Making Industrial Processes More Cost Efficient

Thermal Power Plant

More efficient operation of coal fired power plants using nonlinear models

Coal fired power plants should be operated in such a way that the emissions are kept clearly below desired limits and the combustion efficiency is as high as can be achieved. This requires a lot of quantitative knowledge of the effects of the process variables and fuel characteristics on the emissions and efficiency. Mathematical models can be developed with different approaches. Physical models are too slow to be used for on-line process guidance, and require too many assumptions and simplifications. It is feasible to develop empirical or semi-empirical models from normal production data of the power plant. This technical communication explains with an example of a coal fired power plant how nonlinear models are an effective means of determining the best operating conditions at any given load for a given type of coal.

Coal fired power plants are looked upon as dirty by the general public, partly because that is how most coal fired power plants used to be operated just a couple of decades ago. Pressure from the public as well as environmental regulations have improved the situation significantly, and today, technology is capable of producing power from coal with very modest emissions with a fairly good combustion efficiency. Emissions can be reduced by either adding new equipment like more efficient burners, or by better operation of the power plant by better tuning of the process variables. This article focusses on the latter. Even if new equipment is added, the opportunity to operate it more efficiently remains. In addition to environmental regulations, power plant managers are under constant pressure to improve their production economics, which has only increased with the sharp increase in fuel prices in the last two years.

Turku receives much of its electricity from Fortum's power plant in Naantali, built in 1964. It has been generating electric power for about 50 years. The Naantali power plant comprises three 46 metre high boilers of the once-through type. Russian coal is the main fuel, while oil is normally used as the starting fuel. Saw dust is also added to coal, which accounts for at the most 2% of the energy.

The Naantali 2 boiler was supplied in 1964 by Gebr. Sulzer AG, and turbogenerators by Kraftwerk Union of Siemens. The boiler can burn upto about 44 tons/hour (worth 315 MW) of coal, producing either 90 MW electricity and 175 MW heat, or 120 MW electricity. The boiler typically produces 117 kg of steam per second at a pressure of 180 bar, and a temperature of 535 °C. The turbogenerator can produce between 40 and 120 MW electricity, depending on the output of steam and district heat. The steam output, meant for industrial consumption, can be upto 50 MW.