Finding the perfect match

Designing a waste conversion plant is a multimillion dollar endeavor. Key among the decisions to be made is matching the engine to the project requirements.

“There is no single, right answer,” says Michael Devine, gas product marketing manager for Caterpillar Inc.’s Electric Power Division, Mossville, Illinois. Fuel, location and serviceability are the key factors that go into the generator selection process, he says. Local conditions become a wild card in any design formula.

That means a company or municipality has to have a good idea of the feedstock going in and what fuel is coming out. Can the system parallel with the grid? Or is the project focused at lowering the amount of energy pulled from the grid and offsetting power? How much does it cost to create that electricity?

A well-designed system can offer a payback period of four or five years. It’s all about the planning.

“Planning is not rocket-science. But you have to do it. Sizing is done up-front before project is started,” Devine says, noting that the process is typically part of the feasibility study work on the project.

Selecting the proper engine or generator is a crucial step in developing a system, says Daniel Dodd, vice president of engineering for Sierra Energy, Davis, California. “Many engine/generator companies have expanded their product offerings to include syngas-compatible engines. It is important to conduct trade-off analyses of the manufacturer’s offerings incorporating capital cost, electrical efficiencies and track-history, to ensure that the system developed is optimized for that individual client,” he says. The company’s FastOx gasifier creates an energy-dense syngas, high in hydrogen (H2) and low in nitrogen (N2).
 

Much to consider

Dodd runs down a tick list of factors to consider, starting with simulation of the syngas composition given the waste materials expected to be converted by the customer. This composition is then sent to the major packaged genset manufacturers who will return an engine frame size, efficiency and other important operating metrics including guaranteed emissions.

Then, a trade-off analysis must be undertaken to compare all product offerings. Evaluation criteria should include capital cost, package footprint (size), guaranteed emissions amounts, electrical efficiency, experience of the genset manufacturer with syngas and the experience of the local distributor with installing and maintaining the proposed genset package.

Selecting the proper engine is very important, both economically and technically, agrees Bill Bonkoski, executive sales leader at GE Water & Process Technologies, Trevose, Pennsylvania.

“From an economics perspective, the gas engine will be a substantial investment for the plant, so you need to ensure it is the correct size to process all of the biogas produced from the plant at maximum efficiency with the appropriate conversion to both electrical energy and heat,” says Bonkoski.

As a rule of thumb, based on plant capacity for receiving source-separated food waste, generator sizing should be 0.5 megawatts (MW) electricity output per 10,000 tons plant capacity.

The generator turndown as well as the number of generators must also be considered to ensure a comfortable level of redundancy and ability to handle any variation in biogas output, Bonkoski says. The engine will then either be full size based upon the expected gas production from the anaerobic digestion (AD) plant and the MW size of the connection to the grid or to the given offtake.

While Dodd cautions that every project and its parameters need to be examined individually, he says they typically recommend that distributed systems (approximately 100 tons per day and less) include a packaged lean-burn spark-ignition genset (provided by manufacturers such as Dresser-Rand Guascor, GE Jenbacher and Caterpillar) as often the most economical solution.

“It is possible to obtain these packaged SI engines in sizes up to 4MWe—and very high electrical efficiencies up to 42 percent,” Dodd says. “Having a bank of these identical genset packages in parallel can make a lot of sense for larger scale projects.

“For utility-sized projects we typically begin looking at Solar Turbines’ gas turbine packages that can be coupled to a downstream steam-cycle to obtain the high combined-cycle efficiencies,” Dodd says.

Although the engine is critical during design and planning, typically the prepackaged genset will be one of the last items to be placed into the system/site.

The desired end-product definitely has a bearing on the design and build decisions. The operator needs to define whether they will be producing electricity, combined heat and power or compressed natural gas.

A waste conversion project must first determine whether the plant intends to convert the gas produced into electricity for on-site use, sell to a private customer or sell it to the electrical grid, Bonkoski says.

An increasing trend is to sell the gas into the natural gas grids. Many factors influence this decision. Once this decision is made, then the engine size can be determined.
 

Keeping the engine running

Devine, who has 30 years of experience directly working with gas engines, says that maintenance is as important as proper construction. “It’s not something that one does and forgets about,” he says. “You need to keep maintenance procedures up. A system only makes money while generating kilowatt hours.”

