Assessing energy efficiencies and greenhouse gas emissions under bioethanol-oriented paddy rice production in northern Japan

Abstract

To establish energetically and environmentally viable paddy rice-based bioethanol production systems in northern Japan, it is important to implement appropriately selected agronomic practice options during the rice cultivation step. In this context, effects of rice variety (conventional vs. high-yielding) and rice straw management (return to vs. removal from the paddy field) on energy inputs from fuels and consumption of materials, greenhouse gas emissions (fuel and material consumption-derived CO2 emissions as well as paddy soil CH4 and N2O emissions) and ethanol yields were assessed. The estimated ethanol yield from the high-yielding rice variety, “Kita-aoba” was 2.94 kL ha−1, a 32% increase from the conventional rice variety, “Kirara 397”. Under conventional rice production in northern Japan (conventional rice variety and straw returned to the paddy), raising seedlings, mechanical field operations, transportation of harvested unhulled brown rice and consumption of materials (seeds, fertilizers, biocides and agricultural machinery) amounted to 28.5 GJ ha−1 in energy inputs. The total energy input was increased by 14% by using the high-yielding variety and straw removal, owing to increased requirements for fuels in harvesting and transporting harvested rice as well as in collecting, loading and transporting rice straw. In terms of energy efficiency, the variation among rice variety and straw management scenarios regarding rice varieties and rice straw management was small (28.5–32.6 GJ ha−1 or 10.1–14.0 MJ L−1). Meanwhile, CO2-equivalent greenhouse gas emissions varied considerably from scenario to scenario, as straw management had significant impacts on CH4 emissions from paddy soils. When rice straw was incorporated into the soil, total CO2-equivalent greenhouse gas emissions for “Kirara 397” and “Kita-aoba” were 25.5 and 28.2 Mg CO2 ha−1, respectively; however, these emissions were reduced notably for the two varieties when rice straw was removed from the paddy fields in an effort to mitigate CH4 emissions. Thus, rice straw removal avers itself a key practice with respect to lessening the impacts of greenhouse gas emissions in paddy rice-based ethanol production systems in northern Japan. More crucially, the rice straw removed is available for ethanol production and generation of heat energy with a biomass boiler, all elements required for biomass-to-ethanol transformation steps including saccharification, fermentation and distillation. This indicates opportunities for further improvement in energy efficiency and reductions in greenhouse gas emissions under whole rice plant-based bioethanol production systems.