Life cycle assessment analysis for cogeneration of fuel gas and biochar

Abstract: Chinese straw biomass gas-carbon cogeneration conversion technology is in the pilot phase, including the cross flow moving bed and vertical flow moving bed technologies. The effects of a variety of product utilization modes after straw pyrolysis are highlighted. For example, the better energy utilization efficiency can be seen: the gas product can be converted to power or biogas and so on, and the carbon product can be turned into different products of energy and carbon-base fertilizer, which can replace part of the fossil energy, reduce CO2 emissions and pollutants. However, there is still not quantitative evaluation about life cycle economy and energy consumption of current biomass cogeneration technology, and the effects of products and byproducts to the environment are not clear. In order to explore the efficiency, economy and greenhouse gas emissions of energy conversion about biomass forming biochar and fuel gas through pyrolysis process, the life cycle assessment principle was applied, and then the 3E (economy, energy and environment) model for the cogeneration of biochar and fuel gas was established. To be more precise, corn straw was selected as the material, and the analysis range was from crop planting phase to the utilization of fuel gas and biochar. There were 4 stages: corn planting, collection, storage and transportation to the factory, conversion to fuel gas and biochar, and product application. In the meanwhile, the advantages and disadvantages of 2 technologies i.e. cross flow moving bed and vertical flow moving bed were discussed. The results showed that the net energy of applying cross flow moving bed technology was 6 542.2 MJ/t, and the energy output-input ratio reached 4.5. Among them, the top 3 energy consumption items were nitrogen fertilizer for planting, fuel consumption for agricultural machines and power consumption for pyrolysis, of which the energy consumption proportions were 30.8%, 20.4% and 17.2%, respectively. The total cost of the fuel gas-biochar cogeneration was 319.4 yuan/t, of which the cost at pyrolysis conversion phase accounted for about 34.0%, and the product revenue and the net profit were 567.6 and 248.2 yuan/t, respectively. The CO2 equivalent emission was 18.05 g/MJ through the energy consumption process, and the amount of reducing CO2 equivalent was 40 g/MJ after biochar was returned to the field to fix carbon. Two biomass pyrolysis conversion technologies have their own features. Therefore, based on the product application characteristics, applying appropriate conversion process is vital. Both technologies have some benefits, such as economic advantages, energy saving and low greenhouse emissions, which have certain application value.