Flow properties of biomass and coal blends
Introduction
Biomass fuels, such as wood, bark, straw, and agricultural by-products, are of significant interest in the context of renewable energy as they currently form the world's third largest primary energy resource after coal and oil [1]. Furthermore, the interest in biomass utilisation is on the rise due to concerns over the impact of conventional fossil fuels (e.g. coal) on the global warming [2]. Co-firing of biomass fuels with coal is presently being considered as an effective means of reducing the global CO2 emissions [3], [4], [5]. The rationale is that the photosynthesis of growing plants removes CO2 from the atmosphere and subsequently eliminates its net contribution to the atmospheric build-up after combustion. The co-firing technology has attracted much attention in recent years and as a result, demonstrations have been performed in a number of utility installations across the world, including Australia, Europe and United States. All types of combustion technologies have been used to co-fire biomass with coal including cyclone boilers, wall-fired and tangentially fired pulverised coal boilers, fluidised bed boilers, as well as stoker-fired boilers. Capacities used in co-firing tests and commercial applications have ranged from 50 to 600 MW. As pointed out by Davidson [2], despite recent progress in co-firing research and development, there are many technical issues which remain unresolved, at least in part. These issues can be categorised under four major headings, namely: (a) fuel processing and handling; (b) fuel combustion; (c) ash deposition; and (d) emissions. Fuel processing and handling issues in pulverised fuel boilers, in particular, seem to have a significant impact on the overall success of co-firing in such systems [1], [2]. Issues, such as bridging and blockage of feeding systems as well as poor pulverisation characteristics of coal/biomass blends, could severely hamper the co-firing of coal and biomass even at relatively low blending ratios.1 Although such problems have been experienced in many pilot-scale [6], [7] and large-scale [8], [9], [10], [11] co-firing studies, there is no fundamental understanding of the underlying mechanisms that cause such problems. The relevant literature on the flow properties of coal/biomass blends is particularly scarce. The work presented in this paper is part of a larger project aiming at addressing the above-mentioned shortcoming. Our aim here was to investigate the flow stoppage problem by analysing a set of results on flow properties of biomass/coal blends. The results of our study on milling characteristics of coal and biomass blends will be presented in our future publications.
Section snippets
Equipment and procedures
The bulk material experiments presented in this study were performed using a Jenike type direct shear tester to obtain the relevant material strength properties, a Wall Yield Locus (WYL) apparatus for measuring frictional characteristics, and a compressibility tester for the measurement of the bulk density. The theory behind these tests is provided in Appendix A.
The Jenike type direct shear testing machine, as described by Jenike in 1976 [12], was equipped with a shear cell of circular cross
Direct shear measurements
The results of compressibility experiments carried out as part of direct shear tests are shown in Fig. 3. As shown, the values of bulk density vary about 150 kg/m3 between coal sample and the 10 wt.% biomass/coal blend. At higher normal stress, bulk density of coal sample reaches around 900 kg/m3, whereas the blend sample reaches around 750 kg/m3. The fact that both curves have roughly the same gradient under the same normal stress values suggests that the change in bulk density due to sawdust
Conclusions
Bulk handling issues, such as bridging and blockage in particular form a major problem in co-firing practise of pulverised fuel boilers [1], [2], [6], [7], [8], [9], [10], [11]. The focus of this paper was to address this shortcoming by providing a set of results on flow properties of biomass/coal blends.
Two types of experiments were carried out to investigate the flow properties of solids as they shear towards each other (direct shear experiment) and as they move inside a wall material (wall
Acknowledgements
The authors would like to acknowledge the financial support given to them by the CRC for Coal in Sustainable Development and the assistance provided by the TUNRA Bulk Solids towards completion of this project.
References (14)
Combustion of agricultural residues
Progress in Energy Combustion Science
(2000)Full-scale co-firing of straw and coal
Fuel
(1996)- et al.
Southern company tests of wood/coal co-firing in pulverised coal units
Biomass & Bioenergy
(1996)- et al.
Impacts of wood cofiring on coal pulverization at Shawville Generating Station
Fuel Processing Technology
(1998)
Cited by (70)
Valorization of biomass to palatinose
2023, Valorization of Biomass to Bioproducts: Biochemicals and BiomaterialsHydrothermal co-carbonization of industrial biowastes with lignite toward modified hydrochar production: Synergistic effects and structural characteristics
2022, Journal of Environmental Chemical EngineeringFlow behavior characterization of biomass Feedstocks
2021, Powder TechnologyCitation Excerpt :By this definition, the commonly used term free flowing, is only meaningful when both the bulk solid and the specifications of the solids handling equipment are given. Co-firing biomass with coal is presently being considered as an effective means of reducing global CO2 emissions [24]. Co-firing is generally viewed as the most cost-effective approach to biomass utilization by the electric utility industry [100].
Flowability characterization of torrefied biomass powders: Static and dynamic testing
2020, Biomass and Bioenergy