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Calculating the Heating Value of Wood Waste for Fueling Heat Treating Operations
Investigating various heat options to determine the profitability of turning if wood waste into heat energy for fueling a commercial heat treatment system.
By John Conway
Date Posted: 5/1/2010
Is it profitable to turn your wood waste into heat energy for fueling a commercial heat treatment system? The USDA Forest Service tasked Conway & Robison with doing research on this question, and our research showed some promising results for those looking to heat treat material to the ISPM-15 standard, the international standard aimed at stopping the spread of wood pests via wood packaging. This article highlights the most important parts of our research. For a more complete copy of the research, contact me directly for more information.
There are a few quick warnings that I want to share from the start. Any person or company building or purchasing a heat chamber should used licensed professionals with experience in design and installation to comply with state and federal regulations. Keep in mind, that the EPA is preparing to make changes on the small particulate exhaust from boiler systems in the next few months. It is not clear if wood stoves are part of this change.
Using wood as a heat source for heat treating pallets has good potential. Burning wood is an excellent source of energy, but there are some limitations. Moisture content reduces the BTU capacity of wood. Wood stove design, fan selection and ability to maintain a consistent efficient burn also affect the effectiveness of achieving maximum BTU output.
Throughout this research there were four different types of heating methods either observed or conceived using wood heat.
Air to Air: This heating method uses heated air from a heat exchanger attached to a wood burning stove which is blown directly into the heat chamber via ducts. Based on the systems we observed, this is a poor method of utilizing wood heat and, in most cases, will not provide adequate BTU’s for ISPM-15 compliance.
Radiant Heat: We observed a radiant heat system throughout the build process. This design utilizes a wood stove (heat source) inside the heat chamber. The fans circulate air around the heat source and the exhaust stack. This method will meet ISPM No. 15 standards; however, proper design is essential to reduce heat chamber treatment times.
Boiler to Air: This method is used in the dry kiln and heat chamber industry using wood, electric and fossil fuel fired boilers. This is an efficient manner of using wood heat, although heat treatment times will likely be consistently longer.
Radiant Heat and Air to Air: This concept incorporates utilizing the radiant heat from a heat source inside the heat chamber and also adding a heat exchanger to the heat source to better capture and distribute heat through the chamber.
Estimated Heat Treatment Times:
The heating value of wood in the report is shown per cubic yard, because wood waste is expected to be measured in this rough manner rather than by board footage. Table 1 contains estimates of heating values of wood by species. The BTU capacity for dry wood ranges from 6,000,000 BTU/yd3 to over 10,500,000 BTU/yd3 from low density eastern hardwood to high density eastern hardwoods. Softwood values are in the middle to lower end of that range.
Moisture content plays a pivotal role in the effectiveness of burning wood. Table 2 contains estimates of the reduction in BTU potential for wood with defined moisture contents. The moisture content is directly proportional to BTU potential of the wood. Reducing the moisture content through air drying is essential to maximizing the BTU output of wood burning stoves.
Wood Stove Selection
Based on our observations, we estimate that using wood stoves to capture radiant heat, a BTU output of approximately 800,000 BTU would be a fair minimum target to heat treat wood packaging materials in trailer load volumes. Wood stoves have varying potential BTU outputs which are reliant on effective firebox size, air input, secondary burn capacity, ability to maintain consistent burn and overall construction/design. Table 3 contains estimates of BTU outputs based on the firebox size of the wood stove, the wood stove design, and the fuel type. This table can be used as a guide to determine the BTU output of wood stoves.
Case Study Summary – Plainview Pallet, Lewiston, Ill.
The Yoder family of Lewiston, Ill. invited Conway & Robison to observe and certify a heat chamber they designed and built starting in January 2009 for Plainview Pallet. This company used a building design based on current heat chambers in operation. It used a “squirrel cage” type blower which provides excellent airflow and built a wood stove to heat their chamber using radiant heat. Plainview Pallet also supplements the wood heat with small propane heaters. This heat chamber is completely off the electric grid, with battery operated circular chart recorders and a propane-driven fan system.
Initially, heat treatment times were over 30-hours (this was in the winter). Plainview Pallet modified its stove adding forced air input, welding tubes through the stove to increase the airflow area around the stove and adding a secondary burn chamber to the stove. The stove was designed using an exhaust stack which snakes through the rear of the chamber where the fans circulate air around it. These changes were made slowly and increases in efficiency were observed with each improvement.
Another large improvement was the used of wood with a lower moisture content. Initially, the wood fuel was dead green cottonwood. Essentially, this wood had almost no heating value because the of the high moisture content. Through the design changes and use of a lower MC% fuel (approx. 50%), heat treatment times have been reduced from 30+ hours to 9 hours. Further reduction in time will be possible with future stove improvements, but most importantly burning even lower moisture content fuel.
Table 4 contains BTU estimates from the Plainview Pallet stove. We believe that the full firebox size is not currently being utilized on the wood stove. BTU output estimates range from 70,000-210,000 BTU maximum initially using 75% MC fuel to 145,000 – 420,000 BTU output with design improvements and use of 50% MC fuel. Table 4 estimates show that BTU output over 700,000 BTUs is possible using 20% MC fuel.
Waste wood can be effective in providing the necessary heat to treat material according to ISPM-15 as long as certain key conditions are met. This includes low moisture content of the wood waste, proper design of the system and flow of the air in the heat chamber. Look at the charts accompanying this article to see how your specific situation compares with our research findings.
John Conway is co-owner of Conway & Robison LLC, an ALSC-accredited inspection agency for HT lumber and wood packaging materials. Previously John was the Director of Research and Regulatory Affairs at the NWPCA, a pallet sales director at Atlas Pallets in Va., and a research associate at the Virginia Tech Pallet Lab. Contact John by calling (540) 898-4193, emailing firstname.lastname@example.org or visiting www.cr-inspect.com.