By Robynn Andracsek, P.E., Burns & McDonnell and Contributing Editor
Wind generation is touted as a clean solution to our focus on greenhouse gas emissions. However, while customers demand electricity 24/7, the wind blows sporadically. This leads to demand for a flexible power plant design capable of following wind generation. A solution is to balance wind generation with blocks of natural gas-fueled reciprocating engines.
Flexible power plant offer numerous benefits. Facilities are easier to permit. Typical plant sizes range from 30 MW to 200 MW using multiple engines from 5 MW to 16 MW each. These plants are modular, easily expandable and require little water. Engines offer wind following, continuous baseload operation and black start capabilities. Plants also provide ancillary services such as spinning and non-spinning reserve and regulation.
We’ll examine flexible power plant design, its permitting requirements and look at Midwest Energy’s Goodman Energy Center (GMEC) in Hays, Kansas.
Relatively new to the U.S. power generation mix is the gas-fired engine, built in large quantities of small megawatt units. These engines are four-stroke, lean-burn machines that are turbo charged, rather than naturally aspirated.
Vendors include Wärtsilä, Caterpillar and M.A.N. Caterpillar’s German-made MAK 34 engine was introduced in 2000 in a 16-cylinder spark gas version producing between 5 MW to 6 MW of generator power. M.A.N. owns Fairbanks-Morse and has a 32-cm bore dual fuel version of their standard diesel engine. Rolls-Royce, Kawasaki and Mitsubishi all have large gas-fired reciprocating engines in development or recent production.
The Wärtsilä 34SG lean-burn gas engine uses the Wärtsilä 32 diesel/heavy fuel engine frame with its advanced integrated lube oil and cooling water channels. The bore is 340 mm to fully utilize this engine block’s power potential. The air-fuel ratio is high and uniform throughout the cylinder, due to premixing the fuel and the air before introducing them into the cylinders. Maximum temperatures and nitrogen oxides (NOX ) formation are low, since the same specific heat quantity released by combustion is used to heat a larger mass of air.
Due to U.S. permitting requirements, the 34SG is normally installed 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).
Three main federal regulations apply 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 a year. As such, a greenfield facility can generally permit eight to 24 engines 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 a year of operation will require a PSD permit.
New Source Performance Standards (NSPS) are applicable to over 80 different 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 HP manufactured after July 1, 2007 and before July 1, 2010 for current installations. The 34SG meets these limits.
National Emission Standard for Hazardous Air Pollutants (NESHAP), also known as Maximum Achievable Control Technology (MACT) standards, 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. Under Subpart ZZZZ, the engines must reduce CO emissions by 93 percent or more or limit the formaldehyde exhaust concentration to 14 ppmvd or less at 15 percent O2. The 34SG meets these limits with an oxidation catalyst.
The 34SG is not subject to 40 CFR Part 75 Acid Rain regulations since each engine generates less than the threshold 25 MW. However, a new unit exemption application be submitted to EPA.
The Goodman Energy Center in Kansas began operating in September 2008 after a 16-month construction period. GMEC was permitted under a state permit and was not subject to PSD.
The plant has run for a variety of reasons, including providing energy when other generating resources were unavailable, providing less expensive energy (even when gas is more expensive than coal, the GMEC heat rate is comparable to that of a coal-fired unit), mitigating transmission issues and managing net hourly interchange in response to wind farm output changes.
Absent from this list is run times to support wind generation. So far GMEC hasn’t had to stabilize voltage, as the area hasn’t experienced wind facility-induced instability in transmission voltages. This is largely because GMEC is not in a control area or balancing authority. Accordingly, GMEC does not have to manage the area interchange at the level of adjustments every few seconds.
Spinning reserve is the operational mode that works with wind generation. When operating in this mode the plant operates at minimum load. Automatic signals from the grid dispatch center increase or decrease plant output depending on grid conditions.
Balancing wind generation is managed hourly. Having a plant like GMEC that can be started quickly and that has good ramp rates (10 to 15 minutes) makes this task much more feasible. With nine units, each rated 8.4 MW, GMEC provides flexibility in following changing wind conditions. The units perform well throughout the output range and very well at output levels above 60 percent.