Decisions, decisions, decisions—from what movie to watch, to what bills to pay, to choosing what kind of engine to run the cogeneration plant. Engines generally come in two types: rich burn and lean burn.

By Diane McDilda

“Because there is more oxygen, or air, in the air-fuel mix, lean-burn engines burn less fuel. As fuel prices continue to increase, the economics of lean-burn engines makes sense for large users. Lean burn is a more recent technology than rich burn. So, where natural gas is used as the fuel, there are undoubtedly more rich-burn engines in stationary power-generation applications,” says Steve Strong, marketing manager for Miratech Corp., the Tulsa, OK–based company that manufactures emissions-control equipment for rich- and lean-burn engines.

Either choice, rich or lean, brings with it the issue of air emissions. The regulations are becoming ever more stringent, and meeting the goals is becoming ever more difficult. Here’s the story of one hospital’s rich-burn engines, the emissions issues it faced, and—spoiler alert—its success. A future article will present the learn-burn side of emissions control.

The Basics
During engine combustion, fuel is burned that generates electricity and heat while producing gaseous emissions, such as nitrogen oxides, carbon monoxide, and organics in the form of volatile organic compounds generically known as hydrocarbons. Rich-burn engines typically run with excess oxygen in their exhausts close to 0.5% oxygen. This is close to the stoichiometric point, the theoretical point where all the oxygen and fuel are consumed. In reality, attaining the absolute stoichiometric point is impossible, meaning products remain in the exhaust that must be treated. Because lean-burn engines run at a higher air-fuel ratio, with greater than 4% oxygen in the mix, emission treatment requires an additional step to meet regulatory compliance.

Catalytic converters came on the scene in the 1970s to treat car emissions from rich-burn engines. They’re aptly named three-way catalytic converters because they control three compounds from rich-burn stoichiometric engines: nitrogen oxides, carbon monoxide, and hydrocarbons. For lean-burn engines, the exhaust must be treated with selective catalytic reduction (SCR), which removes nitrogen oxides. SCR requires injecting either liquid ammonia or urea into the exhaust, which reacts with and reduces nitrogen oxides. Separately, an oxidation catalyst oxidizes carbon monoxide and volatile organic compounds.

An air-fuel–ratio controller adjusts the mixture before it enters the engine. Typically, oxygen levels are measured upstream of the catalytic converter and the ratio tweaked to get it as close to the stoichiometric point as possible. But newer technology includes an additional sensor downstream of the converter. The second sensor allows more precise tweaking of the air-fuel mixture, taking into account and optimizing the performance of the catalytic converter. This not only improves fuel efficiency; it’s better for the catalytic converter. While dual sensors have been installed in cars and other mobile air-emission sources, they’ve been late arrivers to most stationary sources.

SACH’s Story
In 1985, the San Antonio Community Hospital (SACH) installed its own cogeneration facility. SACH operates a 280-bed facility located in the city of Upland, CA. The city is in San Bernardino County, just east of Los Angeles County.

With cogeneration, or combined heat and power (CHP), the engine heat generated as part of electricity production isn’t wasted; it’s put to use. A study performed by Energy and Environmental Analysis Inc. in 2003 estimated that of the different business sectors analyzed (including hotels, hospitals, and restaurants), hospitals make up the majority of CHP facilities. It’s well suited for hospitals since they are large users of both electricity and heat for sterilization, laundry, and food preparation. A facility that incorporates CHP can reduce its electrical demand by 40% by eliminating electricity used to heat water.

More and more hospitals are choosing to generate their own power for a variety of reasons. “Hospitals are becoming increasingly aware that they need to have their own energy onsite,” says Tom Beach, technical consultant for the California Cogeneration Council. “With rolling blackouts, 9/11, and earthquakes, people [have realized] that they can’t entirely count on the grid. And nobody foresees the price of energy going down in California. Large institutions like hospitals with power demands 24 and seven can really benefit from making their own electricity and thermal energy.”

