No abstract

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. b s t r a c tThe study draws upon the milling theories developed for the ore processing industry (Von Rittinger, Kick and Bond theories) in order to define a method for characterising wood chip and pellet energy consumption during milling.Energy consumption during wood milling depends on three main factors: the material moisture content, the particle size difference between the feed and the milled product, and the material itself. The latter may be characterised by a single parameter based on an adaptation of Von Rittinger's constant.A relation characterising wood pellet energy consumption as a function of the particle size distribution of the pellet ingredients and the milled pellets is proposed. This is characteristic of each type of pellet for each moisture content value considered. ª 2013 Elsevier Ltd. All rights reserved. IntroductionThe international drive to cut greenhouse gas emissions, carbon dioxide in particular, has prompted many electricity producers to convert some of their pulverized coal or gas plants to biomass co-combustion plants. There is even an example of a plant being converted to run on wood pellets exclusively [1]. Adding biomass to fossil fuels has a number of advantages. From an environmental point of view using biomass reduces fossil CO 2 , SO x and NO x emissions rapidly. Moreover, from a technical point of view boiler efficiency increases and lower fuel cost makes the plants more costeffective [2e4]. Some power plants use biomass (generally wood-derived) in pulverized form, which requires milling before use to produce wood particles that have the requisite aerodynamic and combustion properties to be used in the process. In such systems the characteristics of the biomass particles are vitally important as they impact upon feeding, combustion kinetics, the combustion residue volume and the hearth temperature [5e8].For most such co-combustion units the specification requires all of the particles to pass through a 6.34 mm mesh and the majority to pass through a 3 mm mesh [4]. However, some consumers sometimes apply stricter specifications [1,9].Grinding chips and pellets can pose problems, however. There is a lack of data for dimensioning the mill when designing power plant supply circuits. Conversely, in existing circuits, acceptance of new materials requires true scale pilot projects. More generally, little research appears to have been done overall into fine milling of biomass. Available online at www.sciencedirect.com ht tp://www.elsevier.com/locate/biombioe b i o m a s s a n d b i o e n e r g y 5 6 ( 2 0 1 3 ) 7 0 e8 1 0961-9534/$ e see front matter ª

To quantify the impact of forest management practices and tree growth rate on the potential uses of Douglas-fir wood, nine physico-mechanical properties were studied on more than 1250 standardized clear specimens. These were collected from trees cut in 11 even-aged stands (six trees per stand) located in Wallonia (southern Belgium). Stands were 40 to 69 years old, and mean tree girth was ca. 150 cm. Mean ring width of the 66 trees ranged from 3 to more than 7 mm. Statistical analysis showed significant but weak effects of ring width on the studied properties. Considered jointly, mean ring width and cambial age of the test specimens only explained 28% to 40% of the variability of their properties. Also, when ring width increases, these properties display higher decreases in juvenile wood than in mature wood. From a technological standpoint, maintaining mean ring width under 4 mm in juvenile wood and 6 mm in mature wood should accommodate all potential uses of Douglas-fir wood. However, considering that density appeared to be the main driver of wood properties, our results and the literature corroborate the importance of genetic selection as a complement to silvicultural measures to improve or guarantee the technological properties of Douglas-fir wood.