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Comparison of the rate of smoke generated from the combustion of biomass fuels by Dr. John Orrin


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Boiling Point
Front cover of Boiling Point issue 54
Issue 54 (2007) Climate change and household energy
ArticleComparison of the rate of smoke generated from the combustion of different biomass fuels
AuthorJohn Orrin
A smoke chamber has been used to measure the smoke generated from seven different biomass fuels. These fuels are wood, peanut shells, cardboard, chipboard, straw, hay and eucalyptus leaves. This equipment allows a direct comparison of the smoke generated between these seven biomass fuels. A sample of each fuel was exposed to a radiant heat source and the amount of smoke measured with time. Analysis of the results showed that fuels such as straw or hay produced high levels of smoke quickly for a given mass loss. More compact (higher density) fuels took much longer to produce the same amount of smoke illustrating the benefit of producing fuel briquettes. This technique could also be used to measure the smoke generated as the composition of the fuel briquette was varied.

[top] [end]Introduction

Smoke has been defined as the products of burning organic materials in which small solid and liquid particles are also dispersed[1]. For communities that rely on open or three stone fires for cooking in poorly ventilated areas, smoke provides a threat due to its toxicity. This often results in smoke related illness[2].

The amount of smoke generated is sensitive to a wide range of variables such as the orientation of the fuel on the stove, the size and geometry of the fuel, the temperature and the ventilation. The smoke chamber however allows the fuel alone to be examined under precisely defined and constant conditions[3].

When a fire is first started or when new fuel is introduced onto the fire, the fuel will be subject to non flaming (that is smouldering) combustion. If the fuel is carbon based, high levels of smoke are produced initially due to thermal pyrolysis and evolution of volatile matter. The combustion products with high boiling point condense as they mix with the cooler air and this produces a mist of minute droplets. These droplets combine together to give larger droplets of the order of 1mm that can then deposit on surfaces to give an oily residue[4].

When smoke is present in a room, the visibility is reduced because light is scattered and absorbed by the smoke particles. By measuring the visibility or optical density within the room, the amount of smoke can be measured.

The optical density is defined as:
(O.D.) = log10 ( Io/I )

Where:
O.D. = Optical Density
Io = initial intensity of light
I = final intensity of light after passing through smoke

If 50% of the light is absorbed by the smoke particles, then:
Io =100 and I = 50 and the optical density = log10 (100/50) = 0.3.

For an optical density per path length of smoke of 0.3, the visibility is about 4 metres5. For a smoke filled room that has been caused by a poorly vented stove, the visibility would be much higher than this value and the optical density lower. For example, a visibility of 10 metres and optical density per path length of smoke of about 0.1 might be typical values.

[top] [end]Equipment Used For The Measurement Of Smoke

Figure 1. The Smoke Chamber used for the Measurement of Smoke
Figure 1. The Smoke Chamber used for the Measurement of Smoke
Figure 2. Diagram of Smoke Chamber used for the Measurement of Smoke
Figure 2. Diagram of Smoke Chamber used for the Measurement of Smoke
The smoke chamber is shown in Fig. 1. The smoke chamber has a fixed volume of 914mm x 610mm x 914mm. As shown in Fig. 2, the smoke chamber contains the biomass sample with holder situated on guide rails, a light source, a photo multiplier unit and a 25 kW per m2 radiant heat source.

The sample of biomass fuel is first placed in a sample holder and the sample holder plus biomass fuel is weighed on a digital balance to three decimal places. The sample holder is then placed on the guide rails away from the radiant heat source. The door of the smoke chamber is shut and the software programme that controls the smoke chamber is started. The photo multiplier unit is calibrated so that 100% of the light passes across the smoke chamber (corresponding to no smoke). Drawing the biomass sample in front of the heat source then starts the test. As smoke is produced, the amount of light reaching the photo multiplier unit is reduced and the computer continuously records the results. When the light reaching the photo multiplier tube has been reduced by 30%, the test is stopped and the sample removed from the smoke chamber. The sample plus sample holder is again weighed and the amount of biomass fuel used is calculated. The test is then repeated for the next fuel.

[top] [end]Results and Discussion

Figure 3. Reduction of Light Intensity with Time (T70 = time at the end of the test)
Figure 3. Reduction of Light Intensity with Time (T70 = time at the end of the test)
. 3 shows the reduction of light intensity with time for a typical biomass fuel. At the start of the test, little smoke is produced. This simulates the situation when fuel is first introduced onto a fire and the temperature of the sample is low. After a short initial period, there is a rapid increase in the smoke produced as the fuel sample temperature increases. The time for a 30% reduction in light intensity was measured for all the fuel samples and the results are shown in Fig. 4. The weight reduction and initial bulk densities are calculated for all the fuel samples and the results are shown in Fig. 4 and Fig. 5.

