title: Advanced gasification applications of direct carbon dioxide utilisation in integrated biomass energy cycles
Creators: name: Greencorn, Michael Joseph
Publisher: name: University of Glasgow
Year: 2023

description: International agreements seek to limit climate warming to no more than 2℃. For this goal to be achieved, drastic reductions in CO2 emissions from fossil fuels must be realised in very short timelines. In fact, most climate modelling predictions indicate CO2 will need to be removed from the atmosphere if the worst effects of climate change are to be avoided. Renewable biomass energy and biomass energy with carbon capture and storage (BECCS) will feature prominently in this substantial decarbonisation regime. Despite this technical forecast, BECCS technologies are unproven at scale. Innovative carbon dioxide utilisation (CDU) strategies are posited as a method for improvement of biomass energy system performance. Partially recycling CO2-rich exhaust gases from a syngas fuelled internal combustion engine to a biomass gasifier has the capability to realise a new method for direct carbon dioxide utilisation (CDU) within a bioenergy system. Simulation of an integrated, airblown biomass gasification power cycle was used to study thermodynamic aspects of this emerging CDU technology. Analysis of the thermodynamic system model at varying gasifier air ratios and exhaust recycling ratios revealed the potential for modest system improvements under limited recycling ratios. Compared to a representative base thermodynamic case with overall system efficiency of 28.14%, employing exhaust gas recycling (EGR) enhanced gasification improved system efficiency to 29.24% and reduced the specific emissions by 46.2 g-CO2/kWh. Although emissions from biomass power cycles can ultimately be considered CO2-neutral over time, this reduction in specific emissions from the cycle can minimise the “carbon debt” effect incurred during the initial deployment of biomass power sources. Further investigation of the EGR-enhanced gasification system revealed the important coupling between gasification equilibrium temperature and exhaust gas temperature through the syngas lower heating value (LHV). Major limitations to the thermodynamic conditions of EGR-enhanced gasification as a CDU strategy result from the increased dilution of the syngas fuel by N2 and CO2 at high recycling ratios, restricting equilibrium temperatures and reducing gasification efficiency. N2 dilution in the system reduces the efficiency by up to 2.5% depending on the gasifier air ratio, causing a corresponding increase in specific CO2 emissions. Thermodynamic modelling indicates pre-combustion N2 removal from an EGR gasification system could decrease specific CO2 emissions by 9.73%, emitting 118.5 g/kWh less CO2 than the basic system. A similar method for improving the efficiency of oxyfuel gasification biomass energy with carbon capture and storage (BECCS) cycles using carbon dioxide recycled from exhaust gases is described and modelled. Thermodynamic simulations show this process can increase the indicated efficiency of a representative cycle by 10.3% in part by reducing the oxygen requirements for the gasification reaction. Exhaust recycling is also shown to have a practical limit beyond which the syngas fuel becomes highly diluted. This diluted syngas results in low combustion and exhaust temperatures which, in turn, negatively influence the gasification process during exhaust recycling. For the system presented here, CO2- enhanced gasification is thermodynamically limited to equivalence ratios above λ = 0.13 and equilibrium temperatures above 576°C. This thermodynamically limited case produced an indicated system efficiency of 26.9% based on supplied biomass lower heating value (LHV). Further simulations using both ideal cycles and detailed numerical models highlight the influence of several operational settings on the thermodynamic conditions of the gasification process. Principally, the coupling between exhaust temperatures, allothermal heat, and syngas quality are shown to govern the performance of the gasification reactions. Although these simulated equilibrium calculations revealed the fundamental thermodynamic benefit of EGR-gasification cycles, variability in typical gasification processes often produces syngas compositions that differ from chemical equilibrium. An examination of the evolution of syngas from a biomass sample during gasification was needed to assess how these differences occur. Particularly, experimental confirmation that the key CO2 to CO conversion process is achievable under mild temperature conditions was required to verify the feasibility of the novel process described in this work. Results of these experimental investigations have shown the CDU conversion of CO2 into CO under process conditions similar to earlier thermodynamic modelling. Compared to pyrolysis of soda lignin as a representative biomass sample, CO2 gasification produced roughly 69% more CO while consuming 1.1 mmol CO2/g biomass. Although this conversion process performs poorly under the experimental conditions, it does illustrate the viability of the proposed technology. Significant improvement in CO2 conversion and CO production is noted as reaction temperature increases, particularly above 700℃. Additional features of lignin pyrolysis are also illustrated that suggest incomplete conversion of pyrolysis products contribute to a product syngas with higher CH4 content than expected under equilibrium conditions.