water bath effective for rock removal. How-ever, the water bath has serious disadvan-tages, including: • If you are using the biomass for fuel, it is best not to get it wetter than necessary. • If you operate in a very cold cli-mate, wet biomass can freeze into lumps. • The cleaning water quickly be-comes contaminated and must be continually refreshed. Treating the wastewater can be costly, unless the facility already has a plant with a large wastewater treatment system. 2. air Density separation Whereas both rocks and biomass are heavier than air, it has been found that heavier particles will fall out of a moving air stream and lighter particles will be car-ried away. Air density separation (ADS) systems are effective at rock removal, but also have disadvantages, including: • • • • A large piece of wood can weigh more than a small pebble, so ADS systems effectively work only over a small range of densities, where the particles are similar in size. Generally, they are capacity lim-ited with low throughputs, requir-ing multiple machines. Large volumes of air are required, resulting in the purchase and op-eration of large fans and motors. Effective air clean-up systems that will meet emission standards are required. A hammer blow to your operating costs. The hammer mill Granulex™ is the new dynamic grinding machine from Buhler. Designed for ultimate power, Granulex™ delivers high capacity grinding up to 15 t/h for wood and 75 t/h for biomass. Heavy design and supreme ease of maintenance minimize downtime, so you can make maximum use of this productivity. It’s an investment in quality that is sure to show a rapid return – and deliver a hammer blow to your operating costs. Buhler Inc., 13105 12th Ave N., Plymouth, MN 55441, 763-847-9900, [email protected], www.buhlergroup.com Granulex TM High capacity hammer mill DFZP. Powerful 500 hp (400 kW) motor for high capacity grinding. Large screen area reduces wear of screens and hammers. Screens and hammers designed for replacement by a single person in less than 30 minutes. Smooth running sliding doors on both sides for fast and easy maintenance. Very high rotor speed for superior grinding effi ciency. Innovations for a better world. 3. screening for size If only one particular size of rock is caus-ing problems, it is possible to screen-out that size of particle and discard it. How-ever, similar sized particles of biomass will also be discarded, which may or may not be acceptable. Generally, I have found that a combina-tion of screening for size and air density separation is the best method of removing rock particles from biomass. First, screen the entire product flow into different size classifications, one or more of which will contain the offending rock particle size. Then process the selected contaminated stream through an appropriately designed air density separator to separate the bio-mass from the heavier non-organics. In most cases, it is fairly easy and not too costly to retrofit a rock removal sys-tem into industrial processes that already utilize multiple levels of size screening, as many of the required components are al-ready in place. One example is a pulp mill that dis-covered that rocks 1/4" to 3/8" were caus-ing substantial wear to the pulp screens. This mill already had multiple levels of chip size screening in place for process purposes. It was found that most rocks larger than 3/8" were being removed by the primary screen and over-thick ADS system, whereas the tertiary screen re-moved sand and grit smaller than 1/8". But rocks between 1/8" and 3/8" were staying in the chip flow to the digester. It was found that these sizes of rocks were concentrated in the chip flow from the secondary to tertiary chip screens, and it was therefore recommended that this contaminated stream be diverted offline to an ADS system where the rocks could be removed. NOVEMBER/DECEMBER 2011 34 Canadian BIOMASS
Canadian Biomass November/December 2011: Page 34
