Biomass, Renewables

Generating Electricity from the Forest Floor

Issue 10 and Volume 119.

By Tim Miser, Associate Editor

A spider web of material-handling infrastructure feeds the stoker biomass boiler at the 160-MWth Port of Stockton biomass plant. The California facility is owned by Detroit Edison Energy Services. Photo courtesy: ESI

It’s an unusual sight—a semi truck raised on a ramp to a near-vertical orientation, its windshield pointing toward the clouds like a space ship ready to ascend into the heavens. In fact, there’s nothing unusual going on here at all, just the very terrestrial spectacle of trucks being emptied of their contents at the Savanna River Site Biomass Cogeneration Facility. Dozens of times a day, these trucks deliver wood chips to the steam and electricity generation facility to be burned as fuel in the plant’s biomass-fired boilers. It takes about six minutes to empty 40 tons of wood chips into the stockpile of biomass on the grounds. The fuel is mostly sourced as waste from the forestry industry, tree tops and other timber deemed unsaleable and otherwise left to rot on forest floors.

Located at an inactive nuclear power plant near Aiken, S.C., the 34-acre Savanna River facility has been in operation since January 2012. It’s the product of an Energy Savings Performance Contract (ESPC) awarded by the U.S. Department of Energy (DOE) to Ameresco to finance, design, construct, operate, maintain, and fuel a biomass cogeneration facility under a 19-year fixed-price contract valued at $795 million.

Your grandpa’s woodstove this is not. The plant mixes woodchips with shredded tires to incinerate them in an 800-degree firestorm of blower-suspended sand, combusting about 285,000 tons of forest residue per year to power a steam turbine capable of generating 20 MW of power. It’s the largest woodchip biomass plant in the state, and the largest energy efficiency project in the federal government’s history.

The Biomass Industry

Biomass isn’t as flashy as other more modern sources of renewable energy. The idea simply isn’t new or exotic enough to get the big headlines. After all, mankind has burned wood for its energy needs since time immemorial, so the concept is well-worn, maybe even a little boring. But precisely because of this long history, biomass power generation is a mature industry, reliant on technologies that are time-tested and imminently dependable.

For all its conceptual familiarity, though, the term biomass can prove deceptively complex, especially in the context of power generation, encompassing as it does a host of technologies and methodologies used to convert biological, mostly plant-based materials into the heat that is required to power turbines that generate electricity.

This complexity is due in part to the fact that, in addition to being burned directly to fire boilers and power turbines, biomass can also be converted into biofuels like ethanol, which in turn are burned to fuel the power generation process. To date, wood products constitute the largest single source of biomass fuel. Broadening the scope of biomass to include biofuels, however, opens up the race to many crops like hemp, corn, sorghum, sugarcane, and bamboo, all grown for the express purpose of later conversion to combustible fuels.

“There are a lot of different ways to classify biomass,” says Mitch Hayes, a professional engineer and vice president at ESI Inc. of Tennessee. “When most people hear the word biomass, they think of woody green material—essentially trees. And in fact, the overwhelming majority of facilities built to convert biomass into energy use trees for combustion. This includes waste wood like tree tops and limbs from logging, sawdust and other refuse from sawmills, and bark from paper mills. In some cases, green trees are even cut down for express use as biomass fuel, but this is very rare. Typically, the meat of a tree is more valuable as a raw product than it is as a fuel.”

But there are also other types of useful biomass, explains Hayes, including agricultural products, animal waste, farm waste, and alternative fuel crops like those left over after sugar cane is rendered into sugar. “These are opportunity fuels made from waste products,” he says. “They are used to make biofuel because they do not provide greater utility as something else like fertilizer.”

The Biomass Market

While the biomass generation industry is mature, the market for biomass fuel itself remains less developed. “These are often handshake deals,” Hayes says. “It’s very difficult to simply go out and locate a market for biomass fuels. You basically need to be in manufacturing, or next door to somebody who is. I think the biomass fuel market will always be an informal one,” he continues. “If it was going to become more formalized, it would have done so in the last ten years when biomass was the king of the green energy revolution.”

