Gasification of woodchips from the San Rossore natural reserve maintenance for CHP application: A case study analysis

Biomass gasification is a developing reality in Europe. This technology allows exploiting residual biomass as well as promoting the decentralized production of electricity and heat (CHP). However, despite the simplicity of the gasification reaction, the installation and conduction of a gasification process require an accurate planning, involving economic, environmental, social, safety and process management issues. As a consequence demo facilities and case studies are required to enhance the development and diffusion of this technology. This work aims at analyzing a complete biomass gasification chain for CHP purposes. Four steps can be identified in the conversion chain: biomass production, biomass conditioning, syngas production and cleaning, syngas utilization. Fuel, electricity, materials and utilities consumption, as well as major airborne emissions, wastewater and solid residues are determined for each step of the chain, providing a comprehensive approach to the problem in order to indentify global index for the technical and environmental performances of the chain. The biomass production step is developed on the basis of an existing scenario (the S.Rossore natural reserve in Pisa.) which provides in field data and information. The performances of the conditioning and gasification plant are obtained from literature sources and data sheets of commercial equipments. Special attention is devoted to the gas clean-up providing a survey of problems and technologies associated with syngas cleaning. Two different gas cleaning systems are taken into account, a “traditional” wet gas clean-up line (based on water scrubbing and filters) and an “advanced” hot gas clean-up line (based on a dolomite bed with air injection for tar cracking and syngas sensible heat recovery with an air heat exchanger). The performance of wet gas clean-up and the hot gas clean up are derived from the work of Sharan et al. (1997) and Van de Beld et al. (1997), respectively. The syngas utilization section includes an IC-Engine for CHP applications, data about efficiency and emissions are obtained both from literature sources as well as data sheets of commercial equipments. First an inventory of the energy and mass fluxes is made; subsequently energy and mass balances are carried out for the two systems. The energy flow analysis shows that for the wet gas clean-up the most significant energy loss is due to syngas cooling to other energy losses. Notably in the case of hot tar abatement part of the syngas sensible heat is recovered thus allowing to reduce the fuel demand of the plant, if a suitable heat demand is available in the plant (for instance additional chips drying). However since part of the syngas chemical energy is lost in the partial oxidation in the dolomite bed, the overall energy efficiency of this chain is lower compared to the wet gas clean-up. The hot clean-up flow-sheet is more complex and expensive than the wet clean-up because it is specifically designed to meet tight IC-Engine specifications (in order to improve the engine lifetime and reduce emissions) which cannot be safely met with the wet clean-up. Although this work cannot clearly state which one is the best syngas cleaning system, it highlights the energy and material fluxes of a complete biomass gasification to CHP chain and can be a useful basis for a more comprehensive LCIA study.