Betting on biomass

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Consider a woodchip heating system if you have high heat loads and fuel costs.

July 20, 2022

Daytime operation of the woodchip boiler heats water in the insulated tank for the night, reducing the size and cost of the system needed.
Photos courtesy of John Bartok

Biomass is a low-cost, environmentally friendly, renewable resource that can reduce your heating bill. It is readily available in most sections of the country as either graded or ungraded woodchips. Biomass heating systems have been improved over the past few years increasing the efficiency and reducing pollution. Biomass heat usually costs less than half that of fossil fuel heat.

Sources

Common sources of chips are from utility line maintenance, arborist tree pruning and municipal tree removal. These chips are usually poor quality having considerable twigs and long pieces of bark. They need to be rechipped to a more uniform size to avoid clogging conveyors and grates. They are usually free as there is limited use for them, usually only landscape mulch and animal bedding. Resizing can be done with a chipper or tub grinder to about one inch square by ¼-inch thick.

Another source is from logging operations, sawmills or land clearing. Using a large chipper or tub grinder to handle whole trees or large logs gives more uniform chips with less bark. Sawmills usually debark their logs before sawing into lumber. These chips are usually available at $20 to $60/cubic yard (cy) to recover the cost of chipping. A local sawmill in Connecticut offers an 85-cy trailer load at $18/cy + $8/loaded mile for delivery.

Demolition waste and wood pallets are a third source. These usually have nails and other non-burnable products that make them difficult to burn unless they have been screened.

Chips should be stored under cover to protect them from rain and snow. Some growers use a low-cost, open ended hoophouse as it also provides some drying that increases the heat content.

A woodchip fire burns at about 2000°F, reducing smoke and pollutants.

Heat value

The heat value of either softwood or hardwood chips depends on its moisture content (mc) when burned. An average value is 3,800 Btu/lb. for 45% moisture content, typical of green wood and 7,400 Btu/lb. for 10% mc dry chips. One ton of greenwood chips, burned at 75% heating system efficiency translates to about 7.6 million Btus. This is equivalent to 73 gallons of #2 fuel oil, 73 gallons of propane or 101 therms of natural gas.

Heating system

A boiler is the most common heating system used to burn chips. There are a limited number and sizes of furnaces available. Outdoor wood boilers are commonly used for small greenhouse operations and are available with outputs up to 1,000,000 Btu/hr. These require a bin with auger feed that supplies the firebox with chips. Control is by thermostat the adjusts the burn rate to keep the water at the desired temperature for distribution to radiators, heat exchangers or root zone heat.

For large ranges, multiple boilers are usually installed to allow staging that conserves energy and provide a backup should one boiler fail. Boilers with output of more than 50 million Btu/hr are available. Many growers install smaller units that may not require as much pollution and safety equipment. Equipment needed beside the boiler includes the chip storage and metering bin, air supply blowers, trash and ash removal conveyor, particulate matter screen and cyclone. Distribution of the hot water is through insulated pipe. Heat is radiated by bare or fin pipe overhead or along the perimeter. Low-cost water to air heat exchangers are also popular. Wood chip heating systems are more expensive than fossil fuel systems as they require conveyors and blowers to handle the fuel.

Heat storage

Biomass boilers burn most efficiently if operated at a constant fire rate. Adding a large, insulated water buffer tank can store excess heat during the daytime operation to be used at night when the heat demand is the greatest. Design of these systems allows for a smaller boiler as the water storage carries part of the nighttime load. Maximum water temperature is around 190°F with heat removal to around 150°F. A lower temperature water can be used if a root zone heating system is installed. A typical storage capacity is one gallon per 100 – 200 Btu/hr of boiler heat capacity. Tanks as large as 300,000 gallons are being used.

Woodchip heating system are a good match to greenhouses that have high heat loads and fuel costs. Check local codes to determine what environmental and safety regulations may apply.

John is an agricultural engineer, an emeritus extension professor at the University of Connecticut and a regular contributor to Greenhouse Management. He is an author, consultant and certified technical service provider doing greenhouse energy audits for USDA grant programs in New England. jbartok@rcn.com