Development of an integrated system model for production of fischer-tropsch liquid fuels from woody biomass.
Thesis DisciplineChemical Engineering
Degree GrantorUniversity of Canterbury
The transportation sector in New Zealand accounts for the highest CO2 emission. Replacing the fossil fuels with liquid biofuels derived from woody biomass as a carbon neutral resource can decrease the CO2 emission and secure future liquid fuel supply. The Fischer-Tropsch (FT) liquid fuel production using syngas from gasification is a promising technology for commercialisation in the next 5 to 10 years in New Zealand. However, in order to achieve the maximum benefits by using the woody biomass for liquid fuel production, the plant design and operation need to be optimised. The objectives of this study were: (1). Develop an integrated system model for production of FT liquid fuels from woody biomass; 2) Finding an optimum plant configuration by using energy and exergy analyses; 3) Performing techno-economic analysis on the plant. The integrated system models for conversion of woody biomass to FT liquid fuels (BTL) were developed based on two different scenarios. Scenario I included biomass pretreatment (chipping and drying), biomass gasification in a dual fluidised bed (DFB) gasifier with steam as the gasification agent, producer gas cleaning and gas conditioning, and FT liquid fuel synthesis. Scenario II included biomass pretreatment (chipping, drying and grinding), biomass densification through fast pyrolysis, entrained flow gasification of bio-slurry, gas cleaning and gas conditioning, and FT liquid fuel synthesis. For Scenario I, it was assumed that woody biomass chips were transported from the biomass field to the processing plant where the chips were dried and then fed to the DFB gasifier. For Scenario II, the wood chips were firstly converted to bio-slurry by fast pyrolysis reactors in the biomass field, and then the bio-slurry was transported to the main process plant. The scale of the fast pyrolysis plant was fixed at 20 MWth thus when the main process plant had greater capacity, more than one such pyrolysis systems were operated simultaneously in different biomass fields. The unit operations of each scenario were modelled in a UniSim simulation environment by using a combination of built-in and user-defined unit operations. In the modelling, energy and mass balances in each operation unit were considered. In addition, chemical reactions in pyrolysis, gasification and FT reactors were also modelled using quasi-equilibrium and kinetic approaches. The system models were solved, and the results were compared with reported data. Finally, the system models were applied for a 100 MWth (based on the lower heating value of biomass feed) plant to analyse energy efficiency, exergy efficiency and economic returns. Parametrical analysis was also performed to investigate the effects of feedstock and operational conditions on the system performance. As part of the energy analysis, the pinch analysis was performed to optimise the heat recovery of the system and steam generation. The simulation results show that the energy efficiency of the BTL plant based on Scenario I varies from 55 % to 61.5 % while it is 53 % for the BTL plant based on Scenario II. For improving the energy efficiency, the exhaust heat should be entirely used for biomass drying and steam generation, and the FT off gas should be used for electricity generation. Also, the steam-methane reforming reactor should be chosen over the high-temperature shift converter for the gas conditioning method in Scenario I in order to achieve higher energy efficiency. The model simulation results also indicate that the exergy efficiency of the BTL plant based on Scenario I varies between 38 % and 48 % while it is 33 % for the BTL plant based on Scenario II. Power generation is identified as the largest source of exergy loss in the system. It is proposed to maximise the liquid fuel yields and minimise the power generation capacity for improving the exergy efficiency of the system. Also, the number of process steps should be minimised in a plant configuration. The developed system models were also applied for techno-economic analysis on the BTL plant based on the two scenarios. A sensitivity analysis was conducted to investigate the effect of various parameters on production cost and total capital investment of the BTL plant based on each scenario. These parameters included plant scale, feed biomass moisture content, unit operations’ conditions, and transportation distance between the biomass field and the BTL plant. From the feasibility analysis, it was found that the capital investment required for the BTL plant based on Scenario I was $NZ187 million which was considerably less than that of $NZ 248.5 million required for the BTL plant based on Scenario II. The production costs of FT liquid fuels produced from the Scenario I BTL plant were at $NZ 1.34/litre for diesel and $NZ 1.27/litre for gasoline. These costs were lower than the costs of corresponding products produced from the Scenario II BTL plant ($NZ 1.95/litre for diesel and $NZ1.85/litre for gasoline). The key factor for the higher production costs in the Scenario II BTL plant is the additional cost for biomass pyrolysis to produce bio-slurry that cannot be compensated by the cost of biomass transportation. At the scale of 100 MWth, the Scenario I BTL plant is competitive for commercialisation considering the actual market prices of petroleum-derived diesel and gasoline at $NZ 1.3/litre and $NZ 1.23/litre, respectively. However, the extra costs of production of bio-slurry may be paid by the cost of biomass transportation at large scale of plant (>150 MWth) when more biomass needs to be transported over a long distance. It should be emphasised that at the time of the study in October 2013, the BTL plant was economically feasible. Unfortunately, the plant is not feasible currently as the price of crude oil has been declined significantly to $US 62.5/barrel from $US 105.5/barrel in October 2013. Therefore, the FT liquid fuel production has to compete against the conventional liquid fuels unless some subsidies are provided by the government.