Abstract
Denmark has a well-developed natural gas distribution grid and during the last decade a large number of decentralised CHP-plants (> 2000 MW total capacity) have been built around the country for industrial applications and district heating. The largest small scale CHP has an installed electrical capacity of 99 MW, while most of the plants have capacities from some hundred kW to 10 MW. The larger plants are all based on gas turbines while the smaller plants predominantly use lean-burn reciprocating engines. On the whole this development has been very successful and is supported by tax incitements for small power producers. Lean-burn engines have, however, some problems, e.g. with regard to UHC, so the emergence of small and micro-gas turbines from a number of producers offers an interesting possibility to extend the use of gas turbines down to the smaller plant sizes.
Two cases have been investigated involving smaller industries with a process steam demand.
The energy systems investigated are based on a 100 kW recuperated gas turbine (Turbec T100) and a 600 kW simple cycle gas turbine (Volvo Aero VT600). The steam and hot water is produced in a Waste Heat Recovery boiler (WHR). To obtain the sufficient steam production and sufficient steam quality it is necessary to use supplementary firing.
The analysis shows that a recuperated gas turbine has no particular advantages for industrial steam production plants. The low exhaust temperature after the recuperator is not sufficient to produce steam of proper quality.
When comparing a CHP plant with a natural gas fired steam boiler, it is found that a simple cycle gas turbine is more suitable for steam producing industrial energy supply systems.
Even if the net electric efficiency is lower for a simple cycle gas turbine, one can get a quite high, marginal net electric efficiency of about 60% (LHV) when replacing a gas fired boiler with a steam producing industrial CHP-plant. The marginal net electric efficiency is based on comparing the actual CHP plant with an existing gas fired steam producing boiler with an efficiency of 90% (LHV). This is a very high net electric efficiency compared to a centralised power plant.
The study shows that there is little doubt that CHP systems based on gas turbines are functional, efficient and environmentally friendly, but it is very difficult to achieve a satisfactory economy with the present relations between gas costs, electricity prices and gas turbine prices.
