Landfill methane gas is a potent greenhouse gas, but, fortunately, it's also one that's renewable and usable for the creation of green energy.

By Peter Hildebrandt

As the push to capitalize on beneficial uses of landfill methane gas grows, the technology to do so is also evolving. A successful example can be found at the Valley Landfill Inc., also known as the Coffin Butte landfill, near Corvallis, OR. Owned by Allied Waste Inc., the Valley Landfill Inc. uses methane created by the landfill to power advanced generator set technology.

The naturally occurring methane gas, produced in landfills through the anaerobic digestion process, is captured and, in most cases, flared off to produce a less-potent greenhouse gas, carbon dioxide. This flaring process has been used for many years to safely dispose of the methane gas. Consuming the methane has the added benefit of converting the methane to carbon dioxide, reducing emissions into the atmosphere, since methane is “about 21 times more powerful at warming the atmosphere than carbon dioxide by weight,” as stated by the EPA. In recent years, landfill methane gas has become increasingly popular as a fuel source. Both the EPA and the state of Oregon categorize landfill methane gas as a renewable energy, and, as a result, some tax advantages or energy credits can be secured for power generation projects using it. 

With PNGC Power providing operational and management expertise, twelve electric cooperatives built the Coffin Butte Resource Project in 1995. The original project yielded 2.46 MW through the use of three Cat G3516C generator sets, delivering enough energy to power approximately 1,800 homes. 

In 2007, two Cat G3520C Low Energy Fuel Generator Sets were added to the facility. Combined, these new engines produce a total output of 3.2 MW, raising the total output of the facility to 5.66 MW—enough energy to power a total of 4,000 homes. This energy is sold back to the grid.

Fill Site With a Long History
The project is operated and managed by PNGC Power, which provides the expertise and employees. PNGC Power is a Portland, OR generation and transmission cooperative, providing power supply and ancillary services to 15 electric cooperative owners located in Oregon, Washington, and Idaho.

Photo: Alan Guggenheim

One of the two new 3520 Caterpillar engines

Photo: Kathi VanderZanden

The Coffin Butte Resource Project is a landfill gas-to-energy plant.

“Regarding the relationship between the landfill gas-to-energy plant and Allied Waste Inc., we’re in the business of producing electricity, and they’re in the business of providing space,” says Steve King, generation resources manager with PNGC Power at Coffin Butte Resource Project. “The division of responsibilities is really at the gate where they sell the gas to us. We operate and do minor maintenance on the well field itself, so we are actually on the landfill. But, Allied Waste is primarily responsible for the installation of all the gas wells and the connection up to the main line.”

The landfill has been on this site since World War II. It became a commercial site in the 1970’s, and the extended life of the landfill is estimated to be over 45 years. The landfill takes in approximately 550,000 tons of waste per year, which comes from twelve surrounding counties within a 75-mile radius of the site.

In 1995 when this plant was built, a 100-year plan was displayed to the operators, according to King. “But, in the landfill business it’s all about what you can get permitted when it comes to accepting solid waste,” he says. “The site is currently permitted for 45 years. As the waste decomposes and settles, they’re also able to go back in and go over areas that they’ve already filled in. The main point for us is that we have a long-term gas contract with the landfill to supply the gas, and in exchange we pay them a royalty. In conjunction with that long-term gas contract we’re assured of longevity.”

Approximately 12 to 15 new gas wells are installed in the landfill annually, so that the amount of available gas is continually being increased. At present, Coffin Butte contains 280 extraction wells on the site, varying in depth from 40 to 120 feet for the vertical wells. The horizontal wells are installed in 40-foot lifts and are contained in something referred to by King as “cubes of butter.”

“The cubes are stacked on top of each other and this is how they determine how they will move to the next lift,” King adds. “They are typically 40 feet high, 100 feet wide, and 200 feet long. As they are filling up an area, a trench is dug and a 6-inch perforated pipe is placed in the trench where it is surrounded with nice, clean round rock. The end of that pipe is hooked up to the main line valve. It is subsequently covered with refuse, and they continue on with these lifts, or cubes of butter, staggering them as they come back across.” 

After a height of 120 feet is reached (approximately three lifts), drillers come back in and drill the vertical wells. That is the point at which the wells average 40 to 120 feet in depth. The wells are connected to the main line, and the valve is used to control the extraction rate of the fuel, landfill gas. If fuel is over-pulled from the gas well, oxygen or nitrogen can be drawn in, creating a variety of problems. 

