoves), and liquefied petroleum gas (LPG) stoves as their primary devices for preparing food. Therefore, the use of wood in the drum oven was modelled as an additional LPG need considering that if the wood was not used for biochar production it would have been used for cooking in 3S-stove.

2.3. Inventory analysis

The life cycle inventory to estimate the climate change effect of the systems was developed using SimaPro 8.0.1 and Ecoinvent libraries. The characterization factors of methane (CH4), and nitrous oxide (N2O) are 28 and 265 respectively, according to the Global Warming Potential for 100 year time horizon, reported by the IPCC Fifth Assessment Report [20]. Carbon dioxide uptake and emissions from plant growth and combustion/pyrolysis were excluded from the inventory in accordance with the IPCC guidelines for greenhouse gas inventory related to annual crops [21]. Methane and nitrous oxide emissions from straw combustion and pyrolysis were included. The fraction of the carbon remaining in the ash was assumed to be sequestered in the soil as was the recalcitrant portion of the carbon in the biochar. The total ash loss during open burning and soil incorporation was assumed to be 20%.

The TLUD drum oven assumed to produce the biochar in both systems can be filled with biomass such as wood, crop residues and livestock manure, and is made from a single barrel with holes in the bottom and top of the drum to mix with the pyrolysis gases and facilitate their combustion. A chimney attached at the top of the drum provides the necessary draft to ensure efficient combustion of the pyrolysis gases. The yield of biochar in the drum oven was assumed to be 25% of the mass of dry biomass input.

A carbon content of 51% in the straw biochar was assumed [6], of which 69% was assumed to remain as stable carbon in the soil after 100 years [5]. The effects of straw biochar on soil properties and crop productivity are reviewed in detail by [10]. According to [10], five assumptions used in this paper for 18 Mg ha-1 straw biochar applications to paddy fields are: 1) 10% ± 6 increase in crop yield; 2) 20% ± 60 suppression of soil CH4 emissions; 3) 30% ± 40 suppression of soil N2O emissions; 4) 30% ± 30 decrease in N fertiliser use, and 5) 50% ± 50 decrease in P and K fertiliser use. These assumptions are based on studies where straw and bamboo biochar was applied to grow paddy rice, and uncertainties for these values were incorporated in both uncertainty and sensitivity analyses.

Data from an unpublished experiment on enriched biochar were used to model System B. The experiment was conducted in a paddy field from February 2013 to September 2014 in Thai Nguyen, Vietnam. The treatment plots were 5m x 6m in area and arranged in a randomized complete block design with three replicates. The enriched biochar was added at a rate of 500 kg ha-1. A control treatment without organic matter incorporation was also carried out. Control plots received mineral fertilizer at 100 kg N, 90 kg P2O5 and 60 kg K2O per ha in spring and 80 kg N, 90 kg P2O5 and 60 kg K2O per ha in summer. The biochar treatment received 50% of the amounts of mineral fertiliser. After transplanting rice seedlings, gas samples (CH4 and N2O) were collected at four growth stages using the closed chamber technique in spring and summer 2014. The gas analysis revealed that CH4 emissions were 125% greater in summer compared with spring in the control treatment. In the experiment, the enriched biochar was applied in each cropping season in 2013 and 2014. Emissions were measured in the second year, after 1.5 Mg enriched biochar had been applied in spring and 2 Mg in summer. System B was modelled assuming that the aged biochar, added to soil in the first year, behaved same as the new biochar in the second year. Table 1 shows the assumptions and inventory calculations of GHG emissions from biochar production in System A and System B.

Pyrolysis in TLUD drum oven

Rice straw

____Y_____

Üpen burning -----T-----

Straw biochar

Cooking LPG

Fertilisation

Mineral fertilisers

Rice straw Ash

Pyrolysis in TLUD drum oven 1

~f-r~r

Manure Lime Clay

Enriched biochar _i_

Open burning i___________

Burning Cooking

Fertilisation

N fertiliser

Fertilisation

Mineral fertilisers

Fig. 1. Life cycle diagrams for System A (straw biochar) and System B (enriched biochar). The dashed line represents an avoided process.

3. Results and discussion

The results showed that biochar production from 1 Mg of rice straw, applied to paddy fields significantly mitigated the climate change effect of straw management relative to open burning. The straw biochar and enriched biochar application did not change significantly the yield of rice in either spring or summer growing seasons.

