Explosion reactivity characterisation of pulverised torrefied spruce wood

Wood and other agricultural powders have been recognised as hazardous for a long time. These kinds of materials are also now being used for power generation in 100% biomass plants or mixed with coal as a way of reducing greenhouse gas emissions. However, safety data for biomass is very scarce in the literature, and non-existent for upgraded biomass products such as torrefied biomass, largely due to the challenges that biomass poses for explosion characterisation in the standard methods (1m3 ISO vessel or 20L sphere). The Leeds group has developed and calibrated new systems for the 1m3 ISO vessel that overcome such challenges and thus, this work presents the first data available in the literature for torrefied biomass explosion characteristics, results for untreated Norway spruce wood and Kellingley coal are included for comparison. Also flame speeds and post-explosion residue analysis results are presented. Results showed that torrefied spruce wood was more reactive than Kellingley coal and slightly more reactive than its parent material in terms of Kst, Pmax and flame speed. The differences between coal and biomass samples highlight that it should not be assumed that safety systems for coal can be applied to torrefied or raw wood materials, without suitable modifications

Comparison of explosion characteristics of Colombian and Kellingley coal

Coal continues to be one of the main fuels used for generation of energy in the UK. Despite government’s plans to decarbonise the energy sector in order to meet GHG emission targets, co-firing of coal and biomass is attractive due to the low investment required and since gas prices remain high, the consumption of coal is still considerable in power generation. Pulverised coal has been known to pose explosion risks since the 19th century. The objective of the present work was to compare the explosibility of two coal samples used in UK power stations which potentially can be used co-fired with biomass. Both samples of coal were fully characterised for their chemical composition as well as particle size and morphology. The 1m3 ISO explosion vessel was used to determine the explosion characteristics: deflagration index (Kst), maximum explosion pressure (Pmax) and minimum explosible concentration (MEC). Flame speeds were also measured. The remaining residues after explosion were also analysed. The results were compared to the explosion characteristics of other types of coal available in the literature. Despite the very similar composition of both fuels, the reactivity of Colombian coal was much higher, with a Kst value of 129 barms-1 as opposed to 73 barms-1 for Kellingley coal (Fig.1). There was significant difference between these two coals as the surface area of Colombian coal was 5 times higher than that of Kellingley coal. There was little difference in the elemental composition, but Colombian coal contained more volatiles and less ash. Thus the results indicate a strong impact of particle surface area and volatile content on the reactivity of coal

Biomass Explosion Residue Analysis

On account of its greenhouse gas advantages there is increasing use of pulverized biomass in power generation. However, there is little information on the combustion properties of pulverized biomass and on the explosion hazards they create in the mills, dust conveyor systems and biomass storage silos. This work uses the ISO 1 m3 dust explosion equipment to study the explosion properties and combustion characteristics of pulverized biomass dust clouds. An unreported feature of this apparatus is that in rich concentrations only about half the dust injected is burned in the explosion. This work was undertaken to try to understand, through measuring the mass and composition of the debris at the end of the explosion, why all the pulverized biomass injected did not burn and the consequences for the measured parameters of flame speed, Pmax and Kst. One possible explanation of the results is that the residue material was formed from biomass dust blown ahead of the flame by the explosion induced wind and deposited on the vessel wall, where it was compressed as the pressure increased in the vessel. The flame side underwent flame impingement pyrolysis and the metal side was heated and compressed in the explosion but not burned. This was supported by photographic and pressure decay data that indicated the presence of a “cake” of dust being formed on the wall of the vessel. The results also show that the overpressures remain high for very rich equivalence ratios of up to 6. The reactivity of biomass was higher than coal for the two types of biomass investigated. No rich combustion limit was found. This challenges the general industry assumption that operating in very rich conditions in mills is safe. An explanation is proposed for the high peak pressures under rich conditions

Explosion characteristics of pulverised Colombian coal, pine wood and their mixtures

Co-firing of coal and biomass is a readily implementable, cost effective option of introducing biomass into the European power generation capability. Pulverised coal and biomass can be blended in various proportions and used as fuels in co-firing plants with a subsequent reduction in greenhouse gas emissions. Coal powders have well known explosion hazards and the explosion characteristics for many coals are available in the literature [1], however, data for biomass explosibility are scarcer due to some characteristics of biomass that prevent their characterisation using the existing standard methods [2]. The explosibility of coal/biomass blends is also unknown. The aim of the present work is to provide explosion characteristics and combustion properties such as flame speeds and burning velocities of coal and biomass blends and compare their reactivity to that of coal and biomass alone