Photo: Miratech

In 1985, the SACH cogen system started with two V-12, 900-kW Waukesha L5790 GSI VHP-series engines, adding a third engine in 2001 to increase the hospital’s overall capacity.

The original cogen system at SACH included two V-12, 900-kW Waukesha L5790 GSI VHP-series engines. Being the norm at the time, rich-burn engines were chosen. An identical engine was added in 2001 to keep up with increased demands, allowing flexibility when an engine needed to be taken offline for routine maintenance and servicing. Future plans for the hospital include an expansion that will likely entail the installation of additional rich-burn engines. By staying with rich burn, SACH can maintain its consistency and avoid Selective Catalytic Reduction and related ammonia  dosing to treat nitrogen oxide emissions.

Bill Persi is the facilities director for SACH. “With three 900-kW engines we have a little redundancy,” Persi says. “Depending on the time of the year and the season we do use all three engines, but maintenance is required every 15,000 hours, and we need to take one generator down.”

The engines are capable of generating 2,700 kW of electricity. During winter months, hospital demands range from 1,700 kW to 1,800 kW, and during the summer months it increases to anywhere from 2,100 kW to 2,200 kW. When maintenance is needed, two engines can handle the winter load. Summer maintenance is usually scheduled for nighttime, when the load decreases significantly.

Even with the cost of the additional engine, the hospital is still saving money by generating its own power. “We still have a savings of about $1.2 to $1.3 million a year,” says Persi. “That’s even taking into account plant operation, manpower, and maintenance.”

The decision to generate power onsite had many advantages but brought with it a problem: namely, staying in compliance with stringent air-quality regulations. Air quality in the area falls under the auspices of the South Coast Air Quality Management District (SCAQMD), an entity that still admits it has “some of the most unhealthful air in the nation.” The SCAQMD manages the South Coast Air Basin, which includes the southwestern portion of San Bernardino County, parts of Los Angeles and Riverside counties, and all of Orange County.

As California began reining in air emissions early in 2000, just about the time the third engine was installed, permit conditions for both rich- and lean-burn engines faced challenges. At SACH, continuous emissions-monitoring systems (CEMSs) were now required. As mandated by the SCAQMD, emissions were to be measured every 15 seconds, with instantaneous data being logged and maintained. This meant any wiggle room in monitoring was gone. Compounds such as nitrogen oxides and carbon monoxide were now clearly visible on the regulator’s radar. Staying in compliance for SACH was further exacerbated as the SCAQMD increased the frequency of inspectors performing spot checks and reviewing data.

Nick Detor is the western regional sales manager for Miratech. “Monitoring is done on a continuous basis. So consistency is important to comply with limits,” Detor says. “Business has increased over the past two years as enforcement has increased. This is especially true in the four counties in the Los Angeles basin. Knowing they could be inspected anytime makes operators of stationary sources more amenable to improving emissions.”

Staying out of Trouble
Requirements vary for rich- and lean-burn engines, but both systems now require CEMSs. “A continuous emissions-monitoring system, or CEMS, is required for large gaseous- or liquid-fueled engines to demonstrate compliance with emissions limits. Specifically, District Rule 1110.2 requires that CEMSs be installed and maintained to monitor the nitrogen oxide emissions on engines with a brake horsepower of 1,000 or greater. In order to control emissions of nitrogen oxides, carbon monoxide, and volatile organic compounds, nonselective catalytic reduction [NSCR] is used on rich-burn engines, and selective catalytic reduction [SCR] with an oxidation catalyst is used on lean-burn engines,” explains Rob Castro, senior air-quality engineer with the SCAQMD.

In addition to adding the CEMSs, it was also mandated that each of the engines undergo a relative-accuracy test audit (RATA) every three years, performed by an outside party. The third-party testing accomplishes two goals. It confirms compliance at the time of testing and checks the accuracy of the data collected through CEMSs.

Before the third generator was installed, the system was already equipped with catalytic converters that were nearing the end of their expected service lives. This made it the right time to replace the catalytic converters for the two older engines and for the newer one as well.

