Life cycle assessment of energy generation from agricultural biomass via innovative energy conversion systems

The fundamental role that energy plays in all activities makes sustainability a crucial goal for the energy sector. Biomass is one of the most important parts of the energy sustainability sector due to the inevitability of biomass existence (linked to the existence of life), the interactions with other sectors (such as food, material, and human health), and the complexity of this source, which can be processed in many ways into different energy intermediates and final uses (heat, electricity, and transport fuels). Biomass can even help reduce oil dependency and global warming. However, it also has some undesirable impacts on ecosystems and the price of food commodities under direct and indirect land use change policies. One way to help minimize these impacts is to extend the range of feedstocks that can be used, particularly agricultural and forestry residues. However, a long-term successful bioenergy strategy must also take all sustainability issues into consideration. Unlike all other renewable energy resources, biomass needs conversion steps to transform raw biomass into a variety of marketable intermediate chemical and energy products as solids, liquids, and gases. The diversity of biomass nature and conversion steps creates the need for specific technologies to be developed for each case. Gasification and pyrolysis appear to be the most promising biomass conversion technologies, due to the fact that they, as highly versatile processes, can convert almost any biomass feedstock into syngas, bio-oil, and biochar with a very high carbon conversion and thermal efficiency. Furthermore, syngas and bio-oil are intermediate products that offer a large range of possible secondary conversion and final energy uses. Pyrolysis-based biochar application to the soil on a stable and carbon-rich substance can have substantial advantages from social, economic, and environmental points of view, leading to such outcomes as soil improvement, climate change mitigation, and bioenergy production, in addition to biochar production. Hydrogen from biomass is an attractive product, due to multiple applications in industrial market (chemical, refineries, metal processing, etc.), stationary power generation, and particularly in transport due to growing demand for zero-emission fuels and the implementation of fuel cell systems. Although the environmental benefits of these products in the application have been substantiated, the sustainability of the entire chain, from the production to the end uses, remains unclear. In fact, it is still to be determined whether the production of hydrogen and biochar is economical and environmentally and socially feasible considering costs linked to environmental impacts of its production process. Furthermore, no link has yet been made between the environmental performance of these products from the above-mentioned processes and the achieved economic performance. This study plans to assess the environmental burdens of the various stages of life cycle of hydrogen and biochar using life cycle assessment (LCA), a well-known technique for assessing the potential impacts associated with a product. In addition, the economic concept of shadow prices is applied to assign relative weights of socio-economic importance to the estimated life cycle impacts. This novel integration of approaches complements the assessment of considered bioenergy systems with the inclusion of long-term global environmental impacts and the investigation of trade-offs between different environmental impacts through a single monetary unit. This study also addresses the risk related to economies of scale for bio-hydrogen from small-scale gasification. With the exception of technologies for heating applications, most commercially available technologies generally suffer from poor economics at small scale. This is a particular problem because of the difficulty in supplying mainly lignocellulosic feedstocks to large plants due to insufficient resource availability, distribution, density, and logistics. Therefore, a techno-economic analysis was conducted on small-scale (100 kWth) system to identify system costs and find options to reduce production cost to the competitive rate in the market. The plant is mainly composed of a gasifier (double-bubbling fluidized bed reactor) coupled with a portable purification system (PPS: catalytic filter candles, water gas shift, and pressure swing absorption). The results show that hydrogen production cost is a function of hydrogen production efficiency and a PPS, which is a vital and high-cost unit in the system to provide purified hydrogen. Distributed hydrogen can be supplied at a competitive cost if the PPS unit cost falls by 50 percent and if the efficiency can rise by 50 percent (for example, increasing the steam-to-biomass ratio up to 1.5). Regarding the environmental impacts, this plant has a significant advantage over conventional hydrogen production technology (steam methane reforming) in global warming impact -0.213 kg CO2eq vs. 0.1 kg CO2 eq – and a relatively high score of hydrogen renewability (75 percent). In particular, the application of byproduct to generate electricity considerably affects environmental performance and has positive impacts per 1 MJ H2 produced on global warming (kg CO2 eq), marine aquatic ecotoxicity (1.4-DB eq), and cumulative energy demand (MJ). On the contrary, the significant negative impact on abiotic depletion (MJ) and acidification kg SO2 eq comes from fertilizer application and consumption in the biomass production phase. Weighing the impact assessment into the single monetary unit using three valuation methods indicates that the societal costs of biohydrogen production are higher than the societal benefits, with biomass cultivation being mostly responsible for these costs. This implies that modification in agri-food production management such as substituting chemical fertilizers with green fertilizer and policies to improve biomass supply chain can decrease environmental burdens, not only in its sector but also in linked bioenergy systems. The LCA has also been applied to a set of 50 vineyards. The results showed that the application and production of fertilizers are mainly responsible for all impact categories. After optimizing inputs by DEA, the on-orchard emissions had the greatest potential to reduce the environmental consequences in vineyards, which are connected to drops in manure and N fertilizer consumption. Furthermore, similar to the hydrogen production cycle, byproduct utilization (vineyard waste) by the installation of gasifiers could play a considerable role in improving the environmental performance of crops produced. In biochar production and application in the soil, expected savings in CO2 emissions can be explained by the substituted amount of heat and electricity production from (bio-oil and syngas) and reduced fertilizer production, amongst other things, but the highest share in total CO2 savings is attributable to the application of biochar in soils. The difference in savings of CO2 emissions can be explained by the different stable carbon content of the produced biochar. The biochar produced from willow can reduce GHG emissions more than pig manure biochar (2.2 t CO2 vs 0.98 t CO2 t-1 of biochar) because the stable carbon content of willow biochar is higher than the pig manure biochar. The results of a monetary valuation of environmental impacts for biochar production from willow and pig manure reveal that biochar application in soil significantly increases environmental revenue related to global warming impact due to C sequestration and reduction in fertilizer consumption. Therefore, biochar production from willow is more environmentally favourable based on all valuation methods.