Modelling of torrefaction and hydrothermal carbonization and heat integration of torrefaction with a CHP plant

Abstract

Biomass provides an excellent opportunity to increase the use of local resources and promote renewable energy generation. The considerable diversity of biomass materials and differences in their chemical structure require detailed evaluation of their properties and impact on the conversion processes. The unstable quality of untreated biomass poses certain problems for biomass utilization in a large scale. Hydrothermal carbonization and torrefaction present two possible ways of improving the characteristics of biomass. Comprehensive understanding and evaluation of all the influencing factors of these relatively novel methods is crucial for the development of the processes. This thesis describes the results of chemical analysis of nine biomasses with respect to thermochemical conversion: the impact of chemical components on thermal decomposition is evaluated. The performance of any conversion process depends not only on biomass properties but also on the process characteristics, and that is why the effect of the main reaction parameters on mass and energy yields during hydrothermal carbonization and torrefaction of coniferous biomass is studied experimentally and expressed with mathematical correlations. Since both processes require a certain amount of heat to be supplied, the development of the technologies in a large scale by means of heat integration with combined heat and power (CHP) plants has significant potential. A model of a torrefaction unit is developed and integrated with two different-sized CHP plants. The mutual effects of the torrefaction process and the operational mode of the CHP plant on the thermodynamic performance of the integrated plant are investigated with six integration scenarios. The analysis reveals notable differences in the impact of torrefaction-CHP integration at different operational modes of the CHP plant, and the influence of seasonal variations in the operation of a CHP backpressure plant is analyzed in detail with a developed multiperiod model. Profitability evaluation of the integrated schemes, together with stand-alone and co-located CHP and torrefaction plants make it possible to assess the economic potential of the integration. This study indicates that the heat integration of torrefaction and a CHP plant can be economically profitable over co-located plants (particularly for the integration options with the longest operation time). Additionally, this work assesses the main economic factors for the profitability of integrated plants and confirms the importance of detailed operational and economic analyses for the assessment of the potential of the integration options.