3.1. Carbon footprint of System A

The LCA results demonstrated that emissions abatement (avoided GHG emissions and C sequestered) are greater than emissions in System A (straw biochar), resulting in a negative value for the CF. The net emissions abatement of this system was estimated to be 0.27 and 0.30 Mg CÜ2-eq per 1 Mg rice straw in spring and summer, respectively (Fig. 2). This is a value near to the lower end of published LCA studies on biochar, which report negative GHG emissions of 0.3-1.25 Mg CÜ2-eq per Mg of dry biomass feedstock [18, 22, 23]. The emissions abatement of System A was also lower than a Chinese study where straw of wheat and maize was pyrolysed and applied to soil instead of being burnt in the fields, gave GHG emissions reduction of 1.06 Mg CÜ2-eq per Mg of dry straw [16]. In that Chinese study, the biochar yield and carbon stabilised in the biochar were assumed to be 30% and 80%, respectively, which are higher than the assumptions in the current study. Those authors also assumed that the biochar was produced in a large industrial facility that captures and burns gases to generate electricity, further decreasing GHG emissions due to the assumed displacement of fossil fuels. In contrast, in this study, biochar was produced in a drum oven where the pyrolysis emissions are directly released to the atmosphere. This demonstrates that the importance of pyrolysis gas recycling for sustainable biochar production should not be neglected.

The contribution of each life cycle process to the CF is illustrated in Fig. 3. The carbon stabilised in the biochar is the main contributor to the climate change effect, accounting for 65% of abatement in spring and 62% in summer. GHG emissions associated with the pyrolysis stage is the second largest contributor (22%) to the net emissions from straw management. Approximately half of the C fixed in the biomass during growth is emitted into the air during the pyrolysis process, while the other half is retained in the biochar [24]. The suppression of soil CH4 emissions is another factor that influences the CF of System A, with a negative contribution of 3% in spring and 8% in summer.

Table 1. Inventory for Systems A (straw biochar) and B (enriched biochar), expressed per 1 Mg of dry rice straw.

Unit Amount Comments/Source

System A

Rice straw kg 950 5% lost during straw removal and transport

Wood kg 143 790 MJ LPG requirement

Biochar kg 262 1% and 3% losses during transport and soil incorporation

CH4 from pyrolysis kg 4.26 [25]

Soil CH4 reduction in spring (and summer) kg 0.6 (1.3) Estimated from [10]

Soil N2O reduction in spring (and summer) g 0.02 (0.06) Estimated from [10]

System B

Rice straw kg 650

Rice straw used for ash generation kg 300 Ash is added to biochar

Wood kg 95.6 530 MJ LPG requirement

Lime kg 19.1

Ash kg 47.8

Manure kg 47.8

Clay kg 95.6

Enriched biochar kg 346 1% and 3% losses during transport and soil incorporation

CH4 from pyrolysis kg 3.1 [25]

Soil CH4 reduction in spring (and summer) kg 12.9 (46.7) Based on field measurements

Soil N2O reduction in spring (and summer) g 0.2 (1.7) Based on field measurements

CO2 from land application of lime kg 3.2 Estimated from [21]

Avoided straw open burning

CH4 kg 2.1 Emission factors from [21]

N2O g 54 Emission factors from [21]

Recalcitrant carbon from ash back to soil kg 14.4 Estimated from [26]

3.2. Carbon footprint of System B

The GHG abatement of straw management in System B was calculated to be 0.61 and 1.22 Mg CO2-eq. per 1 Mg rice straw in spring and summer, respectively (Fig. 2). The difference is due to the assumed greater suppression of CH4 emissions from soil in summer than in spring. Enriched biochar increased the abatement by over 120% and 300% in spring and summer seasons, compared with straw biochar. This is because of the larger area treated with enriched biochar, due to the lower application rate. The suppression of CH4 emissions from soil is the major contributor to the decrease in GHG emissions with a reduction of 44% in spring and 69% in summer (Fig. 3). This is followed by the carbon stabilised in biochar, contributing 30% and 17% to the abatement in spring and summer.

Several studies have discussed the effect of biochar amendment on CH4 emissions from paddy rice. CH4 is the main GHG emitted from paddy fields [27-29]. Suppression of CH4 emissions from paddy soils with biochar addition were most likely due to decreased methanogenic activity because of increased soil aeration and soil pH [7]. Soil aeration may be enhanced further through enriched biochar addition than straw biochar. Chia et al. [13] investigated the characterisation of a wood-derived biochar enriched with clay, minerals and chicken manure, and the results showed a range of pore sizes at the interface of the organic and inorganic phases which probably occurred during the heating phase when the manure started to break down during torrefaction. These additional pores enhance the overall soil surface area when enriched biochar is applied as a soil amendment, which will improve microbial activity and population, promote water holding capacity in sandy soil or increase aeration in clayey soil.

0.0 -0.3 -0.6 -0.9 -1.2 -1.5 -1.8

Spring Summer System A

Spring Summer System B

40% 20% 0% -20% -40% -60% -80% -100%

1 ¿¡*1 :::::::: :■:::■:. :::::::::::::::::

□ Ira

Spring Summer System A

Spring Summer System B

□ 3S-stove displaced

■ Open burning

■ Biochar production

r Fertiliser displaced ^ Carbon stabilised « Net soil emissions

Fig 2. Carbon footprint (CF) of management of 1 Mg rice straw in System A and System B. Error bars correspond to 95% confidence interval for Monte Carlo simulations.

Fig. 3. Contribution of life cycle processes to the carbon footprint of management of 1 Mg rice straw in System A (straw biochar) and System B (enriched biochar).

