Gas Technology: Gas turbines spell opportunity for industry
Getting Much More for Your Energy Dollar
Today many industrial energy users understand the opportunity implicit in cogeneration. The idea is that all thermal methods of electric generation also produce waste heat – a lot of it. Combined heat and power (CHP) installations capture that heat and use it as a valuable byproduct rather than rejecting it as waste. Gas turbine CHP plants are an increasingly popular way of meeting the electrical needs of industrial and institutional facilities, as well as their requirements for steam or hot water.
Not a New Technology
Gas turbines in various sizes have been available for more than 50 years. Fuels can include oil or industrial byproduct gases, however most installations use natural gas. Units can be used for direct-drive applications, such as for pipeline gas compression, but the most common usage is for electric generation. Within these systems, a portion of the mechanical energy generated by combustion is used in the combustion air compressor section of the turbine, and the remainder of the rotational energy spins a generator for electric generation.
With all gas turbines, a significant amount of energy is produced in the form of combustion waste heat. In typical electric utility gas turbine generation installations, this waste heat is sometimes simply exhausted. These are called single cycle installations. In other utility installations the heat is used to fire a heat recovery steam generator (HRSG) to generate steam for additional electric generation. These heat recovery combination systems are called combined-cycle plants.
Utilities Use as Peaking Generation
Utilities often use gas turbines for peaking generation or even for longer duty operation when they are combined-cycle plants. They are attractive to utilities because they are largely factory assembled and can be erected in a matter of weeks or months rather than requiring years as with more traditional forms of generation. Further, they can be started quickly and brought on line, often in less than one minute.
Increasingly, owners of industrial and institutional facilities are also looking at owning gas turbine equipment for on-site electric generation, and they are more likely to take full advantage of the byproduct heat output. These are called combined heat and power (CHP) installations.
Wide Range of Potential Uses
Examples of facilities that might be able to effectively use both the electrical and thermal outputs of a gas turbine are large hotels, resorts, prisons, universities, laundries, food processing plants, pulp and paper mills, and other industries that use either steam or hot water in significant quantities.
Another potential application is using the heat to supply absorption chillers to provide chilled water for comfort cooling or process applications. In addition to the energy-saving potential of cogeneration gas turbines, they also provide energy system security by having an onsite source of electric power in the event of a utility service outage.
Higher-Grade Byproduct Heat
Typically, gas turbines generate higher-temperature waste heat than engine-powered CHP systems, making high quality steam generation possible and broadening the range of potential applications. Although the electric generation efficiency of gas turbines has been steadily improving, the proportion of byproduct heat is still very significant. An example might be a natural gas-fired turbine rated at 5 MWe. The turbine exhaust can be diverted through a steam generator and can produce 23,000 lbs. of 150 psig steam per hour, enough to meet the heating and hot water needs of a good-sized industrial or university campus.
Several manufacturers, including Solar, Kawasaki, Opra, and Dresser-Rand, offer gas turbine electrical generation packages that range from 500 kWe to 10 MWe and even larger — sizes ideal for many institutional and industrial applications. Units much larger than this and microturbines rated below 500 kWe are also available.
Solar Offers Range of Units
Chris Lyons from Solar Turbines was recently a presenter at a Technology & Market Assessment Forum, sponsored by the Energy Solutions Center. Solar is a major manufacturer of gas turbine-generators for the industrial market and offers 11 models ranging in size from 1.2 to 21.7 MWe in capacity. He explains that the generation efficiency of gas turbines is generally expressed as their “heat rate”, the required number of Btus needed to generate one kWh. In the Solar family of gas turbines for electric generation in the industrial range, heat rates range from 8,774 to 14,025. Generally speaking, the larger turbine packages have lower heat rates, meaning they are more efficient.
Lyons points out that Solar has a world-wide presence in both the oil and gas industry and the power generation markets. “We have 43 service centers, including 13 overhaul centers and 19 parts centers. We have over 13,800 turbines installed in 98 countries.” Solar is a division of Caterpillar.
Potential Benefits to Economy
Lyons stresses the potential benefits of CHP. “DOE has set the goal of doubling the use of CHP in the U.S. In this way, an additional 25 million tons of CO2 could be avoided, grid reliability could be increased, line losses could be reduced by 5-20% and the utility investment to meet power production costs could be reduced by $136 billion. The advantages are enormous.”
Obviously, the more hours an industrial owner plans to operate a CHP plant, the more priority should placed on a low heat rate. However, most gas turbines are most efficient near their rated capacity. For this reason, your system efficiency may be highest if you have multiple smaller units, allowing you to operate several units closer to their optimum heat rate.
Sizing to Thermal Demand
In order to maximize CHP energy efficiency, owners generally size the system to match their need for process or comfort heat and cooling, and purchase the necessary additional electric energy. In some situations where there is high heat energy utilization, operators may be able to generate more electric power that can be used on site. In many such situations, the remaining electric energy can be sold back to the electric utility.
The advantages of CHP are becoming understood. In another recent presentation at a Technology & Market Assessment Forum, Kawasaki spokesman Mario DeRobertis noted that the largest market share is in gas turbine units 10 MW and below — the industrial market. He pointed out that Kawasaki has sold over 8,000 units — mostly into CHP projects. He indicated that owners typically experience thermodynamic efficiencies from 75% to 82%. Kawasaki offers base-load units sized at 600 kW, and 1.5, 1.7, 3.0, 5.5, 8.0 and 17.9 MW.
Documented Installation Shows Benefits
A well-documented installation of CHP in an industrial facility was a DOE project at a Frito-Lay food processing plant in Killingly, Connecticut. DOE notes that food processing is a high-growth industry with tremendous potential for CHP because of its need for significant amounts of both electrical and thermal energy. In this project, a 4.6 MW Solar Centaur 50 combustion turbine was installed, combined with a Rentech heat recovery steam generator. The system was commissioned in April 2009 and one year of data collection was completed in May 2010 with funding from DOE and a consortium of Energy Solution Center utility members.
A recent DOE report states, “Operating data show the CHP system providing over 90% of the electrical demand and about 80% of the steam load for the facility. The performance of the CHP system has been excellent, with virtually no interruptions in operation and an overall CHP efficiency of 70%.” On two occasions during the one year data collection period, the CHP system helped the plant maintain operations during extended periods of grid power outages. DOE indicates “This information will be valuable for understanding the role of CHP in providing economic and reliable energy services for small- and medium-sized industrial facilities.”
Time to Take a Hard Look
Industrial and institutional energy users that have both high electrical and thermal energy requirements need to take a hard look at gas turbine packages complete with heat recovery — CHP. The technology is mature and reliable. Natural gas prices are low and promise to remain attractive into the future. The door of opportunity is open.
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