Abstract
Biomass, a renewable carbon source, is mainly used to produce heat and electrical power, but it can also be used to produce fuels and chemicals, with properties similar to those of fossil origin. In particular, biofuels can play an important role in the transport sector, leading the transition from fossil to renewable fuels. Currently, biofuels are more expensive than fossil fuels and this will remain so unless the costs of mitigating climate change are going to be factored in the cost of fossil fuels. As a consequence, biofuels production technologies should be strongly investigated to reduce production costs.
This thesis investigates two vital and compelling applications of biomass gasification, which is considered one of the most promising technologies for the thermo-chemical conversion of biomass into intermediate fuels. The first part studies the steam gasification principles of biomass residues to enhance the hydrogen yield in producer gas, which, in air-gasification, is limited to 15-20 vol.%. The second part of this thesis probes fundamental catalyst properties for converting hydrogen-rich syngas to liquid fuels via the Fischer-Tropsch reaction. The third part focus on different technological solutions for the coupling of the two processes. A union of these studies puts forward a potential pathway for installing commercial second-generation bio-fuel plants or forming an essential part of bio-refineries.
In the past few years, gasification has gained high interest mainly because of significant subsidies applicable to renewable energy technologies, especially for electricity generation. However, the constant decrease of public subsidies has led to an increasing interest towards other alternatives, other than CHP use. In this context, a compelling motivation exists to investigate the effect of using steam as a gasification medium for enhanced hydrogen yield. Studies were performed with three different biomass feedstock – wood chips and two residual biomass, vineyard pruning, and barks. The selected feedstocks possess a large range of ash content and a significant variation in elemental composition, thereby covering a variety of biomass reactants used in commercial fixed bed gasifiers. The results show that a producer gas with high hydrogen content (46-70 vol.%) and optimal composition for the upgrade to biofuel through FT synthesis could be obtained. Moreover, the char consumption rate was found to be a crucial factor in the hydrogen generation rate. At high temperature, the char begins to gasify, increasing the gasification rate and increasing the hydrogen content in the producer gas. This research’s immediate outcome could enable the retrofitting of existing biomass gasifiers with steam generated by waste heat or from renewable feedstock.
The second part of this thesis relates to the development of novel combustion synthesized catalysts to convert syngas to liquid fuels via the Fischer-Tropsch reaction. The recent developments on the combustion synthesized (CS) supported catalysts have highlighted the influence of metal-support interaction (MSI) to develop metallic catalysts specific to reactions such Fischer Tropsch. MSI is largely responsible for the availability of active metal sites and the stabilization of nano-crystals against sintering. Earlier studies showed that cobalt crystallite sizes, degree of reduction, and the MSI could be tailored by varying the synthesis equivalence ratios (‑), i.e., by varying the ratios of metal precursor (NO3-, oxidizer) and citric acid (C6H8O7, fuel). 20 wt.% Co/Al2O3 catalysts were synthesized in the fuel-lean regime (i.e., ϕ = 0.6 and 0.3), which eases the synthesis process compared to the uncontrolled synthesis reaction under fuel-rich conditions (ϕ>1). The synthesized catalysts were tested in a fixed bed reactor with the weight hourly space velocity of 2000 mL h-1 gcat-1, a temperature of 230 °C, and 30 bar pressure. Larger fractions of waxes were obtained as the equivalence ratio increased while lower equivalence ratios promoted C5-C11 range hydrocarbons. This was attributed to lower crystallite sizes (approx. 10 nm), higher metal dispersion and, lower MSI at higher equivalence ratios. The space time yields for ϕ = 0.3 and 0.6 catalysts were observed to be 0.32 and 0.36 gC5+ h-1 gcat-1.
Additionally, the large data sets obtained from extensive experiments were used to develop thermodynamic models to simulate the integrated biomass gasification and Fischer-Tropsch (IGFT) process. While extensive literature exists on IGFT modeling, a comparison of the whole process in terms of different type of gasification and cleaning and refining systems is still missing. The third part of this thesis’s modeling work evaluates and presents a comparison between air and steam gasification coupled to two type of producer gas cleaning, i.e. hot (dry) and cold (wet), and FT synthesis. This allows to evaluate the feasibility and overall performance in different operating conditions of several IGFT configurations. The results highlight the higher energy and exergy efficiency of the steam gasification based configurations and of the hot gas cleaning system, that generates lower exergy losses.