Pyrolysis for producing Bio-oil as complement to CHP plants : Integration of biomass pyrolysis and bio-oil production in CHP plants for improved operational flexibility and energy system resilience. A techno-economic assessment.

Sammanfattning

This thesis work investigated how integration of pyrolysis into Mälarenergi’s CFB unit at their pilot plant, could improve capacity utilization and flexibility during underutilized periods. This study is a techno-economic study which was conducted by calculating thermodynamic mass-and energy balances which were assumed to be at steady-state condition, and through an economic assessment of three operating cases: 90 MW, 67.5 MW and 45 MW biomass feedstock input to the pyrolysis unit with 7000- and 5000 operating hours per year. Possible bio-oil upgrading methods involved an AC-field upgrading method and a catalytic hydrotreatment upgrading method. Three different feedstock options were used for the economic assessment: recovered wood, industrial byproducts and forest chips, and pellets. The results showed that the higher annual bio-oil production came from 90 MW case at 7000 operating hours, producing 85 428 tons of bio-oil per year. Annual energy output depends on the chemical composition of the biomass. Biomass composition 2 gives a significantly higher annual energy production compared to composition 1 because it has a higher carbon content and lower oxygen content. Unlike the other cases, the 45 MW is thermally self-sufficient which means that it does not require additional biomass or oxygen feed. The sand within the CFB is enough to support the pyrolysis heat demand. Upgrading improves the energy density of the bio-oil. AC-field upgrading method seems like the easiest upgrading because it does not require additional equipment or hydrogen unlike the catalytic hydrotreatment upgrading method. The latter produces the best quality product but at a greater cost with an economic uncertainty due to the hydrogen demand. The economic assessment showed that operating at 7000 hours lowers the production cost per kg bio-oil compared to operating at 5000 hours. The reason is that the fixed costs are distributed over a larger production which makes it cheaper. Recovered wood gave the cheapest and most favorable production cost. In conclusion, pyrolysis integration can make the CHP a polygeneration system. The economic viability depends on upgrading methods, operating hours, feedstock price and whether the bio-oil could be sold above its break-even production cost.