(10f) Biowastes-to-SNG Via Gasification. An Integrated Economic, Environmental and Process Efficiency Evaluation

Biomass is a promising renewable energy source for electricity, heat and transport biofuels production. Electricity and heat can also be produced from other renewable alternatives (such as solar or wind energy), but biomass is the only available renewable option to produce transport biofuels that can be stored in large quantities. This is of a great importance since the transport sector represents a significant share of the world's energy consumption, which is almost exclusively fuelled by fossil oils. In 2005, transport sector accounted for 27% of the global CO2 emissions.

However, biomass availability is also rather limited and stochastically distributed, especially in densely populated regions where biomass production could compete with other land uses, notably agriculture. In fact, this is the case of the so-called first-generation biofuels (e.g. bioethanol and biodiesel). However, when taking into account emissions from transport and conversion treatments, life-cycle analyses reveal that first-generation biofuels frequently exceed the emissions threshold of fossil fuels. Second generation biofuels are now being developed as a possible better alternative to the first generation, as they can use non-food crops or biowastes from different origins in order to avoid land competition for food production. Second generation biofuels are also expected to achieve higher efficiencies in larger conversion plants. Biowastes-to-biofuels conversion involves several challenges. Firstly, the existing technology must be re-designed and optimized to become cost and efficiency competitive with fossil fuels. Moreover, due to the wide diversity of biowastes and biofuels that can be obtained, the most sustainable conversion routes must be properly selected. Hence, an inherent challenge is to develop a reliable model to evaluate the sustainability of any process.

In this paper we present the design and evaluation Synthetic Natural Gas (SNG) production from woody wastes via gasification and later catalytic conversion of syngas.

Evaluation is made in terms of process efficiency, economic and environmental impact, and these 3 key parameters are later integrated and compared with other biofuels and fossil fuels.

In the biowaste-to-SNG process, pre-treatment steps are required in order to enhance the low energy density of the feedstock prior to gasification. However, since pre-treatment is directly affected by local conditions, the Dutch Friesland province is taken as a case study. The production chain is modeled in Aspen Plus and Aspen Icarus for efficiency (exergy) and economic evaluation, whereas a LCA is conducted for the environmental impact analysis. Moreover, all analyses are conducted under two different scenarios, a ?full-renewable? and a ?semi-renewable? production plant. In the first case, energy demand of the plant is covered by heat integration but extra biomass is also needed to fulfill heat and electricity demand. In the second case, the extra amount of biomass is substituted by a fossil fuel, i.e., natural gas from the grid. Both analyses are extended until the final use of SNG in vehicles. Results will show whether a fully or semi-renewable production plant is preferred and if SNG can become a primary biofuel for the transport sector. The methodology used will be also used to create an own sustainability model that could be applied to evaluate other biomass-to-biofuels conversion routes which will be discussed in this paper as well.