Consumer’s guide to generators & engines

Running a generator for a waste conversion process is like hiking in the woods. If you have a good map and know where you are going, the trek will be a pleasant experience. Blunder blindly into the forest and there are many a big bad wolf out there to get you.

Whether municipal or private, the investment in a generation system represents a huge capital outlay. Getting the most out of a system involves a host of issues—most of which can be engineered properly if they are defined ahead of time.

Depending on the source material and the required output, there are different approaches and degrees of difficulty to generate fuel economically.

Different feedstocks in a waste conversion system will affect the engine differently, and variations in feedstock need to be accounted for. Some operations are consistent with feedstock but most will see a range of materials running through the system. For instance, restaurant grease and grocery produce department waste both make good fuels for waste conversion, but each have different characteristics that may require system modifications.

Here is where the engineering departments—both at the vendor and at the site—need to be involved early.

“There is a recipe someone will come up with,” says Michael Devine, gas product marketing manager for Caterpillar Inc.’s Electric Power Division, Mossville, Illinois. “A combination of biomass being served to the anaerobic digester that will yield an acceptable methane to inert gas ratio in the biogas Getting it right is half the battle.”

The engine needs to know what is coming through the system. “Engines like consistency,” Devine says. “The key to success with any of these projects is up-time,” he emphasizes.

“It is key to ensure that the biogas quality going into an engine is consistent and that any mentioned contaminants are removed prior to biogas utilization in the engine,” says Bill Bonkoski, executive sales leader at GE Water & Process Technologies, Trevose, Pennsylvania. Having a robust preconditioning process ensures that the maintenance requirements, such as oil changes and frequency of wear parts change out are minimized.

“Adopting a philosophy of performing preventative maintenance will ensure that engines are always available when they are needed, and costly unforeseen repairs can be avoided,” Bonkoski adds.

Natural gas production is fairly simple. A system typically can produce what is known as “pipeline quality” gas economically and with consistency, sources say.

Davis, California-based gasification technology firm, Sierra Energy, provides an online calculator (www.sierraenergycorp.com/calculator), which calculates electricity outputs for different waste materials in its FastOx gasification system. Daniel Dodd, Sierra Energy’s vice president of engineering, says for postrecycled municipal solid waste (MSW), with a FastOx gasification system, a project would be looking at net export of 650 to 1,000 kilowatt hours net per metric ton of material gasified. His estimate is for small-scale projects (with a genset efficiency of 35 percent).

Within reasonable variances, most engines are designed to self-adjust. Material is mapped electronically. A move from 55 percent methane to 45 percent is within the tolerance range. Wider swings would require a change in the air-to-fuel ratio.

“It’s a pay me now or pay me later decision,” Devine says. “You can clean the fuel and spend money at a landfill taking siloxanes or hydrogen sulfide out,” he says.

Cleaning siloxanes can add a considerable chunk of change to the cost of a system. For a 1-megawatt system, it can run anywhere from $750,000 to $1 million for a cleaner.

Biogas—whether it comes from a landfill or an anaerobic digester on a farm or industrial site—is a bit more of a challenge, Devine notes.

Typically, dust, water, dirt and other contaminants will enter the fuel stream.

“Siloxane content is a critical variable in landfill and wastewater treatment plant biogas,” Devine says. Hydrogen sulfide is another.

While high-compression-ratio engines are preferred in biogas applications because they are more efficient, sites using high-compression-ratio engines will likely need to provide fuel siloxane treatment.

The British thermal units (Btus) produced per cubic foot—or the fuel’s heating value—will influence the size of the fuel delivery system.

Typically biogas originating from anaerobic digestion (AD) will contain 60 percent methane, 35 percent carbon dioxide and the balance in other gases.

“The volumetric energy content in the fuel is critical,” Devine says. So an operation with 30 percent methane content will have to double the gas flow to deliver the same energy…and this will expose the engine to twice as much impurities.