Part of the informality of the market can be explained by a simple reality; industries that create materials which are not useful to them are accustomed to landfilling them as waste. They do not consider these materials to be commodities, and so have to be convinced that they are better off paying a biomass facility to haul away their waste than they are paying a landfill to inter it. “This is what kills a lot of biomass startups,” says Hayes. “As soon as a biomass operation offers to haul away an industry’s waste biomass for a particular fee, the landfill that had previously been accepting that waste underbids the biomass operation, offering to accept that same waste for less money. The company generating that waste then opts to dispose of their biomass at the cheaper rate, and the biofuel company fails to progress from the planning stages to actual operation.”

Political climates and economics can also drive industry away from biomass and toward other fuels like natural gas. “Trends in power generation are cyclical,” say Hayes. “When natural gas prices are low, natural gas becomes the preferred fuel. When various political and economic forces align and those prices rise again, biomass might again look like a more attractive option. Just two or three years ago, 80 or 90 percent of the phone calls ESI received were about biomass. Now those same calls are about natural gas.”

Hayes also sees other challenges to the biomass market. “Because of the variability of terrain across the country, and the differences in biomass potential that result from this variability,” he explains, “it is not possible to franchise a single biomass solution across all locations. It’s simply too geographically dependent. Biomass resources might prove abundant in a wood basket that is hundreds of miles in diameter, but those same resources are much scarcer in places like West Texas and Arizona.”

Biomass as Renewable Energy

Renewable energy gets a lot of attention these days. With the Environmental Protection Agency’s (EPA) announcement in August of the Obama administration’s Clean Power Plan and its attendant regulation of carbon emissions from existing power plants, any source of energy that is cleaner than coal makes for interesting conversation. Solar and wind power take the lion’s share of the headlines it seems, but across the country biomass facilities are playing a valuable role in the generation of clean energy.

Twin biomass-fueled bubbling fluidized beds (BFB) power the award-winning 20MWe Ameresco facility at the DOE Savannah River Site.Photo courtesy: ESI

Wind and solar energy resources have a reputation of being very clean, but too often this perception fails to take into account the manufacturing processes required to produce wind turbines, solar cells, and batteries, which are themselves carbon-intensive. “Once you account for these variables,” says Hayes, “solar is not nearly as clean as we’d like to think. The reality is, as long as we want to turn on the lights, run our air conditioners, and plug in our iPods at night, there’s no such thing as clean energy. The best we can do is to become clean-ER.”

Hayes further notes that power can be generated from biomass around the clock, as a mature technology that is not dependent on weather conditions to provide electricity. “The intermittency issues of wind and solar create real problems for those technologies,” he says. “By the time enough batteries are manufactured to bridge these intermittency issues, the environmental footprint of these so-called clean technologies is much larger than that of biomass. Additionally, no quantity of batteries will make wind and solar energy dependable enough to meet the needs of places like hospitals, which require uninterrupted power every day, all day.” Hayes also notes that biomass requires trees to be grown, which is never a bad thing. “True, those trees are later harvested,” he continues, “but that harvesting process is not nearly as invasive as drilling and fracking for natural gas. In this context, it’s easy to see how biomass is a high-value green energy fuel.”

The Savanna River plant serves as a good example of the environmental advantage of biomass. Intended to replace older, less efficient coal-powered assets, the biomass facility is by itself responsible for a reduction in particulate matter emissions amounting to 400 tons per year. Sulfur dioxide (SO2) emissions are reduced by 3,500 tons per year, and carbon dioxide (CO2) emissions are reduced by 100,000 tons per year.

Biomass Technologies

Woody biomass is typically burned in one of two ways, using a stoker technology for dryer biomass, or bubbling fluidized bed (BFB) technology for woody biomass with higher moisture content. “All wood has some percentage of moisture in it,” says Hayes. “When a tree is cut down, it’s somewhere between 35 and 55 percent moisture, just as human beings are about 70 percent moisture. After trees are cut, they can actually become wetter if they are in a wet climate like the Pacific Northwest. Then they can reach 65 or 70 percent moisture, because they are no longer expelling moisture, but they continue to collect it. On the other side of this spectrum, in very dry climates like the American Southwest, cut trees might drop to 20 or 25 percent moisture.”

