The obvious co-generation opportunities of the past — where “free heat” and high electric bills or incoming power limitations made co-generation a good investment — have mostly been exercised. The electricity from byproduct heat or steam that couldn’t pass the payback hurdles then is now due for a second look, thanks to rising electric rates and the carbon footprint or CO2 tax cloud over industry.
When you realize cooling 12,000 SCFM of 1900°F hot gas to 400°F can get you approximately 20,000 pph of 600 psig superheated steam and a net electric output approaching 1,500 kW, your plant’s specific conditions may come to mind. It isn’t cheap kilowatts, but the new environmental concerns, rising electric rates and your site specific conditions may make it a good investment.
Venture Engineering studied the options for co-generation at similar industrial facilities in two states. The client has waste heat at over 1500F from multiple sources. That hot gas is currently cooled and cleaned before release to the atmosphere. If economical, the exhaust gas would pass through an intervening heat recovery steam generator before the final gas clean-up equipment. The recovered heat would reduce the current cooling load and be used to produce high pressure steam for a steam turbine generator. Electricity produced would be used inside the fence to supplement power from the utility and would reduce the plant’s carbon footprint.
When considering cogeneration, some key decisions need to be made:
- Vertical versus horizontal HRSG, as a function of space availability and access requirements.
- Synchronous versus induction generator, which has several impacts, but most importantly whether or not the generator is self exciting and can be used as stand-alone power source during power outages. At some sizes the choice is made for you because the generators are only synchronous.
- Condensing turbine versus backpressure turbine – a back pressure turbine simply acts as a pressure reducing valve, and needs at least 75 psi of drop and 2000 pph of steam to be practical. A condensing turbine uses a condenser at vacuum conditions to maximize pressure drop and thereby the energy recovered from the steam. The condensing turbine has more stages and generates more power in comparable steam inlet conditions.
- If using a condensing turbine, water cooled versus air cooled condenser, which has several impacts, including gross electric production and property required.
- Power export to the utility grid, or not, which has several impacts, affecting the electrical gear and system control.
- Use of vendor provided stand-alone controls, and degree of integration and monitoring to and with the plant control system.
- HRSG operating conditions: Metallurgy of the HRSG and associated piping changes with increasing temperature and pressure conditions.
- Water conditioning and treatment that may be needed to address the quality requirements of cogeneration steam, which is usually higher pressure and temperature than plant process steam.
- Foundation requirements; i.e., piles, floating slab, integration with existing.
Key components of the co-gen facility include:
- Heat Recovery Steam Generator (HRSG)
- Steam Turbine Generator (STG)
- Turbine Exhaust Steam Condenser
- Condenser Cooling (Wet or Dry Tower, Plant Process or Waste Water)
- Improved Water Treatment for Water/Steam Meant for the Steam Turbine
Like most 20th century production facilities, the process modifications and the addition of emission control devices has left only minimal room for new equipment. The resulting space constraints will require the steam turbine generator to be on an elevated platform with the condenser hanging below. The heat recovery boilers will be of a vertical design, also due to space constraints. A partial turbine bypass for supplemental process steam is also included.
Economic considerations include the capital and operating cost of the new equipment, and the cost of lost production during tie-ins, versus the current electric cost and the cost of an expanded electrical system. Space availability is the first hurdle for the project and if satisfied, the economics would be considered second. Preliminary equipment sizes, layout options and present worth analysis were performed.
Coordination with facility environmental staff is also important to determine the impact of the project on plant permits.
The bottom line: For the two facilities that Venture looked at, a steam cogeneration plant has a four year payback at an electricity cost of 10¢ per Kw/hr. If more real estate were available, a more favorable return would be possible. Consider cogeneration if your facility has the potential to generate 5-20 MW or more.
For more information on steam cogeneration, contact Robert Gambon at (612)231-5890 ext. 311.
Industrial Heat Sources for a chart listing Typica Industrial Waste Heat High/Medium Temperature Sources.