Integrating bio-oil production from wood fuels to an existing heat and power plant - evaluation of energy and greenhouse gas performance in a Swedish case study

Abstract

Combined heat and power (CHP) production in combination with a district heating (DH) grid gives an energy-efficient use of wood fuels. The heat demand in the DH grid will, however, decline in the coming decades, and operators are seeking additional heat sinks. In this case study, the integration of a pyrolysis unit into an existing CHP plant was investigated as a possible solution. The retrofitted pyrolysis unit makes use of excess heat and yields a liquid bio-oil. Pyrolysis integrated with CHP production was shown to give a net energy yield of at least 80%, and to decrease the net heat output to the DH grid. The carbon footprint of the delivered heat was very low at maximum 1.6 g CO2eq/MJ. Prolonging the operation of the pyrolysis unit to periods without heat delivery to the DH grid would increase the use of existing installations, but at the cost of energy yields decreasing to 63–70%. Up to 2.8 PJLHV/yr crude bio-oil could be produced at the investigated CHP plant. The bio-oil was shown to have a low carbon footprint, 1.7–4.0 g CO2eq/MJLHV, which makes it attractive for the rapidly expanding transport biofuel market.