(58d) Lessons in the Operation and Control of Steam Boilers for Power Production and Heating

Thermal energy for electric power generation is produced from a growing number of sources, including solar heating; waste, biomass and biofuels combustion; combustion of traditional fossil fuel sources; and heat from nuclear reactors. Thermal energy is converted into useful electric power by creating high pressure steam in a boiler and then dropping the pressure across a steam turbine to drive an electric power generator. Even when fossil fuels are used in conventional combustion turbines, the waste heat is increasingly used in a combined cycle heat recovery process to create steam to drive steam turbine generators. In northern climates, boiler plants are often used entirely for heating.

When creating steam for a turbine driven electric generator, the boiler plant must produce steam at a narrowly specified pressure and temperature. While there are numerous commercial designs, most boiler plants include a superheater, steam drum and economizer. Feed water is preheated in the economizer, which is essentially one or more heat exchangers located downstream in the exhaust gas to maximize energy recovery. The heated water then enters a boiler where the relevant thermal energy source further heats it to steam. In boiler plants serving steam turbine generators, the steam then flows into superheater coils nearest the radiant energy source to create the high temperature superheated steam employed in steam turbines.

The University of Connecticut (UConn) has a combined cycle cogeneration plant (Cogen) that generates all the electrical power for the campus under normal operating conditions. The UConn Cogen plant has a capacity of 25 MW of electricity and 200,000 lbs/hr of steam. The electric demand of the campus is met with water-cooled electric generators on three 7.5 MW gas turbines and one 5 MW steam turbine. Three heat recovery steam generators (HRSGs) located after the gas turbine generators each contain 600 psi steam systems and 125 psi steam systems. The 600 psi steam is used to run the steam turbine generator, while the 125 psi steam is used for campus heat in the winter and running chillers for campus cooling in the summer.

To provide students a motivating experience in plant operation and control, we have created dynamic simulations that mimic some of the critical process units from the UConn Cogen plant and that are typical of steam boiler plants. The simulations provide a visual presentation to the student that is similar to what the plant operators see in the UConn Cogen control room.

These simulations enable students to obtain hands-on experience in plant operation and control without risking the safety of people or equipment, wasting expensive fuel, and without distracting operations personnel from their daily jobs. Visits to the nearby Cogen facility help students make the connection between the computer simulation challenges and real world plant operations.

One unit the students study is liquid level control in a steam drum. This is a classical operating challenge in steam boiler plants because of the need to balance the interaction of control drum inventory, steam production rate and feed water makeup. Control strategies include conventional feedback loops from the PID family, as well as the potential to explore the benefits of cascade and feed forward control strategies on steam drum operation.

A second unit the students study is the operation of a heat recovery steam generator (HRSG) unit. The challenge with this operation is to run water through an economizer, steam drum and then superheater to maximize steam production at a specified temperature and pressure. Control challenges include controlling water flow to the desuperheater, as well as controlling temperature and pressure of steam exiting the HRSG.

Modern control installations are computer based, so the interactive color-graphic animations offer students a challenge that rivals that of a real plant. The simulations offer a virtual-world experience that contains a mix of theory and application as they provide an instructional companion useful in the classroom and for homework.

The courseware materials are currently employed in the process dynamics and control course at UConn and Kenai Peninsula College in Alaska. Supporting written materials and slides under development will permit others to exploit the simulations within their existing curriculum. A longer term goal it to expand the effort into the design class and across to mechanical engineering curriculum.