Today we have access to a number of well-proven, highly reliable biomass boilers such as those made by Viessmann, Schmid, Fröling, Maine Energy Systems, and Windhager.
These high-efficiency boilers have heat outputs that range from 24,000 to over 6,000,000 BTU/hour. They can be deployed alone or in staged groups of two to four boilers. All integrate well with conventional oil or LP fired boilers that work in peaking and backup situations.
Some of these boilers are specifically designed for burning wood pellets while others offer the flexibility of burning either wood PDCs or pellets. Some boilers can be set up to burn green chips but the related material handling systems are much heavier.
It is important to note that choosing a wetter fuel directly impacts boiler efficiency and emissions. As a result, additional equipment may be necessary to pass local emissions regulations.
Wood Pellet Systems
Pellet systems are relatively simple because of the refined nature of the pellets themselves: Pellets are made from sawdust and dried, pulverized wood chips, all compressed to become two and a half times the density of wood chips. The small size and density of wood pellets provides some nice flexibility in system design:
- Storage silos can be smaller and located fairly far away from the boilers
- Wood pellets contain about 7% moisture so they store well, and with their small size and high density, pellets are easy to deliver by blower truck. This dryness makes them very stable and easy to ignite.
- Pellets flow like water from an open pipe and are easily transported pneumatically from storage to the boiler. Direct feed flex-augers are also used.
- All of the pellet boilers that we offer make full use of these wood pellet advantages. As a result they are fully automatic, very low maintenance, clean-burning, highly efficient, and reliable.
There is very little for an owner to do other than to empty the ash once a week.
PDC boiler systems are quite similar to pellet systems except they are a bit heavier because they burn a bulkier fuel with a higher moisture level.
- Due to their shape, wood chips are unable to flow like pellets so an agitator of some kind must be used to get them moving. Our most popular method is to use a sweeper arm that circles beneath a stack of PDCs, sliding them into an open auger channel which pushes them to the boiler through enclosed augers.
- PDC boilers are highly efficient and clean-burning, rarely needing expensive pollution control equipment (ESPs) to comply with local emissions regulations.
- PDC silos and boilers are able to handle and burn wood pellets as an alternative fuel.
- Similar to wood pellet systems, PDCs are never visible in the boiler room because they are fully enclosed in pipes and bins at all times.
The main reason someone would prefer PDCs over pellets is fuel cost. PDCs cost 35% less than pellets on a "per BTU delivered basis".
Green Chip Systems
Green wood chips are the original fuel for large, automated boiler systems. With moisture levels as high as 50%, green chips can contain equal parts of wood and water—water that must be evaporated by the burning of wood in the boiler, before any heat can be realized for your building.
- Green chip boilers are designed to drive out the moisture in the chips as they slowly travel towards the fire.
- Heavier, wetter fuel requires heavier equipment and more emissions controls. Systems that burn cheaper, less refined fuels are more costly to build than those that burn wood pellets or PDCs.
- It is often assumed that green chips can be bought for very little money. However, owners usually pay more per ton to a reputable chip supplier who provides quality control, ensuring the absence of oversized chips, rocks, and excessive amounts of bark in deliveries.
- Burning a less consistent fuel like green wood chips requires more on-site supervision and maintenance. As a result, ongoing labor adds to your total costs.