Capture more turbine energy with CHP
In the evolving world of combined heat and power (CHP), the use of natural gas-fired industrial turbines rated at 1 MWe and higher with heat recovery has become increasingly attractive. These machines are built for long running hours, heavy loads and high fuel efficiency. Technology for extracting high temperature exhaust heat has also improved, so total system efficiencies of 80% and higher are often achievable.
CHP Is On the Rise
CHP applications powered by natural gas are burgeoning in popularity. One reason is the continued attractive price of the fuel. Ed Mardiat is a Principal and Director of Project Development for Burns & McDonnell, an international engineering, architectural and consulting firm. Mardiat has been extensively involved in a wide range of CHP projects.
He notes, “The abundance of and lower price for natural gas have dramatically helped CHP economics. The other related factor is environmental regulations such Industrial Boiler MACT [Maximum Achievable Control Technology] requirement, which is forcing many of the older coal-fired assets to be replaced with natural gas-fired units.”
Energy System Security
Mardiat also points out that there is growing interest in micro grids. “These in combination with on-site CHP systems can improve energy security. Many industrial, institutional and government facilities are exploring these alternatives.”
Turbine CHP can be used for a wide range of beneficial purposes. It makes the most sense to use turbines where the need for heat is large, the preferred thermal output is steam rather than hot water, the run-hours will be long, and the output is generally at the upper end of the turbine power range.
In a recent presentation at a Technology Marketing & Assessment Forum (TMAF) sponsored by the Energy Solutions Center, Chris Lyons from Solar Turbines and Mike Devine from Caterpillar discussed the opportunities for both large engines and combustion turbines for CHP applications. They noted that natural gas-powered CHP of both types is attractive now because of the low and stable price of the fuel, the need to control emissions, the proven nature of both technologies, high reliability and low life-cycle costs.
Choosing Between Turbines and Engines
Turbines are often the choice where the thermal load requirement is high, steam is preferable to hot water, and where the electric requirement is continuous and relatively even. These units produce large quantities of waste heat at temperatures ranging from 700° to 1,000° F – a range where high-energy steam can be efficiently extracted using a heat recovery steam generator (HRSG).
In certain applications, CHP using larger industrial turbines is an excellent fit. According to Lyons, common applications include food processing, dairy plants, breweries, pulp and paper, pharmaceuticals, district heating and cooling, colleges and universities, healthcare facilities, hotels and resorts, and penal institutions. “This is an ideal choice where there are major thermal or chilling loads.”
Take Advantage of Higher Temperatures
Mardiat from Burns & McDonnell echoes this opinion, saying, “Because turbine simple cycle heat rate efficiency is lower than reciprocating engine systems, with exhaust temperatures in the 700 to 1,000 degree F range, industrial, institutional and government facilities that require an abundance of steam for heating or process loads provide the best fit for this prime mover technology.” He points out that for most applications, it is important to make full use of the thermal output. “The highest efficiency and most economic CHP systems use 100% of the waste heat to provide steam or hot water.” This then results in very high total system efficiencies.
Range of Sizes Available
Solar offers turbines ranging in size from the 1.2 MWe Saturn 20 unit to the two-shaft Titan 250 unit rated at 21.7 MWe. In some situations, owners prefer to have multiple units to further increase system reliability, and to allow the selected units to operate near their peak efficiency.
Lyons points out that the smaller turbines do not have efficiencies as high as with larger units, ranging from 25% to 39%. “But in CHP applications, the overall thermal efficiency can range from 70% to 90%.” He suggests that most buyers size the units to match their thermal load requirement. “However the decision is also very dependent on overall electric rates and factors such as the need for plant reliability.”
Longer Service Intervals
Today’s combustion turbines have high reliability and designers have extended required service intervals. For Solar Turbines, Lyons explains, “Most of our turbines are designed for 30,000 hours between overhauls. However some of the smaller units go well beyond 40,000 hours.” He notes, “From time to time there are issues that might require attention to prevent a premature failure.”
Generally, combustion turbines operate at their peak electrical efficiency near full load. When at part load, a greater part of the fuel energy goes to the thermal load and less to the electrical. Lyons emphasizes that for combined heat and power situations, the efficiency at part load is still quite good. In situations where the turbine operates at a low level for an extended period of time, a supplemental heat source to the HRSG may be desirable.
Frito Lay Takes the Step
An example of an ideal installation of a combustion turbine in a CHP installation is at the Frito-Lay plant in Killingly, Connecticut. This large scale producer of potato and corn snack products uses daily a quarter-million pounds each of corn and potatoes in a 24-hour manufacturing operation, with a peak electrical demand of 3.8 MWe and a minimum of 1.5 MWe.
