Commercial and Institutional Biomass Projects: Dos and Don’ts

There are many important factors to consider and key steps to take when pursuing an on-site biomass heating project.
By Bede W. Wellford | January 08, 2020

If you’re reading this, you may have already been thinking about a biomass heating system for your facility. The first step should be clarifying your reasons for such an investment, and developing a clear understanding of the project’s objectives. These may include facility economics (fuel cost savings), environmental drivers (reducing carbon footprint), investment in local community (via locally sourced wood fuel), or some combination of these and similar motivations. Local and state incentives, such as New Hampshire’s and Massachusetts’ thermal renewable energy certificates, may also play a role in your strategy.

The following viewpoints will primarily focus on biomass thermal heating systems; specifically hot water biomass boilers. It should be noted that cogeneration and “trigeneration,” including cooling, are options, particularly for larger facilities and loads. However, this would require another article and additional expertise to address the various options, load matching considerations, etc., as would process applications that require or already use steam.

Factors to Consider
Once the decision is made to investigate the feasibility of a biomass boiler project or move forward, the first questions to ask are: What will you burn? What fuels are available? What are the implications of that fuel selection for the project? Many of the system decisions will flow naturally from the fuel choice. Pellets are simple to store and retrieve but cost more than chips. Dry chips are available in some areas and may allow for some system economies vs. forest chips. When it comes to forest chips, qualify the supplier—in general, they must screen the chips to be suitable for use in modern automatic equipment.

Somewhere along the way, usually early in the process, identify an internal management champion for the project. Encourage this person, or persons, to take ownership and become a clearinghouse for ideas, questions and plans.

Another factor for consideration is how will the heat be distributed. Whether a building or campus, hot water is the most efficient option. In a retrofit, if the existing system is steam, the feasibility of conversion to hot water should be evaluated. Understand up front that many biomass projects that require or incorporate such a conversion die on the design floor due to cost. Planners are encouraged to look at hot water conversion as a separate item from the biomass boiler. In most commercial and institutional settings, hot water will pay for itself over a longer period provided a correct path, but short-term paybacks are not realistic.

The next task is to size the biomass system and any ancillary systems with respect to the known heating design load. In this determination, include the need for backup or redundancy. For systems that will combine biomass with fossil fuel boilers, it is common to find an economic sweet spot with the biomass supplying less than the design load. You may have heard of the 60/90 rule, which holds that sizing the biomass boiler at 60% of the design load will offset 90% of fossil fuel use. The exact ratio varies with load profiles (I have worked with projects where the ratio of biomass capacity ranges from 60% to 100%), but in general, this approach has merit. Fossil fuel backup or trim generally has a far lower cost per Btu; the inescapable fact of the matter is that it costs more to move wood fuel through the system, beginning with storage and retrieval, moving it through the firebox and finishing with ash removal. It is vastly different from hooking up a pipe for fuel oil, natural gas or propane.

Another sizing consideration is the need for regular maintenance (cleaning) of the biomass boiler. The nature of the fuel means that even with automatic de-ashing and pneumatic tube cleaning (soot blowers), you will be looking at shutdown for cleaning every 600 to 800 hours. In a system where the biomass boiler is sized at 75% of the load, size the fossil fuel backup at 100% so that it can carry the facility in the dead of winter when the biomass boiler is being cleaned. I note that we also have systems without fossil fuel back up or trim, and they generally employ multiple biomass boilers, often a smaller boiler used early and late in the swing seasons, and a larger boiler for the midwinter design load.

Dos and Don’ts
Do utilize a buffer tank or thermal storage. This should be installed between the biomass boiler and the building or system loop, sized at 10 to 20 liters per kilowatt of output capacity of the largest biomass boiler. We use buffer tanks for three purposes: to anticipate load (think morning warm-up when the fire may be out), to store heat at shutdown (think a typical school operating schedule where the load goes to zero in the afternoon, but the fire is still burning), and to modulate the boiler firing rate as loads decrease during the day. Some control systems use the buffer to optimize combustion under a wide range of conditions. Understand that efficient wood combustion has a maximum turndown ratio of 4:1. Our approach is to use automatic ignition with a buffer tank to allow for efficient use when loads go lower than 25% of boiler rated output. All systems can benefit from the lead and lag capabilities inherent in thermal storage. The only exception to this recommendation is for process applications where there is a relatively constant and consistent load.

Another sizing consideration is for the fuel bunker or silo. How often will you want a fuel delivery? How much lead time will the fuel supplier require? Are there cost considerations attached to the delivery capacity? A good example is a pellet silo. I advise commercial clients, almost regardless of the boiler size, to install a silo that will hold between 30 and 40 tons of pellets. The largest pellet delivery trucks have a 25-ton capacity, so it is desirable to have that room available in the silo with reserve of at least a few days in order to take advantage of full truckload delivery rates.

Do identify qualified trusted boiler suppliers. Insist on ASME H-stamped pressure vessels to comply with codes and manage liability. Safety certification to UL2523, the only ANSI safety (fire, mechanical and electrical) standard for biomass boiler packages, is a plus. This certification may be provided by UL, ETL (Intertek), CSA or any number of ANSI-accredited agencies. Last year, the Biomass Thermal Energy Council issued an efficiency and emissions test protocol with a goal to eventually have an ANSI standard for performance—this process will certainly take time. Meanwhile, particularly for European-designed products, ask for the certified test results in accordance with EN 303-5, currently the only internationally recognized performance standard. For the products being considered, request references for where they have been installed and operating, and ask who provides the supply chain for factory support and parts.

Do identify qualified designers, contractors and subcontractors. I have had the pleasure of working with experienced biomass design engineers and design-build contractors on numerous biomass projects. I have also worked with general contractors (and subs like electricians) who have never installed a biomass boiler, but have solid reputations and stepped up to understand what is necessary for success. There is no magic formula, but insist on quality, and you shouldn’t be disappointed.

Two specific design recommendations include: For hydronics, tie the buffer tank into the loop via closely spaced tees and tie the fossil fuel back up in the same way, downstream of the buffer tank connection. For controls, use outside air temperature setback on the building loop, but not on the buffer tank.

Do specify factory commissioning and start-up services. We use this process as a final quality control point for the product and its installation, and generally bundle the operator training into the visit, which may take several days for a chip system. Completed sign off from our technician and the operator or owner is then the start date for the warranty period.

Do understand the ongoing maintenance and operational requirements of the system. I have heard promoters describe the maintenance as “no more than the oil boiler,” which is nonsense. It is not onerous, but understand that it does involve some additional attention, even with the most refined and automatic systems. Identify an internal champion in facility management to take responsibility for the system. Be prepared to touch your boiler on a regular basis, and insist on factory training for the operators.

Finally, as for the “don’ts,” I will not provide any. Experienced educators and coaches know that negative feedback is not useful. “Don’t do that” is not helpful input for the learner; focusing on what not to do leaves one paralyzed. “If I don’t do that, what do I do?” Instead, stay positive and pay attention to the “dos.” If you follow this path, you won’t have to worry about the “don’ts,” and you will be able to move deliberately through the phases of the project with confidence.

Author: Bede W. Wellford
Sustainable and Renewable Business Development Manager, Viessmann Manufacturing Co. (U.S.) Inc.