Figure 4. Results showing the time for the smoke chamber to fill with smoke for different fuels (for 70% transmission of light).
Figure 4. Results showing the time for the smoke chamber to fill with smoke for different fuels (for 70% transmission of light).
Figure 5. Bulk Density and Average Rate of Mass Loss for Different Biomass Fuels
Figure 5. Bulk Density and Average Rate of Mass Loss for Different Biomass Fuels


It was found that all the biomass fuels tested fall into one of three groups.
  • Group 1: This group included straw, hay, peanut shells and dry cardboard. All these biomass fuels produced smoke quickly (in the range 64 seconds to 93 seconds). These fuels had the lowest bulk density (ranging from 1.28 x 10-4 g/mm3 to 1.78 x 10-4 g/mm3 ). This group of fuels also had the highest rate of weight loss (ranging from 0.011 g/s to 0.022 g/s). With the exception of dry cardboard, these fuels had the highest surface area to volume ratio exposed to the radiant heat source.
  • Group 2: This group comprised wood (pine) and chipboard. These two fuels took much longer to produce the same quantity of smoke that caused a light reduction of 30%. The time for wood (pine) was found to be 317 seconds and for chipboard 250 seconds. Therefore the time required to produce smoke for a given mass loss was in the region of 3 times longer than for group 1 fuels. As can be seen in Fig. 5, the initial bulk density of group 2 fuels is 4 to 7 times higher than for group 1 fuels.
  • Group 3: In this group, Eucalyptus leaves and wet cardboard both contained a large quantity of water per unit mass. As the fuel was exposed to the radiant heat source, large quantities of steam (not smoke) were produced and the time to produce smoke was therefore much longer than the other groups of fuels.

Fig. 6 shows the relationship between the average rate of mass loss of the biomass fuel and the time to produce smoke. Group 1 fuels produced smoke quickly with cardboard producing almost twice the same amount of smoke for a given mass loss as peanut shells. Group 2 fuels took much longer to produce smoke but had a much lower average rate of mass loss. Group 3 fuels (cardboard (wet) and Eucalyptus leaves) were outside the curve shown in Fig. 6 due to the high water content of these biomass fuels.

Figure 6. Time to Produce Smoke (70% Transmission of Light) against Rate of Mass Loss
Figure 6. Time to Produce Smoke (70% Transmission of Light) against Rate of Mass Loss


[top] [end]Conclusions

The time required to produce a fixed quantity of smoke has been measured for a number of different biomass fuels. Each biomass fuel has been subject to the same radiant heat source and the same radiant intensity. This simulated the conditions whereby a fuel is first introduced onto a fire and produces smoke by non flaming (smouldering) combustion. The fuels were broadly classified into three groups. The first group (straw, hay, peanut shells and dry cardboard) had a low bulk density and would produce a large amount of smoke quickly if allowed to smoulder on an open fire. The second group (wood and chipboard) took much longer to produce the same amount of smoke for a given mass loss. The third group (wet cardboard and air dried Eucalyptus leaves) contained large amounts of water and took very much longer to produce a given amount of smoke again for a given mass loss. This last group would have produced large quantities of water vapour (not smoke) for a given mass loss. The results indicate the benefits of making briquettes of the fuel. Samples of briquettes could be tested using this equipment and the composition of the briquette varied to find the minimum rate of smoke production under non flaming (smouldering) conditions.

[top] [end]Notes and References

  1. Gross, D., Loftus, J.J., and Robertson, A.F. (1967). “Method for measuring smoke from burning materials.” Symposium on “Fire Test Methods – Restraint and Smoke”. 1966, ASTM STP 422 (ed A.F. Robertson), pp.166-204. American Society for Testing and Materials, Philadelphia.
  2. www.itdg.org/html/smoke/smoke_report.htm
  3. www.firetestingtechnology.co.uk
  4. “An Introduction to Fire Dynamics”. Dougal Drysdale. 1990. ISBN 0 471 90613 1.
  5. “Generation of Heat and Chemical Compounds in Fires” Archibald Tewarson. SFPE Handbook of Fire Protection Engineering, 1988, Section 1/Chapter13 p.1-194.

[top] [end]Download the original article

pdf file link Comparison of the rate of smoke generated from the combustion of different biomass fuels by Dr. John Orrin (526 KB)

[top] [end]Contents: Boiling Point 54 - Climate change and household energy

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Theme Editorial - Carbon finance for clean cooking – time to grasp the opportunity - BP54:Health and Greenhouse Gas Impacts in Africa - BP54:Carbon Finance for Healthy Kitchens - BP54:Critique of GHG stove assessment methods - BP54: Practical Action CO2 offsetting experience - BP54: Credible Carbon Offsets for African Households - BP54: GTZ News - BP54: Practical Action News - BP54: Marine conservation and energy efficient stoves - BP54: Can Carbon Finance Clean Cooking? - BP54: Rates of smoke emissions - BP54: A Polyethylene Dome for Biogas Plants - BP54: HEDON news



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