Handling the Gas
A primary concern with the use of methane from landfill gases is maintaining a high Btu rate. “This is a huge challenge for us,” King says. “It’s critical enough to our process that we have an employee with the responsibility of adjusting the gas wells. A reading at each one of the wells is taken twice monthly, and we must comply with any guidelines for how we adjust those wells. Well adjustment is critical.”

The oxygen can do a number of different things—it can destroy the methanogenic process and create the potential for an internal landfill fire. EPA guidelines are designed to prevent the intrusion of oxygen as much as possible. If a location in the landfill is found to have increasing oxygen, the first action taken is to decrease the flow. This, in turn, decreases the vacuum in the well, resulting in less pull from that well.

It is something of a balancing process, due to the fact that there is a push to optimize the methane gas coming from the well. The flow objective is to collect as much of the methane gas as possible and avoid pulling any oxygen. Oxygen can be drawn from the outside slope of where the pipe is located or alongside the pipe itself. The remedy is to simply decrease the flow.

Levels of Hydrogen Sulfide and siloxane are also managed at the plant. According to Caterpillar’s gas product marketing manager, Mike Devine, “There are two basic approaches for dealing with fuel contaminants in landfill gas-to-energy projects. The first is to pre-treat the landfill methane, taking out many of the contaminants, like H₂S [Hydrogen Sulfide] and siloxanes, to produce fuel that meets the engines’ operating requirements. The second is to forego all but the simplest pre-treatment and install engines specially modified to burn impure fuel, yet still deliver acceptable component life and maintenance intervals.” In essence, engine designers like Caterpillar accepted the realities of the corrosive fuels introduced to their engines, and then modified the design of critical components and systems to counteract the effects of many of these contaminants. 

At Coffin Butte, these contaminants are controlled through maintenance on the engines. Typically, cylinder heads are pulled off each year in order to clean out the siloxane deposits where it tends to build up in the combustion chambers. “For H₂S, a pretreatment is a good option,” King says. “But, currently, we are not set up for that.”

Coffin Butte is comparing the older G3516 Cat equipment with the newer G3520C engines and tracking operations to determine how closely the new engines are running, compared to the projected specifications. “This mostly involves the efficiencies of the engines,” King says. “We were advised that the two new engines would use the same amount of fuel as the three older ones, yet the net output for the newer ones would be higher. The G3516s are approximately 32% efficient, and the G3520s are about 40% efficient at the site conditions; this is what we are currently trying to evaluate. The testing phase ran from October 2007 to January 1, 2008, when commercial operations officially began. A few issues were being worked out onsite during those first weeks of operation. But, the main comparison we are studying with the two sets of engines is fuel consumption. We can maximize our economics.”

Solution to Higher Tech
From an operator’s standpoint, King finds the new Cat engines are operator-friendly and easy to set up, monitor, and control several important parameters. “These landfill gas facilities have really evolved into state-of-art systems, away from the ‘mom and pop’ operations of the past. Now it’s a complex system with a complete SCADA system involved, making it easy to monitor from home or from other remote locations.”

The sensors monitor close to 80 different indicators on the generator sets, according to Marty Hopkins, sales representative with Peterson Power. These include individual cylinder temperature, bearing temperatures, water temperature, and many other points of information obtained through the Cat control panel. The information is typically fed through the switchgear, from which the engine control module (ECM) exports data directly into Coffin Butte’s supervisory control and data acquisition (SCADA) system. “Their SCADA system pulls in all the information off of our Cat switchgear, as well as pulling information from our extra CCM [customer communication module],” Hopkins says. “The CCM allows the information that is available in the engine to be exported to the customer’s building management, or SCADA system.”

Due to systems on the engine, King and his staff are better able to control the output now, especially with the fluctuations in Btu content, due to oxygen intrusion occurring which can happen simply because of barometric pressure changes throughout the day. “If we’re not out there adjusting for that, sometimes we lose our power output,” King says. “In those cases, it’s subtle enough that daily change is not needed. But, what we’re finding is that the new control systems on the G3520s are running very steady; the engine is able to compensate for slight variations by taking in more fuel automatically. That’s controlled through the air-fuel ratio controls.”

SCADA system monitoring makes it easy to monitor the landfill gas facilities from home, or other remote locations.

To maintain stability while running, an operator programs the system, and the engine adjusts fuel flow needed to maintain full load based on the level of methane entering.  Additionally, the fuel filtration system is an integral part of the process. Blowers at the power plant pull in the fuel, and filters remove water from it. At this point, the fuel is then directed to the engines.

A new SCADA system for the G3520s was installed at the same time that the new generator engines were installed, in late 2007. King is looking into upgrading the SCADA system for the G3516 engines and integrating the entire system, as currently they are separate systems. In case one of the engines goes down, they also have a new alarm system, which sends a text message to the operators’ cell phones.

