Biomass CHP

The thermal energy created by combusting biomass can be turned into electricity via a biomass CHP plant. This works by using some of the heat created to drive a turbine and the excess thermal energy is then used to provide either process heat or district heating.

The design of a biomass CHP plant should focus on meeting the thermal load requirement, as any plant will produce roughly 5 times more heat than electricity. The size of the biomass boiler will typically be 6 times the electrical output. This means that a 6MW biomass boiler can produce roughly 5MW of heat and 1MW of electricity.

We work with two principle CHP technologies that work effectively with our combustion biomass boilers, namely a Steam Turbine and an ORC Turbine.

Steam Turbine
The traditional technology used in thermal power plants involves the production of superheated steam. Pressure is produced at 28 to 36 bar at temperatures between 320°C and 360°C. This steam then drives the turbine which transfers the energy to the generator via gears and coupling. With the single-stage turbine used in small power plants, it is possible to obtain an electrical efficiency of 12-14% of the input energy.
The steam that emerges from the CHP unit is again condensed to water at a low pressure and temperature level, and is then fed back to the steam boiler. Waste heat from the condensation of the steam is frequently fed into a heating circuit such as district heating.

Steam turbines are generally suitable for biomass CHP plants with an electrical output of greater than 2MW.

ORC Turbine
The Organic Rankine Cycle (ORC) turbine is based along similar principles to a steam turbine but uses an organic silicon oil as the operating medium rather than steam. This oil has a lower evaporation point than water. As a result, operation at a lower temperature of 300°C and a working pressure of 1.0 bar is possible. The operating medium circulates in a closed circuit. The oil is vapourised by heat exchange with the thermal oil from the biomass boiler. The expansion of the oil within a stress free vacuum drives a slow-moving axial turbine which in turn drives a generator that is directly coupled to the turbine. The electrical efficiency is around 17% of the total energy input which is about 3% higher than traditional steam turbines.

The advantages of the ORC system are that it performs well under partial load and, because of the low working pressures and temperatures, it is not subject to the regulations of the pressure equipment directive (PED). ORC units are suitable for biomass CHP plants with an electrical output as small as 200kW. Single ORC units go up to 2MW in size and multiple units can be installed to increase capacity.