Small-Scale Biomass CHP Plant and District Heating

CHP potential in Finland has been evaluated to be 80 MW of electricity and 214 MW of heat with 6000 hours of annual peak load time based on the district heat production in biofuelled CHP plants in the year 2000. Part of this capacity is already built when this report has been written. The total amount of possible CHP units is 51. 90% of the CHP would be smaller than 10 MW in thermal effect. Most of the potential CHP plants (53%) would be 1-5 MW in thermal effect. An optimisation model was constructed describing production, distribution and consumption of heat. The energy production in CHP plants is based on simulations done for three different size plants, thus giving a wider perspective on the behaviour of such a plant. The results indicate, that the economically feasible scale for biofuel-fired CHP plants remains relatively large; when a biofuel-fired boiler is among the options for heat production, no investment is made in the smallest of three CHP plants. Specific investment costs for the two larger plants could be only slightly higher until no investment in them would be made either. Possibilities to increase the power production and the power-to-heat ratio of 1-20 MWe steam process CHP plants using biomass fuels were studied with simulation and optimisation tools. The basis for the simulations and optimisations was the data collected from Finnish and Swedish small-scale CHP plants between 2002 and 2004. Four existing CHP plants from Finland and Sweden were selected to represent a BAT technology in the different sizes of CHP processes. The results showed that the addition of a feed water preheater, a steam reheater, and a two-stage district heat exchanger is profitable with the current electricity price of 30 /MWh. This increases the power-to-heat ratios from 0.23-0.50 to 0.45-0.50 depending on the case plant. The considered integration of a gas engine needs electricity prices over 42-45 /MWh but increases the power-to-heat ratios to 0.59-0.78. The addition of a fuel dryer was connected to the integration of gas engine and gas turbine, as this provided extra heat to the flue gases that could be used for fuel drying. ORC process plant is evaluated as one possibility for small-scale biofuelled CHP power plant. Simulation is first modelled for a BAT technology ORC plant and secondly to explore possibilities of improvements by adding some new properties to the basic plant model. Possibilities to improve the basic process are to make the process starting point (pressure and temperature) higher after the boiler, to change the temperature of the preheated air and to use double circle organic process. Simulation results indicate that the most simple structure is the best choice for an ORC plant subjected to power-to-heat ratio. Only the combustion pre-heater has a positive effect on electricity efficiency when compared to the basic ORC plant, although pre-heater does not affect electricity production at all but it improves boiler efficiency. Any changes in the organic cycle, superheating or double cycle structure, only decrease the power-to-heat ratio.