Biomass upgrading technologies for carbon-neutral and carbon-negative electricity generation

Abstract

Biomass is increasingly becoming an internationally traded commodity fuel. In this context, biomass upgrading technologies such as torrefaction and hydrothermal carbonization (HTC), which increase the energy density and improve the storability and handling of the biomass, have recently gained attention. This work provides a techno-economic assessment of commercial-scale HTC plants and their competing technologies. In the first part of this work, energetic efficiency, GHG emissions and costs of HTC are compared to those of wood pelletizing, torrefaction and anaerobic digestion. Moreover, the substitution of fossil fuels by the aforementioned biofuels in existing power stations is analyzed. The second part focusses on the potential role of biomass upgrading technologies for bioenergy with carbon capture and storage (BECCS). Torrefaction and HTC cause the biomass to lose its fibrous structure, thus facilitating entrained flow gasification. In order to investigate the merits of this conversion pathway, torrefaction or HTC followed by entrained flow gasification is compared to the direct fluidized bed gasification of raw wood. The analysis is based on flowsheet simulations created with Aspen Plus. Exergy analysis is employed to locate thermodynamic losses within the respective processes. Exergoeconomic analysis is applied to the HTC plant design to reveal potentials for reducing the biocoal production costs. A simple model of the entire supply chain is developed in order to assess the costs and GHG emissions related to biomass and biofuel transport and storage and their dependency on the plant capacity. The results indicate that HTC can only be economically competitive with conventional wood pelletizing if waste biomass is used as a feedstock. Depending on the remuneration for waste disposal, relatively large processing capacities of up to 100 kt/a of feedstock are required year-round to make HTC an economically viable proposition. Potential feedstocks include park and gardening waste and empty fruit bunches from palm oil production. Exergy analysis reveals that drying of the feedstock or biofuel is the most significant source of exergy destruction in all the analyzed processes generating solid biofuels. HTC and anaerobic digestion also suffer large exergy losses through their waste streams. Measures to improve the efficiency and cost of HTC include efficient heat recovery, drying in superheated steam, and using the waste water to produce biogas. Integration of HTC with a rankine-cycle CHP plant may reduce the biocoal production cost and increase operability by omitting the complex heat recovery scheme required for a standalone HTC plant. IGCC power plants with carbon capture are more efficient when employing entrained flow gasification fired on torrefied wood or HTC biocoal than when using fluidized bed gasification of raw wood. However, the higher efficiency of the IGCC cannot compensate for the conversion losses of the biomass upgrading. Moreover, the carbon capture rate for scenarios with biomass upgrading is only 66–69%, compared to 82–86% for the direct fluidized bed gasification of the raw biomass. The unit cost of electricity generated by the BECCS plants is strongly dependent on the CO2 price. The results indicate that if the carbon price is sufficiently high to incentivize CCS from fossil fuels, then favourable BECCS configurations are also close to economic viability.