By Charles Loos, P.E., Lee Enterprises Consulting
From an operations standpoint, a wood-fired power plant can be described as a fuel handling system first, and a power plant second. The biggest challenges stem from the fuel itself – procurement, supply, handling, and combustion. By contrast, the steam turbine and power cycle seem relatively trouble-free.
Fuel-related problems top the list of headaches at a wood-fired plant. For example, established fuel sources can dry up, even as unforeseen types of “opportunity” fuels become available. Trucks, conveyors, feeders, and boilers struggle to handle fuels they were never designed for. Trucking costs swing wildly as diesel prices rise and fall. Private truck fleets leave to pursue other markets. Fuel piles self-combust and boiler emissions go out of range until woodyard operators learn to segregate various fuels and blend them to keep the boiler happy.
This article unpacks some common fuel-related problems and gives developers, engineers, and financiers tools to minimize future impacts.
Fuel Supply Realities
Most wood fuels (forest slash, mill waste, agricultural waste, construction waste) are unwanted byproducts from other businesses. Fuel “suppliers” focus on getting rid of waste, rather than providing quality fuel. Furthermore, fuel-generating activities are often mom-and-pop in scale, and constantly moving from one forest, orchard, or housing development to the next. Such activities are highly seasonal and sensitive to boom and bust markets. Therefore, wood-fired power plants generally rely on multiple, short-term, opportunistic fuel supply contracts with local small businesses.
As the years go by, the original fuel mix envisioned by developers inevitably changes. For example, California plants enjoyed a temporary glut of fruit tree wood as farmers ripped out drought-stricken orchards. Later this caused fruit pit fuel supplies to drop. Forest fires, pine beetles, and logging have caused wood shortages. Construction waste volumes follow urban sprawl and the boom and bust cycles of home construction. In order to survive, biomass plants must adapt to an ever-changing palette of opportunity fuels.
Regardless of the fuels originally designed for, soon comes the day when cheap non-compliant fuel becomes available. It may be wet or dirty. The particle size may be too big or too small. But it’s oh so cheap, maybe even free.
One plant took on tons of cedar bark because it was cheap. It arrived in slabs several feet long (the plant was designed to process wood chips less than 4 inches in size), filthy with dirt. As the cedar bark progressed through the wood handling system, it separated into stringy ropes of bark. Of course, it balled up the works, especially the screw conveyors. The mud and dirt overloaded the boiler bottom ash system.
They made it work of course, by blending small amounts into the regular fuel stream. And therein lies a lesson about fuel pile management. The segregation and judicious blending of different fuel types are crucial. Whatever the method used to reclaim fuel from the pile – reclaimer or front-end loader, the best woodyard operators have the eye and feel of a sculptor, artfully delivering just the right blend to the boiler.
Another plant accepted free planer shavings from a nearby mill. The dry, fluffy shavings burned too hot and fast. The answer was, once again, to blend with other types of fuels.
At the best-run plants, the reclaimer or loader operators enjoy close-knit relationships with the boiler operators and are respected and appreciated.
Sadly, I’ve seen plants where the loader operators are part of a different union or department, and at odds with the boiler operators. I’ve even seen plants where the loader drivers didn’t have radio communications with the control room.
A crack woodyard crew, who has judiciously squirreled away piles of different fuels, can help a plant successfully blend and burn opportunity fuels.
The Thirty-Thirty Rule
A heuristic I’ve developed over the years is the thirty-thirty rule, which places a cautionary upper bound on plant size and fuel trucking radius. Simply stated, the potential for fuel shortages becomes much more acute for plants over 30 MW, and/or plants where fuel is trucked from more than 30 miles away. In other words, plants over 30 MW often self-create a chronic fuel shortage.
Similarly, if fuel must be trucked from more than 30 miles away, trucking time and costs often reach an economically painful zone, especially when diesel fuel prices rise. Longer trips also require more trucks since each truck makes fewer deliveries per day. The moment the plant can no longer keep the large fleet of independent truckers busy (due to plant curtailment or the advent of closer fuel sources), truckers quickly move to other markets and may be reluctant to return when required. Trucking fuel farther than 30 miles is often both expensive and unforgiving.
The thirty-thirty rule is cautionary, not definitive. Numerous larger plants break the rule quite successfully. If, however, a proposed plant breaks the thirty-thirty rule, extreme diligence must be exercised when developing fuel supply plans. Remember also that multiple plant developers are all targeting the same fuel supplies.
Call to Action
How to build fuel resilience into a wood-burning plant? Many of the most effective actions cost very little, aside from diligent engineering, permitting, and development. Specific measures, ranked roughly from lowest to highest cost, are listed below.
- Remember the thirty-thirty rule, and exercise extreme diligence if you exceed it.
- Empower woodyard operators to segregate mounds of premium and experimental fuels. Align their performance bonus metrics with those of the boiler operators. Forge one team working toward efficiency and emissions compliance. Eradicate any “us vs. them” friction between the woodyard and the control room.
- Craft air permit applications to accommodate the widest possible range of fuels. Clearly state requirements for emissions exemptions during startups and shutdowns. Request the ability to burn experimental fuels on a limited-hour basis. Beyond the air permit, make sure the entire suite of other permits allows flexibility in ash volumes and compositions.
- Visit other biomass plants in the area, and listen to their lessons learned. Listen to woodyard and boiler operators, not just managers.
- Provide space in the woodyard for piles of specialty and experimental fuels
- Include an auxiliary hopper that lets loader drivers dump fuel directly onto the main fuel conveyors, thus allowing manual blending of specialty fuel with the normal fuel stream.
- Leave space in the woodyard for additional experimental or temporary fuel handling equipment such as dryers, hogs, and truck dumpers. Stub out compressed air, water, and data lines for easy connection later. Include space for future switchgear sections, and include spare buckets in the motor control centers.
- Design wood handling equipment and boilers for the widest possible range of fuel sizes, types, and moisture content. Size the conveyors and equipment for the highest volume (generally the wettest) anticipated fuel. Test burn all the fuel types during acceptance tests, and insist the woodyard and boiler suppliers demonstrate the capability to handle these fuels.
Reduced to a single sentence, the message is: “Remember the thirty-thirty rule, bake flexibility into the design and permits, and empower your woodyard operators”. Twenty years from now, the plant owners will be grateful.
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