The Basics of Biomass - an Interview with Bede Wellford
Bede Wellford brings a lengthy track record of pioneering energy work to his role as Renewable Sales Manager for Viessmann US. As early as 1983, Wellford was a key player in the new field of energy recovery ventilation, serving on committees that helped set industry standards at UL (Underwriters Laboratories), CSA (Canadian Standards Association) and the Home Ventilating Institute. In the 1980’s and 90’s, Wellford tackled indoor air pollution issues, involving radon, formaldehyde and moisture in both manufactured housing and stick built homes, including working with Canada’s R-2000 program. He worked on residential ventilation and air quality standards, first at CSA and later as part of the ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) Project Committee revising the international ventilation standard, SSPC 62. Beginning in 1995, he again led efforts to establish commercial energy recovery standards and certification at ARI, now AHRI (the Air-Conditioning, Heating and Refrigeration Institute). After a career move into manufacturing dehumidifiers and designing solar thermal systems, Wellford became interested in automatic pellet fuel boilers, initiating his current work in biomass technology. Today, he is recognized as one of the go-to authorities on biomass applications. In this interview, he shares his insights on what biomass is, why it matters, and how it can be effectively applied as a practical alternative to fossil fuels.
Let’s make this clear right at the start: What is biomass?
Technically, it’s fuel from anything that was once alive, like grasses, grains, or trees. But when we’re talking about boiler applications, we’re really talking about woody biomass, either pellets or chips. Each has its pros and cons. Chips are generally cheaper, and because they are less processed, they bring us closer to the goal of net carbon neutrality. Pellets cost more, but by virtue of being “densified,” they require less storage and less in transportation costs.
Why should anyone consider biomass?
Frankly, if you have access to pipeline natural gas, it currently doesn’t make strict economic sense. But if you’re in an environment with easy access to forestry products, it’s a powerful alternative to fossil fuels. In New England, we could probably meet as much as 20% of our space heating needs with biomass. Consider the impact on local economies: Experts estimate that for every dollar spent on oil, eighty cents goes out of state. With wood, nearly one hundred percent of that dollar stays in state. And we’re talking about a near carbon neutral resource that can be harvested sustainably. A well-managed forest needs harvesting; decaying wood releases greenhouse gasses—carbon dioxide and methane—back into the atmosphere, and represents a wildfire danger. Creating a market for “low value timber” helps to incentivize and fund proper forest management.
Is biomass good for more than space heat?
Absolutely! I’ve worked on applications in which the biomass systems, in addition to delivering space heat, served domestic hot water needs, provided heat for dishwashers and, clothes washing and hot-water coil driven laundry driers, and supported hot water processes in dairy and other food industries. Looking ahead, we’re going to see more biomass use in combined heat and power applications—such as steam turbines or Rankine cycle engines for electricity, and for air-conditioning through absorption chillers.
Realistically, can you apply biomass as a retrofit?
Most of my biomass projects are retrofits. In many ways, a biomass system looks and works like a conventional one, except that the boilers are little larger, and you need space to accommodate the ancillary systems: storage for the pellets or chips; room for the augurs that move the fuel; space for the buffer tank. You also need a way to collect and dispose of the residual ash.
Suppose I’m interested in biomass—where do I begin?
Every discussion of a biomass project starts with the fuel. Once you know what you want to burn, that choice drives all the other design decisions: what boiler to use, what the boiler room will look like, how much storage space you’ll need, how to manage transportation/fuel delivery, etc. You have to know your fuel source and how to manage it—with chips, it’s all about supplier discipline. The top suppliers will screen for size and deliver chips with a consistent moisture content. But if you want the cheapest fuels, you’ll have to settle for less control over the fuel and be prepared to accommodate the inevitable inconsistencies. You’ll have to add a screen between the storage floor and the augur to remove oversized chips; you’ll need a boiler that can accept green or dry chips.
Tell us more about these boilers. What distinguishes an excellent biomass boiler?
The biggest difference between a biomass boiler and a fossil fuel boiler is in the combustion. Unlike conventional boilers, biomass boilers combust in two stages. The first stage “gasifies” the fuel, turning wood fiber into gases. In the second stage, the boiler introduce more air, adding oxygen to burn the resulting gases. Here’s the key: You want to keep these stages as discrete as possible. The more separate they are, the more efficient your burn and the less particulate matter you drive up the flue. When these two stages mix—when you get “reflux”—you lose efficiency and get a dirtier burn.
This need to separate the stages is one of the reasons biomass boilers are bigger than conventional ones—that, and the need to move out the ashes. All biomass boilers face these challenges. Excellent boilers make it easier to master these stages. Flue gas recirculation, for example, allows you to control the primary stage fire, to cool it down so that you can gasify the wood without burning the gases prematurely, and to keep the fire on the grate under fusion temperature to prevent clinkers.
Other features you should look for: Automatic ignition allows the fire to go out and reignite as needed (as opposed to a stand-by or smoldering fire strategy employed by some suppliers). You want a high-quality refractory brick that not only insulates, but reflects heat back into the fire for greater efficiency. High quality insulation of the heat exchanger keeps the energy in the water as opposed to the boiler room. Accumulated fly ash reduces heat transfer efficiency; you want pneumatic tube cleaning to clear the fly ash automatically, and to reduce the frequency of tube brushing. Viessmann’s Pyrot and Pyrotec boilers have all these features, and are among the cleanest-burning biomass boilers in the world
What’s the most common mistake people make when designing a biomass system?
Not including a buffer tank in the design. It’s tempting to leave it out—including a buffer tank adds to the initial expense, and some people, even smart ones, believe there’s sufficient volume in the heating water loop to exclude it. But to get the efficiency you want, a buffer tank is essential for three reasons. One, wood takes about a half hour to reach peak heat production; the buffer stores energy to meet demand instantly—in the early morning, for example. Two—the flip side of this issue—is that it takes thirty to forty minutes for a wood fire to die down. When happens when you have no load? You can either dump this energy up the flue, or you can capture it in your tank. Number three, and perhaps the most important point, is that even with controls to the fuel intake and fan speeds, a biomass boiler can only achieve a 4:1 turndown ratio while maintaining high efficiency and low emissions. With a buffer tank, you can both modulate the combustion and serve lesser loads with stored energy, effectively giving you a broader turndown ratio, which means you can synchronize your boiler response to system needs without sacrificing combustion efficiency—that’s where you get your optimal seasonal efficiency.
Any other recommendations?
Yes. Size your boiler for 60% - 80% of the design load. A boiler sized for 60% of the load will offset 90% of your fuel use. For the remaining ten percent, add a fossil fuel backup boiler; it’s a cheap way to meet peak capacity, get redundancy, and handle your DHW needs in the summer. While we are on summer domestic hot water, consider using solar thermal as the prime mover. Also, work with a manufacturer that has a robust education and tech support structure for your contractor. A good one will come on site at the receipt of goods to show how all the pieces work together, and will remain available to answer questions and troubleshoot problems as the project moves along.
Think long term. Consider a biomass boiler in the context of extended payback, cash flow, life cycle cost, or net present benefit. These systems are rarely going to break even in two to four years. Simple paybacks of ten years are typical, but cash flow with financing or performance contracting can be positive immediately. And the long term assurance of lower fuel costs, and the economic benefits to the owner and the community over a 30 year system life, are persuasive.