AFR: I read “Waste not, want not” in your January 2015 issue with interest. I have a few comments regarding wood moisture content for industrial power plants. You stated, “If that biomass, made up of green hard and softwood logs, dead round wood, and mill by-products, were air-dry, we could cut consumption by a third, leaving trees to turn into more valuable lumber and furniture – and fuel to heat our own rural homes.” It is doubtful that much lumber or furniture-grade wood is going to the biomass plants, as they don’t pay enough. There could be some competition with firewood buyers in some areas.
Regarding lowering the moisture content to reduce fuel consumption in the biomass power plants, that is not a possibility. Air-drying domestic firewood down to 20 percent is advantageous to minimize wood use, emissions, and chimney fires. But industrial biomass plants must use biomass fuel of the moisture content they were designed to burn. In the past, plants like the one in Brooklyn were designed to burn hog fuel – sawmill waste comprising sawdust, shavings, and bark that has been ground to a consistent size for handling. Hog fuel typically has a moisture content of 45-55 percent MCWB (moisture content wet basis, i.e., percentage of water per unit mass in a wet sample).
Twenty years ago, hog fuel was readily available and cheap in the Maritimes and across this land. It could usually be had for the cost of trucking – say $10-15 per tonne. Forest chips were rarely used as a primary fuel. Today, with fewer sawmills and more demand for sawmill waste (from users such as pellet plants), we have switched to using large volumes of forest chips in power plants. (Green, forest chips also range in moisture content from 45-55 percent.) Chips are augmented by mill waste when it is available.
When a biomass plant is in the design stages, the design engineer will always ask two questions: “What fuel do you intend to burn, and what is the moisture content?” Most older plants were designed for hog fuel, and even new plants are generally designed for green forest chips since it is readily available on a consistent, year-round basis.
The engineers, therefore, have designed the combustion cells of industrial plants to handle fuel which has roughly 50 percent water by weight. To be able to burn such wet fuel, they typically install a lot of high-temperature refractory, and often have a large drying zone that precedes the actual combustion zone. The combustion cell is very well insulated, and designed to achieve the high temperatures that are ideal for efficient combustion.
If you change to a drier fuel, it will often ignite as soon as it enters the drying/combustion cell. You will then get a much larger, hotter fire, and the cell temperatures can easily exceed the limits of the refractory. That will destroy the refractory, and they will have to shut down the plant for major repairs. I have seen plant operators on P.E.I. spraying water on wood chips that were too dry, to try to reduce their cell temperatures.
Some years ago a power plant in Grande Prairie, Alta., lost its local hog fuel supplier and found a new supplier in B.C., which offered them drier fuel (residue from Mountain pine beetle wood). The plant operators thought that made more sense than trucking fuel composed of roughly 50 percent water by weight. Their fuel cell refractory was fried in due course, and a major repair job ensued, costing several million dollars. The plant sat idle for a number of months. No one was happy – neither the plant owners, the buyers of steam and power, nor the biomass system manufacturer. The manufacturer had a clause in the warranty for the combustion system specifying that only wet fuel was to be burned, and requiring the plant owner to consult with the manufacturer if any changes were to made in the biomass used. The warranty had been voided.
Most biomass plant operators are aware of the moisture content design parameters of their combustion cells, and they are unlikely to deviate from the recommended fuel moisture content.
Hunter River, P.E.I.
(Thanks for your thoughts on this, Bruce, which go a long way toward explaining the limitations of our existing biomass power plants. Clearly they cannot just flick a switch and start burning dry fuel. But shouldn’t we be moving toward a better system? At a certain point we stopped using leaded gasoline, and we mandated catalytic converters, despite resistance from the automobile industry.
As you know, the Scandinavians leave biomass piles to dry in the woods – and to drop needles and leaves – for half a year or so, often covered by biodegradable tarps that can go right into the chipper. Of course, in our region we’re talking about stems, not slash. We know that round wood stacked at roadside for the summer months, even uncovered, will lose a lot of its moisture – probably not down to 20 percent, but enough to reduce transportation costs and improve combustion efficiency.
A friend was telling me recently about the boiler that was used at the Maritime Forest Ranger School in Fredericton. The 8-foot wood that fueled it was dried for as long as a year, for a target moisture content of 25 percent. (Apparently that boiler ended up at the Marwood plant in Traceyville, N.B.) Probably it would be easier to arrange a year-round supply of dry biomass when buying from local landowners, and operating on a more manageable scale – rather than working with a large and complex supply chain that has a mind of its own. Perhaps you would agree that small or mid-sized co-gen plants, with more sophisticated technology, is the direction we should be headed. DL)