Alternative generation sources such as wind are becoming more options for power utilities who need to build a flexible portfolio that can supplement traditional fossil fuel generation.
Today’s electric power industry is challenged by growing client needs, environmental regulations and an increasing focus on global warming. Wind generation is touted as a clean solution. The reality, however, is that while customers demand electricity 24/7, the wind blows sporadically.
Wind resources can be balanced with a flexible power plant capable of following wind generation. Such a plant must be capable of operating at reduced loads, cycling frequently on and off, and delivering the dispatching ability that wind lacks. Simultaneously, such a plant must complement the clean power attribute of wind energy. One solution is to balance the peaks and valleys of wind generation with blocks of efficient, lean-burning, natural gas-fueled reciprocating engines. Typical plant sizes range from 30 megawatts (MW) to 200 MW, using multiple engines from 5 MW to 16 MW each. This type of flexible power plant has numerous additional benefits (see Table 1).
The gas-fired engine, built in large quantities of small MW units, is relatively new to the U.S. power-generation mix. These engines are four-stroke, lean-burn machines that are turbo charged, as opposed to naturally aspirated. There are a few vendors of these types of engines. Wärtsilä is one vendor. Other vendors not as prevalent in the U.S. include Caterpillar and MAN.
Rolls-Royce, Kawasaki and Mitsubishi also have large gas-fired reciprocating engines in development or recent production.
The Wärtsilä 34SG lean-burn gas engine uses the frame of the Wärtsilä 32 diesel/heavy fuel engine with its advanced integrated lube oil and cooling water channels. The bore has been increased to 340 mm to fully utilize the power potential of this engine block. The Wärtsilä 34SG combines high efficiency with low emissions. The air-fuel ratio is very high and is uniform throughout the cylinder due to premixing of fuel and air before introduction into the cylinders. Maximum temperatures and nitrogen oxide (NOx) formation are therefore low, since the same specific heat quantity released by combustion is used to heat a larger mass of air.
Due to permitting requirements, the 34SG is normally installed in the U.S. with a selective catalytic reduction (SCR) system for control of NOx and an oxidation catalyst for control of carbon monoxide (CO) and volatile organic compounds (VOC). Table 2 shows the nominal, controlled emissions of the 34SG engine in parts per million (ppm). Not shown in Table 2 are emissions of particulate matter (PM10), which can be 3 to 5 pounds per hour depending on loads and circumstances.
There are three main federal regulations applicable to the 34SG engines: New Source Review (NSR), New Source Performance Standards (NSPS) for Stationary Spark Ignition Internal Combustion Engines (Subpart JJJJ), and National Emission Standard for Hazardous Air Pollutants (NESHAP) for Stationary Reciprocating Internal Combustion Engines (Subpart ZZZZ).
In attainment areas, NSR is implemented through the Prevention of Significant Deterioration (PSD) program. For a greenfield, reciprocating engine facility, PSD is triggered if emissions of any single criteria pollutant exceed 250 tons per year (tpy). Therefore, a greenfield facility can permit eight to 24 engines, depending on limits and run times, without need for a PSD permit. For a brownfield facility that is already a major source for PSD, a single engine at 8,760 hours per year of operation will require a PSD permit. A PSD application consists of the following and takes nine to 18 months to permit:
- Determination of Best Available Control Technology (BACT) on a case-by-case basis, taking into account costs as well as energy, environmental and economic impacts
- Demonstration that the increase in emissions will not cause or contribute to an exceedance of the National Ambient Air Quality Standards (NAAQS) or PSD increment
- Analysis of the impairment, if any, to visibility, soils, vegetation and growth
NSPS are applicable to more than 80 types of equipment, including Subpart JJJJ for Stationary Spark Ignition Internal Combustion Engines. The Wärtsilä engines are subject to the NSPS Subpart JJJJ limits for non-emergency spark-ignited natural gas engines greater than 500 horsepower manufactured after July 1, 2007, and before July 1, 2010, for current installations. The applicable limits are as follows:
- 2 g NOx/horsepower-hour (160 parts per million by volume, dry (ppmvd) at 15% O2)
- 4 g CO/horsepower-hour (540 ppmvd at 15% O2)
- 1 g VOC/horsepower-hour (86 ppmvd at 15% O2)
The 34SG meets these limits.
NESHAP, also known as Maximum Achievable Control Technology (MACT) standards, are also applicable to numerous industries and types of equipment but specifically address hazardous air pollutants. The NESHAP for Stationary Reciprocating Internal Combustion Engines (Subpart ZZZZ) is applicable to spark ignition, four-stroke, lean-burn (4SLB) stationary, reciprocating, internal combustion engines, such as the 34SG. Per Table 2a of Subpart ZZZZ, the engines must reduce CO emissions by 93% or more or limit the concentration of formaldehyde in the exhaust to 14 ppmvd or less at 15% O2. The 34SG meets these limits with an oxidation catalyst.
The 34SG is not subject to the 40 CFR Part 75 Acid Rain regulations, since each engine generates less than the acid rain applicability threshold of 25 MW. However, a new unit exemption application form must be submitted to U.S. Environmental Protection Agency
Case Study: The Goodman Energy Center
The Goodman Energy Center began full commercial operation in September 2008 after a construction period of 16 months including installation of Wärtsilä 34SG engines (see Figure 1). The facility was permitted under a state permit and was not subject to PSD. During the 2008 summer months, the plant was run primarily for peaking and test purposes. Either the capacity/energy was needed and couldn’t be found elsewhere, or the cost of energy from the energy center was lower than what was available in the market or under other supply contracts.