This is not necessarily awful—as long as the design parameters have taken this into account and the system is constantly adjusted for the differences.

The ratio of methane to free inert gases, including carbon dioxide and free nitrogen (nitrogen not entering naturally from air in the fuel), is another variable. Inert gases will inhibit fuel ignition, causing lean misfire and resulting in loss of power and increased exhaust emissions.

The carbon dioxide (CO₂) content, too, must be monitored. CO2 affects the flame speed in the cylinders and the temperature must fall below a certain maximum level to keep exhaust and valve temperatures in a safe operating range and minimize maintenance.

For FastOx gasification categorizes equipment in the following modules: feedstock preparation (shredding and sorting), oxygen generation, FastOx gasifier, gas conditioning, end-product generation and utilities/control room, according to Rashael Parker, Sierra Energy chief marketing officer.
 

Beware contamination

Most feedstocks will have contaminants and will produce hydrogen sulfide and siloxanes. Unfortunately, high-efficiency, high-tech engines are weaker at handling such contaminants.

“There are engines designed to handle those contaminants,” Devine says. These engines have hardened components and modified cooling systems.

Demand is solid. “We are definitely seeing an upward trend in the renewable energy from waste market,” Bonkoski says. Legislation banning organics as well as regulations to have renewables in energy producers’ portfolios are driving the market, he adds.

“In the last five years we’ve noticed a significant increase in engines and generators tailored for renewable energy from biogas,” agrees Parker. She notes several drivers pushing this demand, the most notable being legislation and incentives for the reduction of climate-warming pollutants.

Bonkoski adds, “With the costs of electricity only increasing, opportunities to provide additional supply to the grid, not only allows producers to gain revenue, but also provide certainty to population bases. “
 

Making it produce

Republic Services has a biodesulfurization plant in Apex, Nevada, that captures methane gas generated at the landfill through a vacuum system. The Apex regional landfill is designed to generate enough electricity to power 10,000 Nevada homes.

The vacuum blower pulls gas from the landfill and pushes it through a cooler which is then circulated to the contactor tower. In the contactor tower a biological reaction occurs between solution and the landfill gas.

The solution then travels back into the plant bioreactor tanks and eventually through a centrifuge which rotates the gas at high speeds removing sulfur from the solution. The booster blower then delivers the treated gas to Energenic’s power plant for further processing. Energenic is a joint business venture between Marina Energy LLC, a subsidiary of South Jersey Industries, Folsom, New Jersey, and DCO Energy LLC, Absecon, New Jersey.

The power plant receives the gas from the biodesulfurization plant through a channel of underground piping. The gas first enters a gas chiller which cools upon entry to the plant. The first stage compressor then increases the gas pressure from 4 to 80 pounds per square inch (psi). The gas conditioning system removes silicon-based organic compounds and delivers the gas to the second stage compressor where gas pressure is increased to high pressure (225 psi).

The combustion turbine and generator combusts the gas, creating exhaust gas that drives the turbine, which drives the generator and ultimately delivers electricity to the NV Energy power grid.

“When sizing generators, the conversion of the energy in the biogas to electricity and heat is key,” Bonkoski says. Typically combined heat and power (CHP) systems convert 40-42 percent of the energy in the biogas to electricity (this can be improved by about four percentage points by utilizing the engine exhaust heat in a GE ORC system). About 43 percent is heat.

Different digestion processes will have different heat requirements, and this will determine the requirement and complexity of the heat recovery system on the CHP system, Bonkoski says. When either upgraded “renewable natural gas” or compressed natural gas is the end product, electricity generators can still be used, however, a model to supply parasitic heat and electricity loads by burning natural gas from the grid will be implemented.

Again, it gets back to the initial engineering. Done properly, a generator’s feedstock analysis and output parameters become a fairly straightforward series of calculations. It might not be as easy as a walk in the park, but with the proper engineering it will be a rewarding experience.

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