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

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

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

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

Flame speed and Kst reactivity data for pulverised corn cobs and peanut shells.

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 flame speed and explosion Kst data for two agricultural waste materials: corn cobs and peanut shells. Both parameters were measured on the ISO 1 m3 dust explosion equipment. Two coarse size fractions of corn cobs (CC) and peanut shells (PS) of size less than 500 μm were tested using the Leeds 1 m3 vessel and were compared with two pulverized coal samples. This is typical of the size fraction used in pulverized coal power stations and of pulverized biomass currently used in power generation. The explosion parameters minimum explosive concentration (MEC), rate of pressure rise (dP/dt), deflagration constant (Kst), peak to initial pressure rise (Pm/Pi), turbulent and laminar flame speeds were determined using a calibrated hemispherical disperser in the 1 m3 vessel. MEC were measured in the range of 0.6-0.85 in terms of burnt equivalence ratio, Øburnt, which were comparable to the coal samples. The measured Kst (25-60 bar m/s) and turbulent flame speeds (~1.3 m/s) were lower than for coal, which was a reflection of the lower calorific value. These results showed that these crop residues are technically feasible power plant fuels to burn alongside coal or as a renewable biofuel on their own

Combustion of Pulverized Biomass Crop Residues and Their Explosion Characteristics

Two Pakistani crop residues bagasse (B) and wheat straw (WS), both with high ash content, were milled to <63µm and the ISO 1 m3 explosion equipment was used to investigate flame propagation in the dispersed cloud of pulverised biomass. Their turbulent flame speed was measured and the Kst (dP/dtmaxV1/3) and comparison was made with two pulverised coal samples. Minimum Explosion Concentration (MEC) values for B and WS were, in terms of the burnt dust mass equivalence ratio (Ø) 0.2Ø to 0.3Ø , which was leaner than for the coal samples. These MEC were lower than had previously been determined using the Hartmann explosion tube, and this was considered to be due to the 10 kJ ignition energy in the 1 m3 equipment and 4J spark energy in the Hartmann explosion tube, which extended the lean limit in the 1 m3 equipment. Peak turbulent flame speeds were 3.8 m/s for B and 3.0 m/s for WS compared with 3.5–5.2 m/s for the two coal samples. The peak Kst was 103 bar m/s for bagasse and 80 bar m/s for wheat straw and the two coal samples had peak Kst of 78 and 120 bar m/s. Overall the agricultural biomass and coal samples had a similar range of reactivity. Thus these agricultural crop residues are a viable renewable fuel for co-firing with coal or as 100% biofuel operation of steam power plants

Agricultural Waste Biomass Energy Potential In Pakistan

Pakistan has a major electricity supply problem with urban areas having a very intermittent supply of electricity. The supply gap at periods of high demand is 6 GW. Pakistan has a large agricultural economic sector and produces a substantial amount of waste material that has little current economic use. This work shows that these agricultural wastes are a significant energy resource that could be used to generate electricity using relatively small biomass generator sets that could take all the waste biomass from the surrounding agricultural area. Pakistan currently imports most of the oil used for electricity generation. The cost of this result in high cost electricity and it is shown that bio-electricity could be generated competitively in Pakistan. It was estimated, based on 30% thermal efficiency of electric power generation, that the annual production of crop residues have the potential to generate 76% of the annual electricity requirements of Pakistan. For this to come from agricultural wastes in farmland, transport costs would have to be minimised. It is proposed that a series of about 10MWe plants should be established (which are commercially available) with all farms in about a 10km radius delivering their agricultural solid waste to the plant at the farmers cost with direct payment by the power generator

Explosion and Flame Propagation Properties of Coarse Wood : Raw and Torrefied

A current production torrefaction process was used and the explosion and flame propagation properties were determined at the particle size of the raw (spruce, pine and fir – SPF) and torrefied biomass. The biomass material as received was sieved to <1mm. Size analysis showed that 10% by mass was <100µm and the torrefied sample had 25% <100µm. The CV for the torrefied biomass was 10% greater than that for the raw biomass. The ISO 1 m3 dust explosion vessel was used, with a modified and calibrated biomass dispersion system that could cope with very coarse particles. The explosions did not burn all the dust that was present at the start of the explosion and the residual unburnt dust was shown to be the original dust. The equivalence ratio, Ø, of the propagating flame was based on the burnt dust concentration, Øburnt. Raw and torrefied samples were found to have minimum explosion concentrations, MEC, of 2.3Øburnt and 1.4Øburnt respectively and this shows that the torrefied sample was more reactive as it had a leaner MEC. The deflagration index, Kst, was higher for the torrefied SPF with a peak at 35 bar m/s compared with 24 for the raw biomass. The peak turbulent flame speeds were similar for torrefied and raw biomass at about 1 m/s. The torrefied biomass was more reactive than the raw biomass mainly due to the smaller particles size and 10% higher CV. The mechanism for coarse particle combustion is considered to be due to the explosion induced wind blowing the finer fractions ahead of the flame which burn first with the coarser fractions gasifying in the rich burnt gases behind the initial flame. The rich MEC was caused by the requirement to have the fine fraction above the MEC when only about 10% of the mixture was fine