Application of enriched biochar has been shown to reduce mineral fertiliser requirements [30] due to the capacity of biochar to improve fertiliser-use-efficiency through retaining nutrient and enhancing nutrient uptake by plants. This was confirmed in the experiment used in this study where there was no significant change in yield with applying 50% less fertiliser in the enriched biochar treatments relative to the control. Joseph et al. [15] reported that enriched biochar increased growth and shoot nutrient acquisition of wheat plants when 100 kg ha-1 enriched biochar was applied. That outcome was attributed to improved phosphorus and nitrogen uptake of wheat plants possibly due to the positive effects of enriched biochar on soil biology, especially enhanced mycorrhizal symbioses. Sarkhot et al. [12] also found that amendment with raw biochar derived from wood and a woody biochar enriched by dairy manure effluent compared with unamended soil resulted in 68% and 75% reduction in net nitrification, and 221% and 229% reduction in net ammonification. In the present study, considering the soil effects of enriched biochar application on fertiliser use reduction and avoided soil N2O emissions led to the estimate of carbon abatement of 74 and 65 kg CO2-eq per FU in spring and summer, respectively. Therefore, if enriched biochar can aid the reduction of fertiliser application in Vietnam, the resulting GHG mitigation potential through saved fertiliser would be large.

3.3. Sensitivity analysis

A sensitivity analysis was conducted on Systems A and B for two rice cropping seasons (Table 2). The baseline figures were changed up and down by the same proportion, so the variation to the CF for minimum and maximum values is almost equal.

The biochar yield in the drum oven was varied from 15% to 35% with a baseline figure of 25%. When 15% of the biomass feedstock was assumed to be converted into biochar, the CF of System A decreased by an average of 36% in both seasons. The climate effect of System B was slightly less sensitive to this factor. The CH4 emissions from pyrolysis influenced the CF of System A by 33% and System B by 11% in spring. The difference between the two systems is due to the large contribution of pyrolysis emissions in the net emissions of System A. The stable carbon content in biochar was changed from 50% to 90%, with an average value of 69%. When 50% of the carbon was recalcitrant, reduction in abatement was 38-41% in System A and 6-12% in System B. System B was highly sensitive to changes in the suppression of soil CH4 emissions. The results of the experiment used in this study showed that enriched biochar amendment decreased CH4 emissions from paddy soil by approximately 10% in both seasons. Enhancing the rate of suppression of soil CH4 emissions from 10 to 20% resulted in a 59% and 81% reduction in CF of System B in spring and summer, respectively. The difference between these values was caused by the higher CH4 emissions in summer.

Table 2. Sensitivity analysis of important parameters on the carbon footprints of the management of 1 Mg of rice straw in System A (straw biochar) and System B (enriched biochar).

Parameters Baseline Min. Max. % change Spring Sun

System A

Biochar yield (%) in drum oven 25 15 35 35 37

CH4 emission from drum oven (kg Mg-1 feedstock) 3.9 2 8 33 30

Carbon content of biochar (%) 51 40 60 25 22

Stable carbon content of biochar (%) 69 50 90 41 38

Suppression of soil CH4 emissions (%) 20 -40a 80 22 34

Decrease in N fertiliser requirement (%) 30 0 60 0 0

Decrease in P and K fertiliser requirements (%) 50 0 100 0 0

System B

Biochar yield (%) in drum oven 25 15 35 27 24

CH4 emission from drum oven (kg Mg-1 feedstock) 3.9 2 8 11 5

Carbon content of biochar (%) 51 40 60 9 5

Stable carbon content of biochar (%) 69 50 90 12 6

Suppression of soil CH4 emissions (%) 10 0 20 59 81

Decrease in N fertiliser requirement (%) 50 0 50 8 4

Decrease in P and K fertiliser requirements (%) 50 0 50 5 3

a Negative value indicates that CH4 emissions increased with the straw biochar addition.

4. Conclusions

This study demonstrated that using rice straw for biochar production and soil amendment led to a lower climate change impact than open burning of biomass in spring and summer rice cropping seasons in Vietnam. The carbon footprint of both biochar systems had a negative value as the GHG abatement were higher than GHG emissions. System B (enriched biochar where lime, clay, ash and manure were added to rice straw before pyrolysis) showed an increase in abatement by 126% and 309% in spring and summer, respectively compared with System A in which un-enriched straw biochar was applied to the fields. That abatement was mostly due to greater reduction in soil CH4 emissions in System B because the larger area treated with enriched biochar, due to the lower application rate. The conclusions in this paper are based on several uncertain assumptions from the literature including emission factors from drum ovens and the effect of biochar on soil CH4 emissions. Therefore, the processes need further empirical testing and systems modelling before widespread adoption of straw biochar and enriched biochar can be expected.

Acknowledgements

This research was funded by Rural Climate Solutions and the School of Environmental and Rural Sciences, University of New England. The Biochar for Sustainable Soils (B4SS) project, funded by the Global Environment Facility, allowed Ruy Anaya de la Rosa to contribute to this study.

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