“This Oregon landfill-gas-to-energy plant is a real success story,” Hopkins says. “They’ve had generator set availability of close to 96%. Employees at the plant have done an excellent job, as the plant runs 24 hours a day. If the engines or support equipment needs attention at four in the morning, these guys are out there.”

Specifically for Landfills
To prepare for the addition of the new G3520Cs, Peterson Power, the Caterpillar dealer consulting on the expansion of the Coffin Butte Resource Project, took decision-makers to several other landfill sites where the new engines were running. Based on an assessment of these sites, PNGC Power came to the conclusion that the G3520C would be a good fit at Coffin Butte, as they concluded these new generator sets best demonstrated the technological advancements that had been developed since their purchase of the G3516 generator sets.

The Cat engines originally installed at Coffin Butte were initially designed as natural gas engines and have been adapted to use landfill gas. According to Devine, “We started to develop engines that were intentionally designed for landfill gas projects in 1984, with our G3516 product.”

“The G3516s are a very reliable, heavy-duty, gaseous-fueled engine,” Hopkins adds. “But, they weren’t initially designed specifically for landfill applications. Caterpillar took many steps in constructing an engine for landfill applications; that was the big step. They had the comfort level of what worked as a mechanical engine and what was very reliable.  But to take the step into what was an electronic engine, was something else. There are numerous advantages, and it was, in effect, something of an education process for our customers.

“The G3520Cs were designed as a landfill package,” Hopkins continues, “including many of the time-proven components, like the corrosion-resistant aftercooler made of stainless steel, the addition of corrosive-resistant bearings and metals in the engine, the elevated jacket water system temperature to prevent corrosion, and the positive crankcase ventilation system. The G3520C landfill engine also includes the ADEM III-based Gas ECM, that automatically regulates engine governing, air-to-fuel ratio, and ignition.”

The advantage of the change in technology is readily seen in the output of each generator set. While each 16-cylinder G3516 produces 820 kW, the 20-cylinder G3520C produces 1,600 kW. The increase in electric output was a great benefit to the Coffin Butte Resource Project. Not only is more power produced, but also component life is improved, and operating costs are lower on the G3520C. “I think this landfill gas-to-energy project is a good showcase, in that it reveals how much more efficient these engines can be constructed,” Hopkins says. “Emission levels on these engines are dramatically lower; these are much cleaner engines.”

Operating at 4,160 V, the engines are located inside the facility, while remote radiators and silencers remain outside. The two new engines run through Cat paralleling switchgear, assigning an individual breaker for each generator set. Each breaker feeds its own individual transformer, paralleled at the utility line. “They’ve also added thermal couples for testing water temperature. They gather that information, as well as their data, from their blowers. They monitor all this information on their SCADA system, which, in turn, gives them a bit more flexibility in being able to view information from a remote location when they’re not on the job site.”

For Coffin Butte, Caterpillar provided switchgear with the generator set engine as a fully integrated, single-source power system. This allowed for a smooth startup from both Peterson Power’s and the customer’s perspective. In the event of a technical issue, only one source needs to be contacted to address the entire generator set and switchgear power system.

Caterpillar has supplied switchgear for more than 20 years. It interfaces easily with the Cat control panel, because they’ve written the software for both, making the integration easy and smooth. “To me, the single source of product support is a really great feature,” Hopkins says. 

Tax Credits Come Into Play—Or Do They?
Because of recent tax credit options that the federal government is offering right now, there’s tremendous interest and speculation about the generation of power at landfills. “While Section 45 credits were set to expire in December 2008, many expect these credits will be extended for two or more years,” Hopkins adds. “But, of course, there is no guarantee of this either.”

Renewable resources, such as deriving energy from landfill gas, have grown more valuable lately, according to Kathi VanderZanden, communications and marketing manager at PNGC Power. “In Oregon and the rest of the country, that’s risen in importance. In Oregon, we have a new Renewable Portfolio Standard law, and such resources are looking more valuable by the minute.”

“We have been offering our members the option to purchase green power as far back as 1995,” King says. “This plant gives owners’ customers the option of helping to support green power projects.

“A number of our electric cooperative owners are able to sell out their shares of Coffin Butte Resource Project power,” he adds. One of our co-ops has regularly been in the top ten, nationally, for selling a percentage of their power to their members, as green power. The actual numbers aren’t huge, but the percentages, based on relative size, remain impressive.”

 Peter Hildebrandt specializes in science and engineering topics.

DE - May/June 2008