Impact of non-central vents on vented explosion overpressures

It is normal practice to use centrally positioned vents or single vents in most experimental work and in the application of explosion venting in industry. This work seeks to investigate the influence of non-central and multiple distributed vents on the explosion overpressure. A 10L cylindrical vessel of 460mm length and 162mm diameter (L/D=2.8) was used for vented explosion with free venting (without a vent cover). Three different vent coefficient (Kv) were investigated, Kv, 3.6, 5.4 and 10.9 for both non-central and 4 hole vents. 10% methane-air and 7.5% ethylene-air mixtures were investigated to determine the influence of the mixture reactivity. The position of the spark ignition was in the centre of the end flange opposite the vent. It was shown for the non-central vent that the flame speed upstream of the vent was lower than for a central vent and this reduced the mass flow through the vent, which reduced the overpressure and reducing the external explosion due to the lower exit velocity of the unburnt gas and hence lower external turbulence. The external flame jets downstream of the vent was influenced by the increase in characteristic length scale of the vent, which was changed by increasing the number of vents

Vent static burst pressure influences on explosion venting

The overpressure generated in a 10L cylindrical vented vessel with an L/D of 2.8 was investigated, with end ignition opposite the vent, as a function of the vent static burst pressure, Pstat, from 35 to 450mb. Three different Kv (V2/3/Av) of 3.6, 7.2 and 21.7 were investigated for 10% methane-air and 7.5% ethylene-air. It was shown that the dynamic burst pressure, Pburst, was higher than Pstat with a proportionality constant of 1.37. For 10% methaneair Pburst was the controlling peak pressure for K Pburst in the literature and in EU and US standards. For higher Kv the overpressure due to flow through the vent, Pfv, was the dominant overpressure and the static burst pressure was not additive to the external overpressure. Literature measurements of the influence of Pstat at low Kv was shown to support the present finding and it is recommended that the influence of Pstat in gas venting standards is revised

Gas explosion venting: comparison of experiments with design standards and laminar flame venting theory

European and USA design standards for gas explosion venting are quite different in their predictions, with the European standards always giving a higher predicted explosion Pred for the same vent coefficient, Kv. The format of the two predictions are different with the US standards following the approach of Swift expressing the vent area as a ratio to the surface area of the vessel, As/Av and the European standard using the vent coefficient approach. Kv= V2/3/Av. It is shown that these two approaches are directly related as As is proportional to V2/3. The reactivity parameter in the US standards is the laminar burning velocity, UL, and in the European venting standards it is the deflagration parameter, KG = dp/dtmax/V1/3. It is shown that these two reactivity parameters are linearly related. The USA standard is shown to be compatible with spherical flame venting theory and with experimental data other than that of Bartknecht. There is also good agreement with the present results for a 10L vented vessel for which the spherical laminar flame venting theory gives reasonable agreement but predicts the Pred to be higher than measured. This is because of the assumption that at the maximum value of Pred the bulk flame area is equal to As which is not valid. The US standard also has corrections for flame self acceleration, which is a vessel size effect, and for the influence of vessel size on the external explosion, which are not factors addressed in the European standards. The European standard is the equation for the results of Bartknecht for a 10 m3 vessel and the results of higher and lower volumes in Bartknecht’s results are all lower that for 10 m3. The experimental results reviewed, for methane and propane maximum reactivity vented explosions, include data for vessels larger than that on which the European standards are based and they all give significantly lower values of Pred than those of Bartknecht.

Comparison of central and end spark position for gas explosion vessels with L/D of 2.8 and 2.0

Current explosion vent design correlations and guidance are based on an experimental data base of centrally ignited vented tests. However, there is evidence in the literature that ignition positions other than central produce higher overpressures. The objective of this work was to compare central and end ignition of vented explosions in a 10L and a 200L cylindrical vessels of L/D of 2.8 and 2 respectively, with vent area coefficients of 10.9, 5.4 and 3.1 for free venting. Methane-air (10% v/v) and ethylene-air ( 7.5%) explosion tests were carried out using a 16J spark ignition at the far end wall opposite the vent and half way along the length of the vessel. The results showed that for both vessels and for both gas/air mixtures end ignition produced the highest overpressures. This was attributed to the higher axial flame speed towards the vent with far end ignition, inducing higher vent mass flows and higher external flame speeds and associated overpressures. The present results and other data from the literature show that the vent design guides may not be based on sufficiently conservative data and need to be reviewed

Effect of steam exploded treatment on the reactivity of pine wood

A commercial thermally treated biomass process known as ‘steam exploded biomass’ provided the treated biomass samples for this project together with the original yellow pine wood. The aim was to investigate the change in pulverised biomass reactivity. The steam exploded biomass is processed into pellets in the normal way and are known as black pellets (BP). The material was investigated using the Hartmann dust explosibility equipment. This enables the minimum explosion concentration (MEC) to be determined together with the initial rate of pressure rise and the flame speed and these latter parameters are measures of the mixture reactivity. BP was found to have a higher reactivity than the raw biomass with a much leaner MEC. A good correlation was found between the initial rate of pressure rise and the flame speed for the raw wood sample. Surface morphology was performed to investigate the effects of the steam exploded treatment. This showed the enhancement of the proportion of fines. The particle size distribution was determined and this confirmed the enhancement of the fineness of the treated sample. The enhanced reactivity of BP was found to be due to the greater proportion of fine particles which had a higher heating rate and a greater release of volatiles. The steam explosion treatment was found to be an effective pre-treatment in facilitating the combustion of renewable fuel and the main effect was that it was more easily milled, changes in the biomass chemistry was of secondary importance

