- Title
- A novel ex-situ calcium looping process for removal and conversion of tars formed during biomass gasification
- Creator
- Yin, Fengkui
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2017
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Biomass is an important primary and renewable energy resource which is widely distributed around the world. Bioenergy derived from biomass is a form of renewable energy that may be used to generate power, heat and to synthesize liquid fuels for transport or chemical production. Among the different biomass utilisation processes, thermo-chemical conversion methods, such as pyrolysis or gasification, are the most efficient way to convert biomass to fuels or fine chemicals. Biomass gasification is generally favoured as it can be used for the production of syngas (H2 and CO) in differing H2/CO ratios, which can be used as a fuel, or for chemical synthesis. However, existing gasification units, suffer from a major technical hurdle that is tar formation. Tar components, especially smaller molecule aromatic ring components such as toluene, xylene and naphthalene etc., are still present in syngas after primary and secondary tar removal treatments. This seriously influences the efficiency of subsequent syngas application in either gas turbines or chemical synthesis. As such tar removal technologies are imperative for the purification of raw syngas generated during biomass gasification. Catalytic cracking is currently considered the most attractive method for tar removal, as cooling of the raw syngas can be avoided and hence minimal energy is lost. A further advantage lies in that tars can be cracked and reformed into light gaseous components, such as H2, CO and CH4, increasing the overall gasification efficiency and improving the lower heating value (LHV) of the product syngas. Among the various catalysts available such as alkali metal catalysts, nickel-based catalysts and transition metal catalysts, calcium based catalysts were selected for systematic investigation. Calcium based catalysts were evaluated as the most attractive due to their low cost and ability for simultaneous catalytic tar cracking and CO2 capture at moderate temperatures. In order to utilise a calcium based catalyst for tar cracking in a biomass gasification process, with a primary focus on syngas production and power generation, two configurations of a novel calcium looping based biomass gasification process were proposed in this thesis; (i) an ex-situ calcium looping (ECL) process for biomass gasification and (ii) a calcium looping based biomass integrated gasification combined cycle (CL-BIGCC). To assess the technical merits of the proposed processes, a fundamental understanding of bio-tar formation during biomass gasification, as well as, bio-tar cracking in the presence of calcium-based catalysts was necessary. Accordingly, a series of experimental and theoretical modelling investigations were conducted to assess the performance of the proposed processes. An experimental investigation of tar cracking behaviour during biomass partial oxidative gasification in the presence of CaO was conducted. The amount of tar cracked was proportional to the oxygen content and CaO loading or Ca/B (mass ratio of calcium to biomass). The effect of oxygen in the gasification environment was more significant for bio-tars cracking in comparison to CaO addition but resulted in higher CO and CO2 yields in the syngas. However, the presence of CaO resulted in greater catalytic conversion of tars resulting in higher H2/CO syngas ratios at lower temperatures. It was also found that the gasification process reaction rate increased for increasing oxygen and CaO content. Therefore, it is crucial to control both the oxygen and CaO content in the biomass gasification process in order to achieve synergetic effects relating to bio-tar cracking and to ultimately produce syngas of the desired compositional requirement. Steam reforming of bio-tars in the presence of natural CaO minerals (stone dust and dolomite) and a novel synthesized dual supported CaO catalyst (CaO-Ca12Al14O33/Al2O3) prepared with and without ultrasonic treatment, was investigated. Of the natural minerals, stone dust showed better tar cracking performance than dolomite. The synthesized catalyst without ultrasonic treatment (CA) exhibited better tar cracking performance in comparison of stone dust and dolomite in the temperature range of 600 to 800 °C. Conversely, the synthesized catalyst with ultrasonic treatment (CAU) performed worse than all other catalysts when the temperature was above 650 °C mainly due to a smaller specific surface area caused by the ultrasonic treatment during preparation. The properties of the natural and synthesised catalyst were also assessed. The CA catalyst had the greatest performance in terms of superior surface area and good mechanical strength due to the core support of Al2O3. This makes it a potential bed material for further study of tar cracking in large-scale fluidized applications. A thermodynamic assessment of the proposed ECL process was completed using the software package Aspen Plus. The ECL process was proposed to overcome the disadvantages of ash separation in traditional chemical looping/ calcium looping biomass gasification processes. Simulation of the ECL process aimed to identify the optimum process operating conditions/parameters for H2 rich syngas production and CO2 separation from a thermodynamic point of view. In order to achieve the highest cold gas efficiency, energy efficiency and CO2 capture efficiency, the simulation results found the optimum operating parameters to be a CaO/biomass mass ratio (1.1), steam/biomass mass ratio (0.3), temperature (650oC) and pressure (1bar). Finally, a CL-BIGCC process was proposed on the basis of complete tar cracking by the ex situ calcium looping process. A thermodynamic assessment of the CL-BIGCC process was also completed using Aspen Plus. To determine the performance of this proposed process, some key parameters such as compression ratio, air/fuel mass ratio, calcium-to-biomass ratio (Ca/B), steam-to-biomass ratio (S/B), carbonator temperature and calciner temperature were investigated. The results show that both power generation efficiency and gas turbine inlet temperature are sensitive to the aforementioned variants. It can be concluded that the most favorable values of compression ratio, air/fuel mass ratio, Ca/B, S/B, temperatures of carbonator and calciner are 5.1, 15, 0.53, 0.17, 650°C and 800°C, respectively. With the above inputs, the net power generation efficiency of novel CL-BIGCC process was found to reach 25%, which is higher than those of other parallel processes.
- Subject
- calcium looping process; tars; biomass gasification
- Identifier
- http://hdl.handle.net/1959.13/1353428
- Identifier
- uon:31096
- Rights
- Copyright 2017 Fengkui Yin
- Language
- eng
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