In addition, the facility has a peak annual steam demand for up to 90,000 lbs/hr of saturated steam at 325°F for process applications. The minimal steam demand is on summer weekends, at 12,000 lbs/hr.
Because the plant was in an area with power reliability problems caused by distribution constraints, the plant had experienced significant interruptions in service. This created issues in terms of lost production, wasted stock in production and required re-inspections. According to Christopher Wyse, Communications Manager for Frito-Lay North America, the area also has high electric rates.
Further, the company needed additional steam generation capacity to keep up with growing production. Wyse indicates that they knew that “CHPs are common in other industries and have a proven reliability record, provided the owner follows a proactive maintenance program.” Beginning in 2006, working with consulting support from Dana Technologies, they investigated and began the engineering and permitting process to install a CHP system powered by a combustion turbine with heat recovery for process steam.
Major Part of Plant Energy Needs
In July, 2008 they began construction. The system uses a Solar Turbines 4.6 MWe Centaur 50 turbine with heat recovery by a Rentech HRSG, along with a Coen supplemental duct burner for additional and standby steam capacity. The turbine can provide nearly 100% of the site electrical requirement, and releases heat to generate 24,000 lbs/hr of steam without supplementary firing. With supplemental firing, the unit produces 60,000 lbs/ hr. This represents nearly 90% of the site steam requirement. The company maintains a connection to the electrical utility and purchases a small amount of power to maintain the connection.
After a short and trouble-free startup and commissioning, the system went into operation in March, 2009. In the first two years since beginning operation, the turbine had 96.5% availability. In that period, the CHP project has saved the company $930,000 in operating costs, as well as assuring improved reliability and reduced emissions.
Taking into account an incentive payment from the State of Connecticut, the payback on the project was calculated at 6.9 years. The current low natural gas prices are rapidly lowering the expect payback period. Also, this does not include the benefits to production from improved electrical reliability. On several occasions the plant was spared significant operating losses during grid power interruptions.
Dual Fuel Where Appropriate
Manufacturers do offer combustion turbine units designed for dual fuel operation – usually natural gas and fuel oil. In most parts of North America natural gas has a significant advantage in price. Lyons from Solar Turbines notes, “Dual fuel applications are more prevalent in the Northeast of the United States, where most of the large users have interruptible gas supply contracts. However in places such as California, Texas, Louisiana, etc. dual fuel is not as common.” Where dual fuel is necessary, most owners have a few day’s supply of fuel oil on hand to use as needed.
Another provider of combustion turbines for industrial and large commercial users is Kawasaki Gas Turbines – Americas. Kawasaki offers base load turbines in seven size classes from 600 kWe to 18 MWe. In addition, Kawasaki has a separate line of standby power turbines in sizes from 750 kWe to 4.8 MWe. The base load units would be the equipment suitable for heat recovery and full-time CHP operation.
As an example of CHP benefits, Kawasaki installed a 1.5 MWe cogeneration plant in 2007 for a major East Coast pharmaceutical company. The unit was supplied with an unfired heat recovery boiler providing over 11,000 lbs an hour of steam to the plant. The plant incorporated a KGTA pre-wired extended electrical skid which facilitated a quick installation on site.
This plant installation provides power plus steam at an overall 80% thermal efficiency rating. This plant also qualified for a state grant for its CHP energy savings. This facility uses Kawasaki Dry Low Emissions (DLE) technology ensuring lowered exhaust emissions, with NOx less than 20 ppm and CO less than 50 ppm.
Consider Your Application
Not every application will benefit from combustion turbine CHP, but if you have major thermal requirements for process steam or absorption cooling, plus a relatively continuous need for electric power, it may be your solution. An onsite plant can improve your power supply reliability, and could also lower your carbon footprint and reduce total emissions.
Help is Available
Ed Mardiat indicates that there are multiple sources for guidance in developing a CHP installation. “The first place I would recommend is the U.S. EPA CHP Partnership, which has developed several resources for CHP screening, spark spread analysis, emission calculators and a database for CHP incentives listed by state. Beyond that, I would recommend the owner find an engineering consultant that has experience preparing CHP Feasibility and Economic Investment Grade Audits at the specific type of facilities being considered.” For large energy users, industrial turbine CHP is here, and it’s better than ever.
– This story appeared in the Summer 2013 Gas & Technology supplement. See additional stories below.