Under what Circumstances is Biochar a Sustainable Use for Rice Residues Compared to Conventional and Alternative Uses?

Abstract

<p>Vietnam is one of the largest rice-exporting countries, and therefore a large amount of rice husk and rice straw is produced annually. To manage residues after harvesting, Vietnamese farmers commonly burn rice residues in the field which emits large quantities of gaseous and particulate pollution to the atmosphere and has a negative impact on the climate and the health of the population. In the last decade, using biomass to make biochar for application to cropland has received growing attention as a possible strategy to mitigate climate change by sequestering carbon from the atmosphere and suppressing soil greenhouse gas (GHG) emissions. Biochar can be produced from various biomass sources including crop residues, and at various scales from large industrial facilities, village scale and even at the household level using small-scale pyrolysis technologies.</p> <p>This thesis investigated the climate change, human health (particulate matter and human toxicity) and economic impacts of biochar production from rice residues for addition to paddy soils compared with the conventional practice of open burning of residues. Life Cycle Assessment (LCA), a methodology that aims to assess impacts of products, processes, or services from “cradle to grave”, was employed to evaluate the environmental and health effects of alternative uses of biomass in rice growing systems in northern Vietnam. For this purpose, different studies related to crop residue management were defined and, for each study, the biochar system and a comparison reference system were modelled. The biochar produced in all studies was assumed to be returned to paddy rice fields from where the biomass was harvested. In study one, the carbon footprints (CF) of two rice production systems were calculated: one scenario in which rice residues are burned and another scenario where these are converted into biochar and incorporated into soil. The functional unit (FU) was the production of 1 kg of milled rice. It was assumed that households used pyrolytic cookstoves and drum ovens to produce biochar. Based on a literature review, I assumed that the agronomic effects of biochar compounds with increasing biochar application until reaching maximum benefits at 18 Mg ha^-1. This amount of biochar would take eight years to be produced in pyrolytic cook-stoves and drum ovens using the rice husk and straw available. Biochar addition reduced the CF of spring rice and summer rice by 49% and 38% respectively, compared with rice produced with conventional residue disposal, after eight years of biochar addition. </p> <p>Study two assessed the CF of two different biochar production systems: one scenario in which rice straw-derived biochar in raw form was applied to the paddy fields, and a second scenario using enriched biochar (biochar made from rice straw enriched with lime, clay, ash and manure). In this study, the management of 1 Mg of dry rice straw was chosen as the FU. Applying enriched biochar showed an increase in GHG emissions abatement by 126% and 309% in spring and summer seasons, compared with using rice straw-derived biochar. This was mostly due to greater reduction of soil CH₄ emissions by enriched biochar, because of the larger area treated with enriched biochar at a lower application rate.</p> <p>Study three involved a comparative analysis of the climate change and health impacts of various biochar-compost (COMBI) systems relative to the conventional practice of open burning of rice husks. Three COMBI systems, using different pyrolysis technologies (pyrolytic cook-stove, brick kiln and the BigChar 2200 unit) for the conversion of rice husk into biochar were considered. In this study, the FU was the management of 1 Mg of dry rice husk. All biochar systems substantially improved environmental and health impacts of rice husk management compared with the open burning of rice husks. The differences between the three COMBI systems in the climate change and particulate matter impacts were not significant, due partially to large uncertainties. The lowest human toxicity effect was offered by the BigChar 2200 system, where biochar is produced in a large-scale plant in which pyrolysis gases are used to generate heat energy. This result highlights the significance of pyrolysis gas recycling for sustainable biochar production. </p> <p>At the household or village level, economic benefits are a key factor that drives producers to adopt a new agricultural technology. The scenarios modelled in study one were used to assess the costs and benefits, and non-renewable energy use of rice production. After eight years of biochar application, the net present value of rice was enhanced by 12% and the energy intensity decreased by 27%, compared with rice production with conventional residue management. The existence of a carbon market that recognises the reduction of soil GHG emissions and carbon sequestration due to the land application of biochar could considerably raise the profitability of rice production. </p> <p>These results indicate that ceasing the open burning of residue in the field and using residues to produce biochar for addition to soils can provide important benefits in climate change mitigation, human health and economic returns in rice cropping systems in Vietnam. This conclusion relied on several uncertain assumptions, including the effect of biochar on CH₄ emissions from soil. The assumed suppression of soil CH₄ emissions is a major contributor to the reduced climate effects for the biochar systems, and sensitivity analysis showed that the CF of biochar systems was highly sensitive to any variation in this factor. Therefore the soil impacts of biochar need to be confirmed by further research to enable more accurate quantification of the climate effects of biochar use in rice production.</p>