Wood pellet milling tests in a suspension-fired power plant

Abstract

This paper investigates the milling behavior of two industrial wood pellet qualities (designated I1 and I2 as per ISO 17225-2:2014) in large-scale coal roller mills, each equipped with a dynamic classifier. The purpose of the study was to test if pellet comminution and subsequent particle classification (i.e., the classifier cut size) are affected by the internal pellet particle size distribution obtained after pellet disintegration in hot water. Furthermore, optimal conditions for comminuting pellets were identified. The milling behavior was assessed by determining the specific grinding energy consumption and the differential mill pressure. The size and shape of comminuted pellets sampled from burner pipes were analyzed by dynamic image analysis and sieve analysis, respectively. The results showed that the internal pellet particle size distribution affected both the milling behavior and the classifier cut size. I2 pellets with coarser internal particles than I1 pellets required more energy for milling, led to a higher mill pressure drop and showed a larger classifier cut size. Comminuted pellet particles sampled from burner pipes were notably Masche et al. / Wood Pellet Milling Tests in a Suspension-Fired Power Plant / 2 finer than internal pellet (feed) particles. At similar mill-classifier conditions, characteristic particle sizes of 0.50 mm for comminuted I1 pellets (compared to 0.83 mm for material within I1 pellets) and of 0.56 mm for comminuted I2 pellets (compared to 1.09 mm for material within I2 pellets), respectively, were obtained. Pellet comminution at lower mill loads and lower primary airflow rates reduced the mill power consumption, the mill pressure drop, and the classifier cut size. However, this was at the expense of a higher specific grinding energy consumption. Derived 2D shape parameters for comminuted and internal pellet particles were similar. Mill operating changes had a negligible effect on the original elongated wood particle shape. To achieve the desired comminuted product fineness (i.e., the classifier cut size) with lower specific grinding energy consumption, power plant operators need to choose pellets with a finer internal particle size distribution.

Experimental and modelling study on the influence of wood type, density, water content, and temperature on wood devolatilization

“…In a pulverized wood-fired power plant, particles fed to the pulverized burners are in as size range of from 10 µm to 5 mm wood particles [27], The millimeter-size wood particles in a pulverized biomass combustor may result in unburned carbon in both bottom and fly ash, and therefore need special attention [28]. In addition, the experiments with millimeter-sized wood particles are relatively easy to be carried out.…”

Charcoal as an Alternative Reductant in Ferroalloy Production: A Review

“…The pre-treatment of herbaceous biomass can be adopted from biomass combustion, where the lignocellulosic materials are first decentralized milled and pelletized, and second biomass pellets are then milled using coal roller mills prior to combustion [79]. A number of studies [80][81][82][83][84][85][86][87][88] have investigated the influence of mill type on both the particle size and shape. Momeni [80] showed that comminuting woody pellets in hammer and roller mills produced significantly different sized particles.…”

“…In addition, it was reported that a high moisture content (>20%) increased the specific energy consumption by 50% [86]. It appeared that different feedstocks (switchgrass, corn and soybean) showed differences in the particle size and shape during comminution and generated particles with various morphological properties [87,88]. Milling of the thermally treated material showed that the energy efficiency can increase twice at temperatures up to 180 • C and decrease at carbonization temperatures above 270 • C reduced by factor of 4 [89].…”

From wood chips to pellets to milled pellets: The mechanical processing pathway of Austrian pine and European beech

“…dc,min represents the narrowest maximum chord length of a 2D particle projection measured from all measurement directions. Recent studies [37,38] suggest that dc,min gives close results to sieving data. Two shape factors provided by the Camsizer ® X2 software were used to characterize the particle shape; elongation ratio (width-to-length ratio) and circularity.…”