by Sanne Kure-Jensen
Biomass heating may be cost-effective where growers burn a lot of fuel, have a long heating season, produce at high volume or need high temperatures. Andy Jones of Intervale Community Farm in Burlington, VT, has had extensive experience with using a biomass heating system. The Intervale Community Farm heats their 30’ by 96’ transplant house each spring, starting in early March. Jones’ mentioned 2012-2013 fuel costs for comparison:
• Propane — $36 to nearly $50 per M Btu (annual contract or spot purchases)
• Fuel oil — $33 to 36 (annual contract or spot purchases)
• Corn and wood pellets — $21 to $26 per M Btu
Intervale Community Farm received a $3,000 UVM Extension cost-share grant to help fund the new biomass heating system. They installed a 165 K Btu hot air furnace and left one of their 175 K Btu propane burners in place as a backup on colder nights and on milder days. The biomass system now provides 80 to 90 percent of heating needs in the farm’s transplant house.
A bin and auger system feeds the heater’s firebox. The farm’s fuel bin must be filled every day or two. The firebox needs to be hand-lit to start it, and the fuel needs to be consistent to avoid clogging the firebox. Burning corn leaves behind clinker. This unburned corn can clog the firebox and reduce efficiency. Higher quality corn makes less clinker. Corn burns hotter than wood pellets. Burning pure corn is not as efficient as burning corn with a wood pellet blend. This produces less clinker, but more ash. High-quality wood pellets have fewer fines reducing risk of clogging the auger and chimney. Jones likes a blend of two-thirds corn and one-thirds wood pellets.
The chimney needs to be vertical rather than venting through a sidewall for optimal draft. Use a triple-wall chimney to prevent burns from bumping into the chimney. Jones recommended using a polycarbonate panel to house the chimney’s roof panel.
Jones shared his financial analysis. The capital investment of $6,500 could be amortized over seven years for 900/year. Stocking the bin, cleaning the firebox, emptying the ash, restarting the fire and tinkering took an average of 15 minutes/day for 75 days/year costing $475/year at $25/hour. The farm saved $1,350 by buying biomass versus pre-buying propane. This was a net wash. Thanks to the cost- share grant, the farm was ahead by $400/year.
Drawbacks to a biomass system include ash disposal. Jones said they add cooled ash to their compost piles.
A lot of potential growing space lost to the heating system. Intervale Community Farm’s gave up 100 square feet to the stove, fuel bin and fuel bag storage. Early on, the farm switched to a larger fuel bin to save on filling time. If the space permitted, Jones would have liked an even larger fuel bin to accommodate bulk bin deliveries for fuel costs savings.
He was disappointed that they had to replace and upgrade worn augers after a year or two.
Even after rerouting the chimney from horizontal to vertical venting, Jones still had some smoke inside on mild days and when first starting the stove. Jones said, “These systems require more tinkering than oil and propane systems.” If growers do not have someone on staff who can tweak heating systems, this choice may not be right for you. These stoves need to run all the time because lighting them cold is tedious and time consuming. This makes them tough to use with oscillating temperatures on sunny days with puffy clouds. The greenhouse can become overheated.
Jones said more automated systems are available now, but the costs are higher. Some biomass burners operate outside the greenhouse and pipes deliver heat inside, saving production space.
Some growers may be able to reduce heating costs further if they can grow their own biomass fuel.
Greenhouse heating with biomass
by Sanne Kure-Jensen