Agricultural waste pulverised biomass: lean flammability and flame speed as a measure of reactivity

There is very little information on the combustion properties of pulverised biomass, particularly for agricultural wastes. This makes burner design and optimization difficult and also has implications on fire and explosion hazard protection, both in storage, milling and particle transport to burner. A modified Hartmann dust explosion tube was employed to determine the Minimum Explosible Concentration (MEC) and the flame speed for three Pakistani agricultural wastes: bagasse, rice husk and wheat straw. The MEC was influenced by the particle size distribution and there was a strong linear correlation between the MEC and the sum of the ash and moisture content of these and other biomasses. Comparison of the results was made with more conventional pulverized biomass. Peak flame speeds were approximately 2.5 m/s. The lean limits for these pulverised agricultural waste biomasses were comparable to pulverised wood but much leaner than those for coal and hydrocarbon fuels, which indicate that these biomasses are highly reactive

Comparison of Explosion Characteristics of Torrefied and Raw Biomass

Biomass is one of the most attractive and versatile sources of renewable energy. However, biomass properties such as high moisture and volatile matter, low energy density and poor grindability affect the overall efficiency of traditional combustion processes, fuel supply chains, storage, handling, by­product management, etc. Pre­treatments, such as torrefaction, can upgrade some of such properties. Dedicated and coal co­firing plants, in which pulverised biomass and torrefied biomass can be used, are exposed to explosion hazards during handling, storage and transport from the millers to the boiler. Data on the explosion characteristics of biomass and torrefied biomass are scarce. This study presents explosion characteristics (maximum explosion pressure, deflagration index and minimum explosible concentration) of two torrefied wood samples and compares their reactivity to that of their corresponding untreated biomass materials and to a sample of Kellingley coal. Torrefied biomass samples showed higher reactivity, overpressures were around 9 bar (0.9 MPa, 1bar=105 Pa) for all biomass samples irrespective of size or sample composition. Derived laminar burning velocities ranged between 0.1­0.12 ms­1, and were much higher than for coal (0.04 ms­1). These differences influence the design of explosion protection measures and can be used to introduce suitable modifications for safe operations with torrefied biomass

Improvements to the Hartmann Dust Explosion Equipment for MEC Measurements that are Compatible with Gas Lean Limit Measurements

Literature results for the MEC of dusts show that the MEC in g/m³ convert to lean equiva-lence ratios of 0.2 to 0.3 for many HCO dusts, compared with 0.45 for hydrocarbons and alcohols. This indicates that HCO dusts are more reactive than hydrocarbons, but it could be that it is the measurements of MEC for dusts that are suspect. The measurement of the minimum explosion concentration in dusts is reviewed together with measurement methods for the lean flammability limit of gases. There is considerable uncertainty over the concentration of dusts at the lean limit in the 1 m³ MEC method, as most of the dust injected in the 1 m³ does not burn. It is shown for gases that a vertical tube equipment operated as a closed vessel will give the same lean limit of 0.46Ø for methane-air as the European standard method, but large spheres will not and thus the use of the 1 m³ dust explosion vessel is unlikely to give MEC data that is compatible with lean limits for gases. A study of the MEC of polyethylene dusts was carried out at various ignition delays in the Hartmann and an ignition delay of 50 ms was recommended for Polyethylene dust to give reliable MEC measurements

Explosion characteristics of pulverised torrefied and raw Norway spruce (Picea abies) and Southern pine (Pinus palustris) in comparison to bituminous coal

Pre-treatments, such as torrefaction, can improve biomass fuels properties. Dedicated and coal co-firing plants, in which pulverised biomass and torrefied biomass can be used, are exposed to explosion hazards during handling, storage and transport from the mills to the boiler. Data on the explosion characteristics of biomass and torrefied biomass are scarce. This study presents explosion characteristics (maximum explosion pressure, deflagration index and minimum explosible concentration) of two torrefied wood samples and compares their reactivity to that of their corresponding untreated biomass materials and to a sample of Kellingley coal. Torrefied biomass samples showed higher reactivity, overpressures were around 9bar (0.9MPa, 1bar=105Pa) for all biomass samples irrespective of size or sample composition. Derived laminar burning velocities ranged between 0.1-0.12ms-1, and were therefore similar to that of coal (0.12ms-1). The differences in explosion reactivity influence the design of explosion protection measures and can be used to introduce suitable modifications for safe operations with torrefied biomass