Types of Biomass Heating Systems
There are different approaches to biomass boiler systems that we have dealt with in the program, which include four general categories: fully automated systems, "surge bin" or semi-automated systems, pellet fired systems and combined heat and power.
Fully automated systems
Fully automated systems are exactly that – a delivery truck drops off the chipped or ground up waste wood with a live bottom floor, and a series of augers and conveyers move the wood fuel from there to the boiler. With computer controls and a laser eye that measures the amount of fuel in the metering bin, the system automatically turns the augers and conveyers on and off as needed to maintain the amount of wood fuel to sustain the boiler's pressure and thus, the temperature demand of the thermostat. In other words, no handling of the wood fuel is required by the boiler operator.
The Darby system is a fully automated, small "district" type heating system, in which three separate buildings were connected to one boiler system via underground piping to connect to their hot water and steam distribution systems. With the knowledge we have developed since the first system was built in Darby, we would not repeat that type of installation, because the interconnection via underground piping of several buildings was very expensive (over $200,000). The boiler at Darby was also sized to meet the peak heat demand of the facility, and in retrospect, we would likely have installed a much smaller system to meet the base load, relying on backup systems for peak demand. The Darby boiler is slightly over 3 million BTU per hour output and serves three facilities totaling over 82,000 square feet. It replaced three fuel oil fired boilers, which are kept in place as backup systems during maintenance of the wood fired system. Darby's storage bin holds two semi-truck sized chip van loads of fuel, or about 50-60 tons, which is also larger than we would likely require today. However, in general we consider a Darby-sized or larger project to warrant at least considering a fully automated fuel feed system.
Semi-automated or "surge bin" systems
Semi-automated or "surge bin" systems require somewhat more manpower than the fully automated system. Thompson Falls schools have installed a semi-automated system to handle the base heat load for four buildings totaling about 100,000 square feet, with a 1.2 million BTU per hour output. This system has a smaller storage bin, shorter, simpler fuel conveyance system, and is designed to meet less than the full heat load of the facility. The rationale behind this is that wood fired boilers operate most efficiently when they are working hard, and that the peak heat load in our region is significantly higher than the load required on most days of the year. The peak load may only be required on one or two days of the year, and sizing a boiler to meet the peak would necessarily compromise its efficiency for the majority of the time it is in use.
In comparison to Darby, Thompson Falls has a small bin that holds about 5-6 tons of fuel, which was designed to hold enough to keep the boiler running in low fire mode over a long holiday weekend covering three of the coldest days of the year, to avoid requiring maintenance attention. Adjacent to the small "surge bin" is a larger storage area, which holds about one chip van load or 25-30 tons of fuel. The facilities manager must use a small front-end loader to transport the fuel from the garage style storage area to the surge bin about once a week on average. The smaller, simpler surge bin system cost about 2/3 of the fully automated version ($455,000 versus $650,000+, which is the cost of a Darby-sized project without the underground piping integration costs incurred at Darby), even if the cost of the front-end loader is included in the cost of the surge bin system. We are monitoring the operations and maintenance time required with this system to better determine the economic tradeoffs in fuel savings, up-front cost savings, and personnel costs of operation. We suspect that surge bin systems are most appropriate for facilities in the range of 25,000 to 100,000 square feet. In retrospect, if we had used smaller, surge bin systems at Phillipsburg and Victor, the economics of those projects would have been much more favorable in that they would have been able to pay the systems off with savings in a very short time frame. Thus, smaller less expensive systems help make more biomass installations possible by improving the payback and reducing the initial cost, as well as helping us stretch our grant dollars further for the brief time they are available.
The third type of system we are working to demonstrate now is a pellet-fired system. Pellets are a more processed fuel, and therefore, more expensive, but the tradeoff is that they are more condensed and uniform, which makes them much more efficient to transport and store. Pellets are currently selling for about $110 per ton or $6.50 per decatherm for a commercial grade fuel, not including the cost of transportation. Recent rises in fossil fuel prices have driven up the demand for pellets rather dramatically, so this is an historic high. About 6 months ago, Eureka Pellet Mills was quoting bulk commercial grade pellets at about $80 per ton, or $5 per decatherm, not including transportation. Because of their uniform nature, the conveyance system for pellets is even simpler and less expensive than a surge bin system. These systems essentially use a grain-type of storage silo and move the fuel to the boiler using gravity. The condensed nature of this fuel also makes the storage requirements considerably smaller, reducing up-front construction costs. Pellet fired systems are available and appropriate for a fairly wide range of facility sizes. Typically they are most cost effective when space for fuel storage and conveyance is limited, and pellets are manufactured relatively close to the end user.
Combined heat and power (CHP)
The fourth type of system is combined heat and power or CHP, in which wood waste is used to generate power, and heat is created as a byproduct of the power generation system. The capital investment required for high pressure steam generation associated with CHP systems is significantly higher, and due to the risks associated with their high pressure (from a minimum of 100 pounds per square inch up to hundreds of psi), the systems require a highly skilled and certified operator, which adds to the operating expense. In comparison to CHP systems, the school heating systems we have installed to date produce, on average, between five and ten pounds per square inch of pressure. Creating power also requires a great deal more fuel than creating heat, and as will be discussed in the fuel supply section below, wood fuel is bulky and difficult to transport. Requiring a great deal more fuel can enlarge the travel radius to the extent that the savings benefits of the system diminish. Finally, CHP systems create heat on a year-round basis, so if that heat is not desirable for certain seasons of the year, an additional expense of installing a cooling tower is required.
There are situations in which CHP is an excellent option, however. Wood products manufacturers often use small CHP systems. They have a large, ready supply of waste wood, and a constant need for both energy and heat (for dry kilns to dry their lumber or other products). Other examples of CHP applications could include hospitals or prisons, which have a constant need for energy and domestic hot water. In these cases, it typically makes the most sense to size the energy production such that the residual heat produced closely matches the constant heat load, so no excess heat or cooling tower are involved.
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