Devine emphasizes that engines are 24/7, 365-day-per-year machines. You can’t forget about them. “If proper maintenance is not performed and the engine fails, it will usually fail big,” he says.

The key, then, is to catch problems before they hit. Especially on smaller operations, it is vital to have someone familiar with the system to keep an eye on maintenance. This could mean a maintenance contract. Not only will a maintenance deal eliminate one concern on the operation, but it will make your banker more comfortable that the financial outline will be realized.

Caterpillar figures that predictive maintenance, service and overhaul intervals will reduce service costs by up to 15 percent.

A lot of tricky factors affect engine life. Bright metals like aluminum and unprotected steel that are vulnerable to acid corrosion should be replaced in biogas systems. It is recommended aftercooler cores, made from aluminum in standard engines, be made of stainless steel in biogas systems. Likewise, connecting rod bearings should use brass instead of steel.

Rashael Parker, chief marketing officer at Sierra Energy, says the mechanical integrity of the metallic components in gensets is typically affected most greatly by the presence of H2S, causing corrosion when the H2S reacts with water condensate to form sulfuric acid.

“It is essential for the process engineers designing the system to provide the genset with clean syngas to not only reduce any H2S content of the syngas far below the genset manufacturer’s requirements, but to ensure that the syngas is dehydrated and then reheated to at least 30 degrees Celcius greater than the previous postdehydration dew point,” Parker says. This ensures that any final moisture present in the syngas will not condense out in the low-pressure areas within the genset.

Valve and valve seat angles should be increased to prevent formation of hard deposits that could prevent proper valve closure allowing combustion gases to escape.

Cooling system modifications typically start with elevated jacket water temperatures to help the engine handle the siloxanes that show up in the inputs. Bumping the temperature from the normal 210 degrees to 230 degrees helps prevent condensation of water which attracts the sulfur, chlorine and fluorine in the fuel. These elements morph into weak but damaging acids.
 

Taking the heat

Bonkoski says that if electricity is the desired end-product, a generator unit to maximize the electricity output should be selected. Electricity generators typically will have a waste heat component as a byproduct of the combustion process. Using it needs to be considered. While he says waste heat can be utilized in the AD process, he advises considering a means to deal with any excess.

“With combined heat and power units, a decision can be made to only take portions of the heat of the jacket of the engine and leave the heat from the engine exhaust to disperse in the environment,” he adds.

Running the engine at a higher temperature keeps water in the fuel entering the engine from condensing on cylinder liners and keeps crankcase ventilation gases from condensing on the engine block. That is bad because it could carry sulfuric acids into the lube oil.

As with any engine, it pays to be aware of oil. A typical 1,500-to-2,000-hour oil change interval may have to be cut in half because the materials in the oil that neutralize the acids will be absorbed more quickly. Acid oil creates engine problems.

Moisture is another concern. “Engines don’t like water,” Devine notes. “In almost any anaerobic digestion, you will have a lot of water.” The relative humidity in the engine needs to be 80 percent or less and it needs to be noncondensing.

Landfills will often add a chiller in the gas stream in order to knock out water as part of the design of a biogas system. In several cases, deliberately designing the pipes to run underground for cooling purposes, with gravity-fed bleeders to take off excess condensate moisture, has proved successful.

Most larger engine providers operate local offices or dealerships. Since there are literally thousands of such projects out there, it would be nearly impossible for a home office to stay on top of everything. A good local dealer with knowledgeable, trained maintenance staff is vital.

Parker advises to make sure the local distributor for the genset has the skillset to provide maintenance support and will do so on a time-and-materials basis for a minimum of 10 years from project initiation. “Obtain the service intervals and current parts costs, and ensure the escalating cost of genset maintenance is taken into account as part of the annual operating costs,” she says.

Bonkoski says typical western country source-segregated food waste generates 115 standard cubic feet per minute (scfm) of biogas per ton received. Biogas is typically 60-65 percent methane. “When considering generator sizing, the potential electricity output of the generator is always the sizing specification based on the biogas fed,” Bonkoski says.

Lastly, since engines are typically a long lead item for AD plants, Bonkoski advises procurement of them should occur as early as possible.

“With respect to installation, the engines will be installed concurrently with process equipment; however, the start-up of engines would typically occur once the AD process has been established and biogas is available,” he says.

The author is a contributing editor to Renewable Energy from Waste. He can be contacted at curt@curtharler.com.