That’s when Persi turned to Val Jensen, the Waukesha product manager for Valley Power Systems. “Newer and stiffer regulations and stricter enforcement are forcing our clients to look hard at how they’re arriving at compliance goals,” says Jensen. “There is still work to be done, but companies like Miratech are working hard to improve their products and achieve compliance goals.”

Exhaust from each of the SACH cogen engines now runs through a Miratech 3-Way Catalyst. It’s honeycombed with a surface coating of platinum group-metal catalysts. It’s known for its durability and easy maintenance. In addition to the catalytic converter, another inclusion in the treatment train was the air-fuel–ratio controller.

Compliance Controls is a joint effort between Miratech and FW Murphy. While Miratech focuses on equipment such as three-way catalytic converters for rich-burn engines and selective catalytic converters for lean-burn engines, Compliance Controls handles air-fuel–ratio control systems for both types of engines.

Jensen recommended Miratech and Compliance Control products be used at the SACH cogen plant. “Different controllers work for different applications,” Jensen says. “Some have a wide range, some a smaller range. Under the conditions here, Compliance Controls’ air-fuel control has performed well.”

The MEC-R air-fuel ratio controller (for rich-burn engines) maintains the delicate balance by measuring both the pre- and post-catalyst exhaust. Based on the readings, a signal is sent to the controller to adjust the mixture ratio. It measures oxygen levels using heated exhaust-gas oxygen sensors (HEGOs). HEGOs are commonly used in cars and other mobile air-emission sources, but the move to adopt them in stationary sources has been slow.

The air-fuel mixture is a delicate balance that can be upset by a change in temperature or engine load, or by a fluctuation in fuel quality. A difference in the mixture of just 1% can impede the performance of the catalytic converter and throw an engine out of compliance, particularly when measurements are collected continuously.

“The controller is matched to each individual engine because they all operate differently,” Persi explains. “Each controller is gauged to operate the engine at certain conditions. We programmed them in, and the controller takes care of it.”

The SCAQMD understands that maintaining continuous compliance is difficult, but with an emphasis on improving air quality in southern California and meeting federal air-attainment goals, it’s a priority. And one way to do that is by upgrading emissions-handling equipment. In many ways this means making stationary-source emissions equipment comparable to that designed for auto emissions.

“Permitting staff have been evaluating rich-burn engine projects equipped with catalytic converters and air-fuel–ratio controllers. Lately, the converters have been upgraded through better construction and increased catalyst. Improvements in the air-fuel–ratio controllers have focused on the pre- and post-oxygen sensors,” says Castro.

The controller’s ability to monitor and fine-tune the engine’s exhaust means operations go smoothly through the engine startup process. This is when rich fuel mixtures can easily result in excess fuel leaving the engine through the exhaust. Catalytic converters require a minimum temperature upwards of 700°F to operate, making the removal of the unburned fuel less likely.

“The MEC-R controller allows you to map and manipulate the system,” explains Jensen. “The emissions evaluate the exhaust and tweak the valve controller.” Jensen says that this can be done through the engine startup process, as the engine revs up.

Regulators understand the predicament, often accepting concentrations above regulatory limits during startup as par for the course. “Since it takes time for engines to warm up, the emissions are not effectively controlled during starts,” he says. “The time period for warmup varies. Depending on the size of the catalyst, the amount of insulation material, the amount of stack flow, and the load conditions of the engine, it could take from 10 to 30 minutes to reach optimal temperature. With newer sensors, we have seen more correct air-fuel–ratios while the engine load is varying.”

In addition to keeping emissions under control, the controller allows smoother operation of the whole system by adjusting the mixture for a variety of conditions. “Before, if we had any problems, if the fuel was little watered down, the engine would shut down. Now the controllers take over,” says Persi.

Mission Accomplished
In California, if a facility is found to be out of compliance, it has 24 hours to remedy the problem. If compliance is not met, the facility has to shut down its plant and purchase electricity through the grid, not an attractive solution.