Development of the Controlled Atmosphere Cone Calorimeter to Simulate Compartment Fires

The cone calorimeter with the controlled atmosphere compartment was used to control the fire air ventilation and to simulate the behaviour of materials in compartment fires, with rich burning under post flashover conditions. The standard cone calorimeter with controlled atmosphere design has to be improved, by compartment wall insulation, to reduce heat losses which reduced the fire temperature. Heat losses from the test section to the water cooled load cell were shown to be significant and the test specimen was insultated from the support. A chimney was added to the cone outlet to enable the measurement of the mean composition of the raw discharge gases. A method was developed for determining the mean gas sample and to prevent back flow of external air. This improved design was used to create under ventilated fires with pine wood where the equivalence ratio was controlled by the air flow into the compartment. These modified procedures for the cone calorimeter greatly extend its usefulness in material testing to conditions close to those encountered in post flashover compartment fires

Burning Properties and Flame Propagation of Varying Size Pulverised Rice Husks

Flame propagation in different size fractions of a rice husk (RH) crop residues were investigated using an ISO 1 m3 dust explosion vessel. This was modified to operate with coarse biomass and for the determination of flame speeds. The flame speed, burning velocity and Kst were found to be greater for the finer fractions compared to the coarser sizes. The MEC were measured at 0.27 equivalence ratio (Ø) for the finest fraction to 1.4Ø for the coarser fraction. The most reactive concentration was measured at lower Ø for fine particles and higher Ø for coarse particles. The maximum Kst for the fine particles was 83 bar m/s and 33 bar m/s for the coarse particles. The size distribution of coarse rice husk particles always has a fine fraction and the flame propagation occurs first in the fine particles, with the coarse particles burning in the hot products of combustion of the fine particles. The fine particle fraction in a coarse mixture has to be flammable and as there is a low proportion of the mixture in the fine fraction, the overall concentration of particles has to increase for the concentration of fines to be flammable. This resulted in the observed lean flammability limit that was richer than stoichiometric for coarse size mixtures

Solid Biomass to Medium CV Gas Conversion With Rich Combustion

A modified cone calorimeter for controlled atmosphere combustion was used to investigate the gases released from fixed bed rich combustion of solid biomass. The cone calorimeter was used with 50 kW/m2 of radiant heat that simulated a larger gasification system. The test specimen in the cone calorimeter is 100mm square and this sits on a load cell so that the mass burn rate can be determined. Pine wood was burned with fixed air ventilation that created rich combustion at 1.5–4 equivalence ratio, Ø. The raw exhaust gas was sampled using a multi-hole gas sample probe in a discharge chimney above the cone heater, connected via heated sample lines, filters and pumps to the heated Gasmet FTIR. The FTIR was calibrated for 60 species, including 40+ hydrocarbons. The hydrogen in the gas was computed from the measured CO concentration using the water-gas shift reaction. The exhaust gas temperature was also measured so that the sensible heat from the gasification zone was included in the energy balance. The GCV of the pine was 18.8 MJ/kgpine and at the optimum Ø the energy in the rich combustion zone gases was 14.5 MJ/kgpine, which is a 77% energy conversion from solid biomass to a gaseous fuel feed for potential gas turbine applications. This conversion efficiency is comparable with the best conventional gasification of biomass and higher than most published conversion efficiencies for coal gasifiers. Of the energy in the gas from the rich combustion 35% was from the CO, 20% from hydrogen, 35% from hydrocarbons and 10% sensible heat. Ash remained in the rich burning gasification zone. As the biomass is a carbon neutral fuel there is no need to convert the gasified gases to hydrogen, with the associated energy losses

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

Steam Exploded Pine Wood: The Influence of Particle Size on Mixture Reactivity

Power generation using waste material from the processing of agricultural crops can be a viable biomass energy source. However, there is scant data on their burning properties and this work presents measurements of the minimum explosion concentration (MEC), flame speed, Kst , and peak pressure for pulverised pine wood and steam exploded (black pellets) pine wood. The ISO 1 m3 dust explosion vessel was used, modified to operate on relatively coarse paticles, using a hemispherical dust disperser on the floor of the vessel and an external blast of 20bar compressed air. The pulverized material was sieved into the size fractions <500µm, <63, 63-15-, 150-300, 300-500µm to study the coarse particles used in biomass power generation. The MEC was measured in the range of 0.6-0.85 burnt equivalence ratio, Øburnt,. The measured Kst (25-60 bar m/s) and turbulent flame speeds (~1.5 - 5 m/s) These results show that the steam exploded pine biomass was more reactive than the raw pine, due to the finer particle size for the steam explosed biomass