Residential biomass fuels are usually chunks and pieces of wood and logs that are burned in small woodstoves and fireplaces, in contrast to medium — and large-scale municipal, industrial, commercial, and power-generating facilities that burn a large variety of virgin and waste biomass fuels such as MSW, RDF, sawdust, bagasse, rice hulls, wood chips, and industry-specific wastes. The residential wood fuels range from about 15 to 50% moisture content depending on the extent of air drying. The moisture content of seasoned firewood is typically about 20%. Most of the older woodstoves consist of conventional downdraft, updraft, or crossdraft fireboxes with fixed steel grates. Airflow is adjusted manually and the flue connections are sized for maximum loadings. Electric air blowers are sometimes used in the larger wood-burning appliances. The ash collects in a compartment below the grate. A common characteristic of the older woodstoves is that they permit long fuel residence times in the firebox to maximize fuel usage. Over the years, hundreds of woodstove mod­els of different designs have been marketed, many of which are claimed to have superior performance. Some of the modifications did effect small in­creases in thermal efficiency, but many were stricdy aesthetic changes.

With few exceptions, it was not until the 1980s when major advances were made in woodstoves in response to government mandates to reduce pollution.

Catalytic woodstoves with secondary combustion chambers are good exam­ples of the application of modem technology to improve operating efficien­cies. These appliances are at the high end of the efficiency scale as compared to noncatalytic woodstoves; fireplaces are at the low end. A comparison of the operating efficiencies of woodstoves is shown in Table 7.3 (Long and Weaver, 1985). The woodstove efficiency in this comparison corresponds to the usable heat over the energy content of the fuel input unadjusted for moisture content. The efficiencies range from a low of 13% for conventional fireplaces to 75% for airtight, catalytic woodstoves with a secondary combustion chamber.

As discussed in Chapter 6, pellet-burning stoves for residential use, or pellet stoves as they are generally called, and pellet fuels made from wood, wood wastes, straws, RDF, waste paper, and other waste biomass have been commer­cially available for several years. Residential pellet standards have been pro­posed by the Pellet Fuels Institute in the United States (Table 6.5), where the annual market has averaged about 35,000 pellet stoves over a 10-year period (Pickering, 1995, 1996). Pellet stoves are marketed in both free-standing and fireplace-insert models. These stoves are equipped with hoppers which hold about 20 kg or more of pellets that are auger-fed, usually from the top, into the combustion chamber. The advanced design units employ forced air flow past the pellets, and passage of the hot combustion products through a heat

TABLE 7.3 Comparison of Woodstove Efficiencies”

Woodstove type Efficiency (%)

75 An airtight stove with a catalytic afterburner and a

smoke chamber such as a double-drum stove or a design that provides for improved burning in a secondary combustion chamber.

63 Any of the openable stoves with tight fitting doors

that would not fall into the airtight catalytic category.

50 Non-airtight stoves with flue connections smaller

than 20.3 cm (8 in.).

38 Non-airtight stove with a 20.3-cm flue connection.

25 Equipped with heatalator or similar device to

improve heating effect.

13 Without heatalator.

“Long and Weaver (1985). The efficiency does not include a correction for moisture content. When adjustments are made for moisture, the efficiency of the average airtight woodstove or fireplace for wood fuel with 20% moisture, for example, would be 50% rather than 63%, or 10% rather than 13%.

exchanger to heat circulating room air, and then through a simple flue to the outside. The advanced stoves are relatively complex and require several mo­tors, fans, and electronic modules to control fuel and combustion air.

Small furnaces and boilers for wood fuels with and without backup oil or natural gas have been designed for burning logs, wood chips, sawdust, and pelletized wood wastes for central space heating in northern climates (с/. Brandon, 1981; Sahrman, 1983). The loading systems are gravity-fed hoppers or screw-fed hoppers (stokers) for chips, pellets, and sawdust. Logs and split logs are loaded manually. When thermal energy storage is employed, provisions are generally made for hot water storage and circulation through radiators when heat is required. One example of the controls for a conventional furnace and boiler is those used for one type of manually loaded round wood system (Brandon, 1981). Regulation of the heat output is achieved by varying the amount of combustion air available. Demand for heat from the building thermo­stat activates a control motor that opens a primary air vent to the furnace; when no heat is required, the vent closes and the fire dies down. If the wood fire is not controlled by the primary air vents, the heat is dumped to the building by switching fans or water circulators. This ensures safe operation by preventing overheating of the furnace or the boiler. Automated feeding systems for particulate fuels are often controlled so that both combustion air and fuel feeding are adjusted with the demand for heat. Controls for advanced space-heating units usually take into account the fact that wood combustion cannot be controlled by instantaneous on-off devices, as an oil or gas burner can. Combustion can be sustained in a low-level, standby mode, or a fast-start device such as an auxiliary oil burner can be employed to renew combustion when heat is needed. There are many variations in designs and controls for these systems. In comparative tests of 10 commercially available units for residential use, the overall efficiencies in terms of wood fuel input over useful output during the heating season ranged from about 40 to 50% for 9 of the 10 units evaluated (Brandon, 1981). This is quite high for small systems.