When wood is fired, this moisture must first be boiled away. “The stoker technology that we use to burn dryer fuel is older than BFB technology,” explains Hayes. “Essentially, it’s a large grate like the one in your backyard grill. Unlike in your backyard grill though, the wood is actually placed on top of the grate. Air can then flow up from underneath the biomass. Combustion must be initiated using another fuel like natural gas, but once the wood is lit, it is self-sustaining.”

BFB technology is used to burn woody biomass with higher moisture content. “A BFB system is essentially a bed of sand that is heated using natural gas to as hot as 1200 degrees,” says Hayes. “The sand maintains a thermal mass that is sufficiently hot to prevent new loads of woody biomass from extinguishing the wood that is already burning.” Hayes notes there is also a technology called a circulating fluidized bed (CFB). Unlike a BFB system in which the sand effectively bubbles at the bottom of the combustion chamber, a CFB system fully suspends the sand so that it flows through a closed loop. “This is typically used when other fuels are employed that have higher heating values like tire-derived fuels or coal,” he says.

Woody biomass is typically chipped to a size that will pass through a sieve with a three-by-three-inch mesh. This helps to control combustion better. It also makes the wood easier to transport. “It would be very difficult to get a limb up into a furnace,” says Hayes, “and to then repeat this 40,000 times an hour.” Chips also help the biomass to flow more readily, though Hayes is careful to note that chips have an amazing ability to bridge a gap by interlocking with one another. “They almost have to be dropped through a near-vertical chute that becomes larger at the bottom to prevent clogs that block the entire system,” he says. “It is very difficult to get wood to truly flow. In fact, the material handling systems are the overwhelming failure point at biomass plants.”

There are alternatives to burning biomass directly, though these alternative methods comprise only a minority volume of the industry. Methods include pelletizing wood products like sawdust for transportation to other locations like Europe, which are in need of alternatives to coal. “The pelletizing process removes a great deal of the moisture from the biomass,” says Hayes, “This is important because shipping costs are based on weight.“

The creation of biofuel represents another alternative to burning biomass directly. “There are several ways to use biomass to create biofuels,” explains Hayes. “The type of biomass feed stock entirely determines the biofuel end product. That means a biofuel plant that is trying to produce a specific type of biofuel is dependent on a specific type of biomass feed stock for its processes. You can’t simply substitute a different type of plant matter and expect to create the same type of biofuel. So depending on the type of feed stock available in a given geographic location, a plant must rely on a specific process to convert that biomass to the desired biofuel.”

A number of processes can be used to convert biomass to biofuel including pyrolysis, biological reduction, and gasification. “The creation of liquid biofuel typically involves putting biomass through a chemical process like pyrolysis, in which the carbon and hydrogen in the wood are separated from the other components in the feed stock,” explains Hayes. “Alternatively, biofuel can be created through gasification, a process in which biomass is heated up in the absence of oxygen so that it cannot burn. This produces a biogas that behaves much like natural gas. Biofuel can also be produced using a biological method in which microbes digest once-living material into liquid or gaseous fuel that is high in energy content.” Hayes notes that the biofuel industry is still relatively young, at least when compared to the ancient practice of burning woody biomass as fuel. “Burning biomass involves processes and equipment that are very mature,” he says. “On the other hand, rendering wood into another type of fuel is a relatively new idea, and so is still evolving”

Never mind the emissions; fossil fuels have another major drawback—they take a heck of a long time to form. Admittedly, with a few sips of Ponce de Leon’s fabled Fountain of Youth, it might be possible to wait the eons required for tectonic subduction to turn decaying crustaceans into combustible fuel, but who has the patience? If only there was a way to harvest energy from biology without all the waiting around. Biomass might just fit the bill.