Landfill methane is an especially nasty 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 use it beneficially becomes stronger, the technology is also evolving to better handle this application. Landfill gas from the Coffin Butte Landfill, owned by Allied Waste Inc. near Corvallis, OR, is used to power advanced generator technology. The Coffin Butte Resource Project is a landfill gas-to-energy plant, that works in partnership with Allied Waste. Both the EPA and the state of Oregon see deriving methane gas from landfills as renewable energy. Therefore, some tax advantages or energy credits exist for such projects. Because methane is a harmful greenhouse gas, using it for energy is desirable. Typically, the gas is either flared or put to some other use. Using it to produce electricity is a generally accepted and popular use.

In 1995, Power Resources Cooperative’s 12 electric-cooperative members built the Coffin Butte Resource Project. (From the north side of the butte, it does look somewhat like a coffin, even more so from the air.) The original project yielded 2.46 MW through the use of three G3516 Caterpillar landfill gas generator/engines installed at that time. That’s enough to power approximately 1,800 homes.

In 2007, two new G3520 Caterpillar landfill gas generator engines were installed, as well. Those engines have a total output of 3.2 MW—enough power for an average of 2,200 homes—for a plant total of 5.66 MW. Between all five generators, enough energy is generated to power a total of 4,000 homes. In reality, the power goes back into the grid. The figures for the homes powered, simply give a baseline equivalency for how much power 5.66 MW will supply.

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.

“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 the 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 12 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,” he adds. “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.”

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.

“The cubes are stacked on top of each other, and this is how they determine how they will move to the next lift,” King explains. “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 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. These are controlled through maintenance on the engines. Typically cylinder heads are pulled off each year in order to clean out the siloxane where it tends to build up in the combustion chambers. “For H2S [hydrogen sulfide], 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 G3520 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 25% efficient, and the 3520s are about 40% efficient; 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,” he adds. “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 Caterpillar engines are operator-friendly, and easy to set up and control several important parameters that are monitored. “These landfill gas facilities have really evolved into state-of-the-art systems, away from the ‘mom and pop’ operations of the past,” he says. “Now it’s a complex system with a complete SCADA [supervisory control and data acquisition] system involved, making it easy to monitor from home or from other remote locations.”

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,” he 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.”

The other aspect still being investigated is the emission rates. These appear to be turning out much, much lower on the G3520s compared to the G3516s. Nitrous oxide is at about 1.5 grams per horsepower hour on their G3516 engines. “Caterpillar has guaranteed us that, with the G3520, it will be less than 0.5 grams per horsepower hour,” King adds. “That was a real trigger for us, as we had to conduct a year-long meteorological data collection and modeling study on air quality, so we had to come in right on target for the EPA.”

A new SCADA system for the 3520s 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. Incase one of the engines goes down, they also have a new alarm system, which sends a text message to the operators’ cell phones.

Specifically for Landfills
In order to facilitate the upgrade to the new G3520Cs, Peterson Power took decision-makers with Coffin Butte to several other landfill sites, where the new engines were being run. Eventually, PGNC Power came to the determination that the G3520C was a good fit for them at the Coffin Butte Landfill. Now, they are online, running and making power from the methane gas at the landfill. The leap from the old G3516 model, an “A” model engine, to the G3520C, involved a change in technology with the passing of time. The G3516 model Caterpillar engines were engines converted to diesel from natural gas and placed in the landfill. “We started to intentionally work on landfill gas projects in 1984, with our G3516 product,” says Mike Devine, gas product marketing manager for Caterpillar.

“The G3516s are very reliable heavy-duty engine,” adds Marty Hopkins, sales representative with Peterson Power. “But, they weren’t specifically designed for landfill applications. The newer G3520Cs have a landfill package, including a corrosion-resistant aftercooler made of stainless steel and an ADEM III-based Gas Engine Control Module, that automatically regulates engine governing, air-to-fuel ratio and ignition.”

“Caterpillar took many steps in constructing an engine for landfill applications,” Hopkins says. “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 change was made from a 16-cylinder, G3516-producing 820 kW to this G3520, which is a 20-cylinder engine producing 1,600 kW. The increase in electricity, of kilowatt output was a real benefit to them. Caterpillar has been involved some 15 years with generation of energy from landfill methane gas. “This Oregon landfill gas-to-energy plant is a real success story,” Hopkins says. “They’ve had an up time of close to 96 percent. Employees at the plant have done an exceptional job, as the plant runs 24 hours a day. If the equipment needs attention at four in the morning, these guys are out there.”

