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Towards a Sustainable Biomass and Waste Refinery Based on Pyrolysis Combined with a Pretreatment Process

Time: Fri 2022-10-21 10.00

Location: D3, Lindstedtsvägen 5, Stockholm

Video link:

Language: English

Subject area: Energy Technology Chemical Engineering

Doctoral student: Yuming Wen , Processer, Energy and Furnace Technology group

Opponent: Professor Tobias Richards, Högskolan i Borås

Supervisor: PhD Weihong Yang, Tillämpad termodynamik och kylteknik, Processer; Pär Jönsson, Processer

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Some of the naturally accumulated biomass and the massive production of waste by human activities have caused serious environmental problems. The degradation of biomass and waste is one of the main greenhouse gasses (GHG) emission sources. Pyrolysis is a technique that can convert the organic feedstock into char, bio-oil, and gas at 350 - 800 °C and in the absence of oxygen. The diversity of the pyro-products makes pyrolysis one of the most promising techniques for biomass and waste refineries. One of the main challenges of the technique is that the unfavorable physical and/or chemical properties of the feedstock would increase the energy and cost required for the whole refinery process. Combining feedstock pretreatment with a pyro-refinery has the potential to make the entire process more efficient from a cost, energy, and climate perspective.

In this thesis, the performance of the peat moss pyrolysis is firstly investigated. It was found that it has a potential to convert peat moss into fuels through pyrolysis. Thereafter, beach-cast seaweed is further considered as a feedstock for the process, which is a high-ash content biomass. Three refinery processes were designed and simulated based on the pyrolysis results. Results showed that it was necessary to have a washing pretreatment for the beach-cast seaweed pyro-refinery. The implementation of washing pretreatment could decrease the direct energy for the whole process from 1485.8 to 1121.0 MJ for treating one ton of dry beach-cast seaweed. The further life cycle assessment (LCA) analysis showed that using the pyro-refinery process with washing pretreatment to treat one ton of dry beach-cast seaweed for the electricity production had the lowest cumulated energy demand (CED). Specifically, it has a value of -3.0 GJ and the lowest global warming potential within a 100-year time frame (GWP100) with a value of -790.9 kg CO2eq compared to the other scenarios of producing liquid biofuel and syngas.

Digestate from anaerobic digestion (AD) requires proper treatment. The third work of this thesis compares the pyrolysis behavior of the organic fraction of municipal solid waste (OFMSW) and its digestate. It was found that the AD process could decrease the pyrolytic activation energy of OFMSW. Due to digestate’s higher ash content, the char yield rate of digestate was higher than that of OFMSW. On the other hand, the yield rates of bio-oil and gas of OFMSW pyrolysis were higher than that of its digestate.

The moisture content of the digestate is hard to be removed by traditional mechanical dewatering techniques due to digestate’s hydrophilic properties. Thus, the use of a pretreatment combined with hydrothermal carbonization (HTC) and mechanical dewatering has the potential to contribute to the digestate pyro-refinery. In the last work, the effect of HTC on the kinetics and thermodynamics of the agricultural waste digestate (AWD) pyrolysis was investigated. It was revealed that the HTC pretreatment could decrease the pyrolytic activation energy of AWD from 182.9 - 274.4 kJ/mol to 144.6 - 205.2 kJ/mol. Bench-scale pyrolysis experiments, process simulations, and LCA were then conducted based on the kinetic prediction results. In the process simulations and LCA, four scenarios of AWD refinery with different pretreatment combinations with HTC and mechanical dewatering were designed. It was found that the different pretreatment processes could benefit the designed AWD refinery for different targets. The implementation of both HTC and mechanical dewatering pretreatment for refinery based on a 650 °C-pyrolysis presented the lowest CED value of 10.3 GJ for treating one ton of AWD. The least emission of carbon dioxide equivalents (-843.3 kg) was achieved in the case of using a 650 °C-pyrolysis temperature with the pretreatment with only a dewatering process when treating 1-ton dry AWD.