Gas on tap

Natural gas has been a popular heat source for veneer dryers, but escalating gas prices have cancelled out some of that heat source's advantages and caused mills to look for alternative fuel sources.   One of the early companies to manufacture its own gas is Tolko Industries Ltd's Heffley Creek plywood operation near Kamloops, British Columbia.   Tolko partnered with Nexterra Energy Corp, Vancouver BC, in designing and building the new operation alongside one of Tolko's veneer dryers. Most of the manufactured gas heats that dryer, although some is used to condition peeler blocks.   The company estimates that the system, producing 38 million btu per hour, will save more than Can$1.5m in annual fuel costs, replacing 40% of the mill's natural gas consumption; this is 235,000 gigajoules per year. It also will reduce greenhouse gas emissions by some 12,000 tonnes per year.   The end product is clean-burning. In addition to replacing natural gas, it can substitute for propane gas and fuel oil in producing hot air and hot water, steam and even electricity.   The automatic plant runs around the clock, using 25,000 tonnes per year of bark and hog fuel with a moisture content of up to 60%.   Two gasifiers produce syngas; an oxidiser combusts the gas; a heat exchanger heats air for the veneer dryer; a boiler heats water for conditioning logs; and, to round out the benefits, VOCs (volatile organic compounds) from the dryer are consumed in the oxidiser.   In a principle rather similar to charcoal production, the wood is 'burned' while starved of oxygen, receiving about a quarter of that drawn to a normal fire. The product is mostly gas with a minor amount of wood actually burned and this produces the heat for the process. A granular ash remains, containing some nitrogen. A farmer whose fields adjoin the mill thinks it could possibly be used in those fields.   The system is much more versatile than burning the residues to produce heat. The syngas, a mixture of carbon monoxide, hydrogen and methane, can be used right along with natural gas.   In the process, the fuel comes from the debarker, dropping onto a 1,000ft-long conveyor belt lined with shut-off pull cords.   The fuel drops into a metering bin where a vertical auger system feeds the twin gasifiers where it is dried, undergoes pyrolysis, and is gasified.   Partially processed fuel is reduced to ash, which is automatically removed intermittently through openings.   The syngas leaves the gasifier at 500 to 700ºF (260-370ºC).   The heat comes down and proceeds either to an air-to-glycol heat exchanger, or to an air-to-air heat exchanger.   The emissions from the next-door dryer are fed into the oxidiser, located between the two gasifiers, for incineration. If the dryer is not operating, an alternative port provides outside air for the process.   The dryer is fed 22 million btus 600ºF (316ºC) hot air, while the glycol transferring heat to the vats is in the 145ºF (63ºC) range in summer and 170ºF (77ºC) in winter, taking 16 million btus. Block conditioning time is 12 hours.   The whole system operates automatically, controlled from an Allen Bradley Panel View Plus 1500. The various variables can be set on the screen and the status of every component is available on that screen.   With its three dryers in the mill, Tolko could possibly add a second system.   Hog fuel storage can hold 48 hours' worth of material and the system retains heat for 48 hours.