Since then, the plant has been run for a variety of reasons, including:
- To supplant other resources that were unavailable or on forced/planned outage.
- To produce less expensive energy. (Even when gas is more expensive than coal, the heat rate at Goodman is comparable to that of a coal-fired unit.) A typical net plant higher heating value heat rate is less than 8,800 Btu per kilowatt-hour for this type of facility.
- As local generation to mitigate transmission issues, usually for planned outages of transmission lines.
- To manage net hourly interchange in response to rapid and unexpected changes in wind farm output.
- To replace generation or transmission schedules curtailed by the regional reliability coordinator.
Absent from this list are extended run times to support wind generation. So far, Goodman hasn’t been needed to stabilize voltage — the area hasn’t experienced instability in transmission voltages because of the wind facilities in the region. This is due largely to the fact that Midwest Energy is not an independent balancing authority but rather part of a larger balancing authority. Accordingly, the center does not have to manage the area interchange at the level of adjustments every few seconds.
Other environmentally friendly qualities of the plant include:
- Nearly-zero water consumption — crucial in arid western Kansas
- Low emissions — the air permit allows over 8,000 hours/year of operation at full load; it is not expected to run that much
- Use of vegetable oil as insulating oil in some transformers, avoiding the risks associated with an oil spill for those transformers
- Use of recycled crushed concrete both as an initial and final area and roadway surface material; all concrete was crushed locally as debris from other construction projects, greatly reducing the hauling of crushed rock material from distant quarries
- Use of flyash as a soil stabilization treatment during construction
Two facilities in Texas, similar in composition to Goodman, are in the final stages of receiving a PSD construction permit. Both facilities are existing PSD major facilities, and the addition of the gas-fired reciprocating engines necessitated a PSD construction permit. South Texas Electric Cooperative (STEC) in Pearsall, Texas, is applying to install 24 Wärtsilä 34SG engines at its Pearsall Power Plant. The draft permit reflects the BACT determinations (in grams per brake horse-power hour (g/bhp-hr)) made by the Texas Commission on Environmental Quality (TCEQ) as shown in Table 3. Ammonia slip will be permitted at a pound-per-hour rate equivalent to 10 ppm. The second facility is GEUS in Greenville, Texas. The Greenville Engine Plant is co-located with the Greenville Boiler Plant and will consist of three Wärtsilä 34SG engines, with a possible addition of three more engines in the future. An early draft of its permit shows BACT rates equal to STEC’s Pearsall plant.
Working with Wind
Spinning reserve is one operational mode that can work with wind generation. When operating in spinning reserve mode the plant is in operation at levels below its minimum load. Automatic signals from the grid dispatch center will increase the plant output if the grid requires additional energy. If system demand is falling and the plant is operating above its minimum load, plant output can be ramped down until the load is balanced or the plant’s minimum load is reached.
Balancing wind generation is managed on an hourly basis, not including any additional balancing for CPS1/CPS2 control performance standards. Having a plant like Goodman that can be started quickly with attractive ramp rates makes this management more feasible. With nine units, each rated 8.4 MW, the energy center provides flexibility in following changing wind conditions. This is due to the units performing well throughout the output range and very well at output levels above about 60%. Furthermore, units can be started or stopped fairly quickly, multiple times per day, with minimal impact on performance. This gives the Goodman center and Midwest Energy more flexibility than they would have with a single large combustion turbine (see Figures 2 and 3).
Keep in mind that the Goodman Energy Center has not been operating long and has limited run times. The best example of a windmill-following Wärtsilä plant with some operating history is Plains End I near Denver, which consists of 20 18V34SGLN engines for a total of 111 MW. This facility has been operating since May 2002. The Plains End II extension was commissioned in summer 2008, consisting of 14 20V34SG engines, bringing the facility to a total of 227 MW.
Plains End is owned by Cogentrix but is dispatched remotely from downtown Denver (about 25 miles away) by Xcel Energy. Plains End I has 5,000 to 6,000 running hours and is being dispatched to help mitigate windmill variability.
A facility such as the Goodman Energy Center complements wind generation by providing rapid cycling in response to wind’s unpredictable nature. Blocks of small, efficient, lean-burning natural gas-fueled reciprocating engines can provide needed flexibility to the U.S. power grid.
Table 1: Flexible Power Plants
- Easier to permit than traditional baseload power, even inside cities and nonattainment areas.
- Modular and easily expandable for future needs, requiring little or no water consumption.
- Engines are capable of wind following, continuous base load operation and a black start.
- Capable of providing ancillary services such as spinning and non-spinning reserve and up-and-down regulation.
Production or absorbtion of reactive power, a valuable commodity in an organized, ancillary-services market and important for stabilization of the transmission system in the vicinity of significant wind resources.
About the Authors
Robynn Andracsek, PE, is a senior environmental engineer at Burns & McDonnell. She specializes in air quality permitting for industrial and utility clients and is a frequent contributor to Power Engineering magazine. She received a bachelor's degree in mechanical engineering and a master's degree in environmental engineering at the University of Kansas.
William N. Dowling is vice president, energy management and supply for Midwest Energy Inc., a customer-owned electric and natural gas utility in central and western Kansas. With more than 30 years of industry experience, his responsibilities include resource and transmission planning, generation and control center operations. He also served as owners' project manager for construction of the Goodman Energy Center.
Wayne M. Elmore develops new power plant projects using Wärtsilä technology for the Midwest and southern United States. With 16 years of experience, he has worked on projects throughout the Americas and in Sweden. He has a bachelor's degree in electrical engineering from Virginia Tech.
Note: An excerpt from this article appeared in Robynn Andracsek's monthly column for Power Engineering magazine in March 2009.