The current method of biofuel densification - pelletizing - can be best described as “compacting dust fines” into a typical 6 mm diameter cylindrical shape, approx 12 mm long. Pellets made from dust fines, if not handled properly, can cause explosions, fires, carbon dioxide off gassing and respiratory problems. Every time pellets are handled there is the risk of more fine particulates separating from the pellet body. Making fuel pucks from larger particulate can minimize these hazards.
Pelletizing consumes large amounts of energy, especially if whole tree grinding is necessary to convert this feedstock to fines. Pellet manufacturing requires higher capital and operational costs per ton of output than fuel puck manufacturing. Hammer milling might not be required for fuel puck manufacturing.
Fuel puck manufacturing is more economical due to lower capital and operational costs, as well as lower repair and maintenance costs and faster start-up times. Die replacement in pellet mills is a significant cost and the dies are more sensitive to feedstock specifications.
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Fuel pucks also can use larger feedstocks of smaller particle size, minimizing dust related hazards. Torrefied fibers also can be briquetted, thus improving overall performance and storage characteristics. Fuel pucks made from the briquetting process have similar heat and density values to pellets and can be bulk or container shipped.
Coal-fired power plants can use their own stationary grinders prior to fuel injection and thereby create fines just in time, thus minimizing hazards and dust mitigation costs. This will reduce the amount of dust in the densified product, reduce the emission of dusts from the fuel in its transfer stages and handling, as well as reduce the exposure time of the fines/dust in the journey to its market. By using JIT grinding methods ahead of the co-fired burners, one can reduce the risks of dust explosions, as well as having better control of micron size materials going into the burners.
The idea of grinding in a JIT mode is not an unknown technology in coal-fired plants. Some co-firing utilities already do this and most grind all of their coal input in a “pulverizer.”
Briquetting process
Briquetting is more cost effective than pelletizing and can minimize dust hazards. It is well known in Europe and has been in use for more than 50 years, both in consumer and industrial co-firing and district heat plants.
Roll press briquetting is the main technology that has been used for decades to compact high capacities of coal fines into briquetted coal shapes.
Dust control by briquetting fines has been used for many years in major paper and tissue mills, as well as printing plants worldwide. Other applications include briquetting-sanding dust in MDF panel manufacturing plants and plywood mills where the fines are very dry and explosive in nature.
Briquetters are simple, robust, use modern PLC controls and are designed for 24/7 operation. They also are available in size and configurations to suit plant requirements.
Fuel pucks, a type of briquetting which has some trademarks associated with it, are used primarily in biomass-fired power generation, co-fired power generating stations, district heat and other industrial heating applications, especially in Europe.
New markets for fuel pucks
North American utilities can use the best technical features of bio-fired European utilities while adding their own improvements to suit local markets. This could involve co-firing with biomass and natural gas, which has already been done in Sweden, further reducing carbon emissions as compared to strictly coal co-firing. The advantage here is access to larger and lower cost gas reserves compared to Europe. There also are advantages in logistics.
Demand for fuel briquettes is large as there are more than 600 coal-fired plants in North America, many in the 600 MW range, which equates to approximately 1 million tons of coal per year per plant. If 5-10 percent of it is exchanged for biomass fuels, it equates to millions of tons of required biomass.
Transport and handling
Bulk densities of densified products do not change with larger particulate size. Briquetters making fuel pucks can use variable types of forming dies to suit feedstock requirements. This is not as easy and more expensive and time consuming to achieve in pelleting.
North American utilities will have advantages due to reduced shipping costs and localized feedstocks, as well as the use of railcars for transport. These could be the same type of cars used for coal. In most cases local utilities will not have to deal with ships and demurrage like their European counterpart. Very expensive pellet ship loading and unloading facilities are few and far between, and utilities closest to these facilities have an advantage. These reduced local handling costs can pay for the extra grinders required for larger particulate densified fuel formats.
Transporting dust in densified pellets from production to end use increases the risks of explosions, fires, off gassing and respiratory hazards. Every time these types of agglomerated dust feedstocks are handled, more fines break off causing dust buildup.
Existing process
Coal-fired utilities need dust particulate size to co-fire and biomass particulate size, which is similar to dust, is highly explosive, more than even coal dust.
Scientific research supports all of this and is validated by real world scenarios, including pellet plant fires, many of which go unreported; ship loading facility fires, where dust is 3-5 percent of total mass and is extracted by expensive collectors along the handling route-silos; co-firing plant explosions; conveyors with static electricity potential; and issues in which pellets are loaded and unloaded “as is” pellets to maximize profits.
Enormous amounts of energy are required to drive a log through a 6 mm hole in the pellet manufacturing process. The big sawmill sawdust pile is history, and mills must now resort to chipping whole logs, which is only the beginning of a long, expensive process of size reduction.
