Techno-economic assessment of poly-3-hydroxybutyrate (PHB) production from methane-The case for thermophilic bioprocessing

- CH4 is abundant and relatively cheap; its use does not adversely impact food supply.
- A full process flowsheet for biopolymer from CH4 is developed.
- Techno economic assessment reveals that biopolymer production from CH4 is competitive.
- Heat removal to allow for mesophilic operation contributes nearly a third of the operating cost.
- Thermophilic methanotrophs could significantly reduce cooling costs.

A major obstacle preventing the large scale production of polyhydroxyalkanoates (PHAs) has been the lack of a reliable, low cost, large volume feedstock. The abundance and relatively low price of methane therefore marks it as a substrate of interest. This paper presents a techno-economic assessment of the production of poly-3-hydroxybutyrate (PHB) from methane. ASPEN Plus was used for process design and simulation. The design and economic evaluation is presented for production of 100,000 t/a PHB through methanotrophic fermentation and acetone-water solvent extraction. Production costs were estimated at $4.1-$6.8/kg PHA, which compares against a median price of $7.5/kg from other studies. Raw material costs are reduced from 30 to 50% of production for sugar feedstocks, to 22% of production for methane. A feature of the work is the revelation that heat removal from the two-stage bioreactor process contributes 28% of the operating cost. Thermophilic methanotrophs could allow the use of cooling water instead of refrigerant, reducing production costs to $3.2-5.4/kg PHA; it is noted that PHB producing thermophilic methanotrophs are yet to be isolated. Energy consumption for air compression and biomass drying were also identified as significant capital and operating costs and therefore optimisation of bioreactor height and pressure and biomass moisture content should be considered in future research.