Techno-economic assessment and environmental analysis of bio-compressed natural gas production from palm oil mill effluent / Nasrin Abu Bakar

Nasrin , Abu Bakar (2024) Techno-economic assessment and environmental analysis of bio-compressed natural gas production from palm oil mill effluent / Nasrin Abu Bakar. Masters thesis, Universiti Malaya.

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Abstract

There were 451 palm oil mills in Malaysia and these mills generated about 60 million m3 of palm oil mill effluent (POME) in the year 2021. Due to its high organic content, POME needs to be treated before being discharged to the water bodies within regulatory discharge limits. Conventionally, anaerobic digestion method via open ponding or tank systems are used for this purpose where the treatment efficiency is low and the biogas produced is not recovered. In the last two decades, capturing biogas using closed anaerobic digester are becoming acceptable practice as an integrated treatment of POME and biogas capture as a mean to reduce greenhouse gases (GHG). The captured biogas is typically used for heat and power generation. In recent time, upgrading biogas for biomethane or bio-compressed natural gas (Bio-CNG) production has emerged as an alternative to biogas utilisation in Malaysia. However, a detailed technical, economic and environmental assessment for commercial Bio-CNG production, is yet to be established in Malaysia, which forms the justification of this work. Potential biogas volume, installed electricity capacity and Bio-CNG production from entire palm oil mills (451 mills) in Malaysia in 2021 were approximately 1648 million m3, 508 MW and 988 million m3 Bio-CNG, respectively. A total of 135 mills were installed with biogas plant in 2021 and therefore only 33% of the full energy potential was realized. In terms of utilization, 87 mills utilise the biogas for electricity generation, 15 mills for steam or combined heat and power generation, and only a single mill for Bio-CNG production. However, 32 mills were just flaring the biogas generated without energy recovery and used it for the purpose of methane (CH4) emission mitigation strategy. As a proof of concept, a 400 m3/hr Bio-CNG plant was developed and evaluated in a palm oil mill located at Kuala Kubu Bahru, Selangor. The Bio-CNG production process which was based on combined biological and physical methods, and membrane technology achieved hydrogen sulphide (H2S) and carbon dioxide (CO2) removal efficiencies of 99 and 92.2%, respectively. The produced Bio-CNG was found to contain about 94 vol.%, 3 vol.%, 0.5 vol.% and 3 vol.% of CH4, CO2, O2 and N2 respectively with H2S at a trace level of 3 ppm, resulting in significant increase of calorific value from 20 MJ/m3 to 35 MJ/m3. These properties are also comparable to natural gas quality. The economic analysis conducted for Bio-CNG plant integrated with existing biogas plant indicated that an approximate 14% internal rate of return with a payback period about 6 years for a mill with 54 tonnes per hour capacity. The Life Cycle Impact Assessment carried out showed that the environmental impacts of the Bio-CNG production were global warming, fine particulate matters formation, fossil and mineral resources scarcity. These were due to the plants’ heavy dependence on the grid-connected electricity. In conclusion, Bio-CNG is technically, economically, and environmentally viable business alternative to biogas offsite utilisation.

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