Pat Runnels, technical service manager with Miratech and Compliance Controls,  educates and trains those responsible for operating rich-burn engines and air-fuel–ratio controllers. “SACH is a little different than most of our clients,” says Runnels. “It doesn’t buy any outside power, so it keeps a close eye on its system. A lot of our customers buy most of their power and generate some just to alleviate their dependency. SACH is self-contained. With the cost to buy power at twice what it takes for them to produce it, it watches everything.”

But staying off the grid and in compliance isn’t the sole driving force behind SACH’s desire to keep its facility up and running smoothly. “SACH has always been proactive,” Detor says. “They want to be good neighbors.”

And it looks like it’s succeeding. The SCAQMD required that each of the engines be taken offline one at a time and be retrofitted with CEMSs. The CEMSs have now been installed, and the engines are all back online. The SCAQMD has inspected the installation, and SACH is just waiting for the results.

John Bustos is the plant supervisor at SACH. Aside from the diligence he and his operators have put into maintenance, he believes that the Miratech equipment has helped them avoid regulatory problems.

Castro and other regulators see the difference when operators take care of their systems and provide routine maintenance. “While one component of achieving compliance is better technology, it should also be noted that engine operators have an important role to play. The majority of compliance problems can be prevented.”

Beyond helping the system stay within strict regulatory compliance, the air-fuel–ratio controller reduces the potential for other catastrophes. Unspent fuel in the exhaust, whether it occurs during startup or while the engine has leveled off, can cause a backfire.

Backfires can happen in different locations, from the engine through the catalytic converter or in the exhaust. The concussion caused by a backfire can reverberate through the system, damaging the equipment and possibly destroying the converter.

The MEC-R also tracks other components besides pre- and post-catalytic converter oxygen levels. It monitors engine load and other operating conditions. This improves performance, saves fuel, and eliminates unnecessary wear and tear, prolonging the life of the engine—all of which adds up to cost savings. The controller comes with a Windows-based platform that is capable of working with most computers, laptops, or networks. The software allows data to be plotted, a feature that can make troubleshooting much easier.

What the Future Holds
The SCAQMD is required to update its air-quality management plan every three years, and a revised draft is currently out for public comment. As it stands, Rule 1110.2 Emissions from Gaseous- and Liquid-Fueled Engines has amendments that not only illustrate changes to the onsite power environment but could affect operations at the cogen facility at SACH.

New phrases such as “rich-burn engine with a three-way catalyst” and “useful heat recovered” are now included in the regulations. But more importantly, allowable concentrations of nitrogen oxides, volatile organic compounds, and carbon monoxide will be sequentially reduced until the year 2011. Other proposed changes include more stringent monitoring and reporting requirements. Third-party RATA frequency will likely increase. Another proposed change specifies that a system found to be out of compliance for a period of 15 minutes must be reported within one hour. It’s also likely that systems required to have CEMSs will have to include air-fuel–ratio controllers with feedback controllers, meaning that SACH, with Miratech and Compliance Control equipment already installed, is ahead of the game.

It doesn’t come often, but one change that could be considered a relaxation of regulations addresses startup emissions. It’s been proposed that startup emissions requirements should be exempt until the engine is sufficiently warm. The catch? The start time cannot exceed 15 minutes, a time frame some systems may have difficulty meeting.

As part of preparing the draft plan, the SCAQMD also reviewed regulatory requirements of other air districts in California and Texas, Lake Michigan, and Midwestern states of Illinois, Indiana, Michigan, Ohio, and Wisconsin. The study determined that air regulations were more stringent in the South Coast basin.

No one expects air-quality regulations in southern Los Angeles or anywhere in the US to loosen anytime soon. “California, New York, and Massachusetts are also leading the way for stationary source emission regulations,” says Detor. “We can see their influence in other states like Texas, Colorado, Wyoming, Montana, and even Washington and Oregon. It’s spreading.”

For those in the business of air emissions in California and other parts of the US, take notice; stricter air emissions are likely on the way and the technology needed to meet them is here. 

Diane McDilda is an environmental engineer and writer living in Gainesville, FL.

DE - July/August 2007