With the landfill, it’s not just the engine: they also have to maintain the well field in order to keep a steady supply of gas. This is a challenge on such a site. “There is not a clean, natural gas pipeline entering the site and feeding the equipment with fuel,” Hopkins says. “This type of gas supply presents many challenges, due to the corrosive nature of the fuel.”

Caterpillar is not involved with pretreatment on this site. As with other landfill sites in Oregon, the water is simply removed. Siloxane involves the buildup of material in the engine, due to the methane. This can be a real problem. From a distributed generation prospective, this is not a lot different than a diesel or natural gas plant, according to Hopkins. “The major problem is that you have an unpredictable fuel supply requiring a great deal of maintenance. If a pipeline breaks and a great deal of oxygen gets into the line, there will be a decrease in the methane content of the fuel. This, in turn, drops the Btu value and the performance of the engine is compromised as a result.”

This is one of the differences between the G3516 and more advanced models. The G3516 can be manually adjusted down to where it can run at a little bit lower kilowatts if the fuel methane content dropped. At a certain level, things begin to become unstable and the engine would have to be shut down. Since the newer G3520 is electronically controlled, it contains fuel sensors, which can adjust the engine by opening up the throttle or decreasing it, based on the fuel quality. An operator will program what the system needs to see, and then the engine will adjust, based on the fuel coming in, to remain more stable while running. The first challenge is the fuel. There are blowers at the power plant which pull in the fuel. The water is removed, and then the fuel reaches the engines.

The engines themselves have remote radiators which sit outside, as do the silencers for the engines. But, the engines are located in a room. They operate at 4,160 V. These two new engines are running through Cat-paralleling switchgear. There is also, actually, an individual breaker for each generator in the switchgear, it feeds its own individual transformer, and they’re paralleled at the line.

These generators aren’t paralleled at the plant and fed that way, but, are instead, fed individually into the utility line. “In this case I think it’s just more of a customer preference, due to the fact that they were more comfortable with that setup,” Hopkins says. “As far as the engine itself, the controls are the big difference. These newer engines are operating at a higher compression—which gives them a higher kilowatt output as well.”

The sensors monitor perhaps close to 80 different points on the generator, according to Hopkins. 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. Caterpillar puts on an additional Electronic Control Module that exports data directly to their SCADA system. “Their SCADA system pulls in all the information off of our Caterpillar 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. They’ve also added thermal couplers 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 term gives them a bit more flexibility in being able to view information from a remote location when they’re not on the job site.”

The challenge with some landfills is that, if they don’t have a collection system in place, it’s hard to do such a project as the one at Coffin Butte. The total cubic feet per minute of methane consumed by the G3520 generators is 500 cubic feet per minute each, and for the G3516s is 300 cubic feet per minute. “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.”

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

The G3516s are perhaps 2 to 3 grams per brake-horsepower hour of nitrogen oxide, and the new ones are half grams of nitrogen oxide. “That’s a big deal, and, in fact, can be a deal breaker in some of these projects, even if the economics are good. If they can’t get an air permit, they can’t do the project,” Hopkins adds. “I would also add that the other 49 states are pretty similar to what we have here in Oregon. They all have a bit different local regulations, but tend to follow closely EPA guidelines. California is much more restrictive, and, at this point, it is very hard to place a landfill project in the state, though they are working hard there to ease things up a bit.”

Caterpillar provided the generator engine with the switchgear, in this case, making this a single-source product. This helped make startup go smoother, and, from Peterson Power’s prospective and the customer’s, if there is a problem, sometimes a technician will go out to a job site, thinking it’s a switchgear problem, when it was really a control panel in the engine or vice versa. What is helpful now is that a single source can be contacted and both ends can be taken care of. Caterpillar has supplied switchgear for more than 20 years. It interfaces easily with the Caterpillar control panel, because they’ve written the software for both. Therefore the integration is easy and smooth. “To me, the single source of product support is a really great feature,” Hopkins says.

The G3516 generators are induction generators, and the G3520 generators are synchronous generators. Because the former are induction generators, they don’t require the same type of switchgear as the others.

Tax Credits Come Into Play—Or Do They?
Because of recent tax credit options that the federal government has right now, there’s tremendous interest and speculation when it comes to landfills. “Due to the Section 45 credits, which are set to expire in December 2008, expectations are that these credits will be extended for two or more years beyond December 2008,” 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, PNGC Power, communications and marketing manager. “In Oregon and the rest of the country, that’s risen in importance,” she says. “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. 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