(85b) Modeling Combustion of Composite Fuels

1.  Introduction

There are three objectives for this current work.  These include firstly to lower combustion level of emissions from coal fired plants and secondly to determine the feasibility of using solid fuel-vegetable-oil-water emulsions for direct combustion.  The third objective is to determine differences between rates of combustion for solid fuel-vegetable oil water-emulsions and solid fuel-vegetable oil mixtures.

2.  Composite Fuels

Composite fuels used with coal can lower the pollution level of a power plant burning this fuel including particulate and sulfur compounds because most vegetable have sulfur concentrations that are low or non-existent.

Previously (1) we found that about 20% of water was acceptable to maintain good stable emulsions on solid combustible fuels. Previously (2) we also found that mixtures of waste oils could be utilized for combustion in power plants when mixed with water and treated.  The cost of oils ranges from less than the price of crude oil to about 20% more than crude oil.

There are many vegetable oils available to create composite fuels.  These include soybean, rape seed, peanut, canola, corn, palm, sunflower cottonseed and a number of others.  Depending on their cost and availability most of these can be interchanged in the composite fuels.  Our experiments and calculations used canola oil.

In this study of composite fuels we included comparisons between Coal Vegetable Oil-water and coal-vegetable-oil systems as well as Wood Vegetable Oil-Water and wood-vegetable oil systems.

Settling when the solid is surrounded by an emulsion is hindered providing some stability to the emulsion.  The smaller the particle size the more that settling is hindered.

3.  Modeling Equations

We propose a linear model:  The models are defined by Figures 1 through 4.  Figure 1 presents the temperatures of the model.  The inlet temperature and the furnace temperatures are set.  Figures 2and 3 provide a graphic of the material and energy balances.  Figure 4 depicts the final material from the combustion.

Figure 1:  Linear model of composite fuel combustion with composite particle temperatures.  The system is solid fuel surrounded by vegetable oil.

Figure 2:  Model of composite fuel particle with mass and energy balances for the flame sheet

Figure 3:  Model of composite fuel emulsion shell with mass and energy balances for the emulsion shell surface

Figure 4: Remainder of composite particle after the shell combusts.

Using these figures, the following combustion equations to predict combustion time were developed. (3,7,10)

Generally it is the evaporated gases that combust.  The modeling equations included vaporization of liquid shell and then combustion the vapor.

For the evaporation of the emulsion shell the combustion constant must be calculated:

K=8kgρlcpgln1+Bo, q     1

Then the decreasing diameter of the shell for a sphere is defined:

D2t=D02-Kt          (2)

In order to solve for the combustion time of a shell we use:

tR-trS=2R2K-2rS2K=tshell     (3)

The combustion for the solid particle remainder is split into three phases, beginning with drying:

msλwX1-X2A(hc+hr)(T∞-Ts)=tdry   (4)

followed by combustion of the volatiles:

lnVVi-k=tvolatile   (5)

followed by combustion of the char:

tchar=0.785 ρcd02DpO2RTMC    (6)

All the equations are used to predict times for combustion in a linear fashion. Our linear model combines the time for a liquid shell to combust followed by the combustion of a solid fuel particle.

            For the liquid vegetable oils the model combines vaporization and combustion.  The parameters used in the emulsion shell combustion and for the combustion of the solid fuel particle can be found in references  (4,5,6,8,9,10,11).

The following figures represent the results of the linear combination model consisting of the central solid and liquid shell. The figures show the times involved in the combustion process.

Figure 5: Linear combustion model for soybean/water-coal droplet with average combustion times

Figure 6: Linear combustion model for soybean/water-wood droplet with average combustion times

            4. Results

The following graph indicates a small representation of the results for combustion of coal and wood particle surrounded by a shell of a vegetable oil emulsion.

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Figure 7: Total combustion time comparison between 50% wood-50% soybean oil/water particle and 50% coal-50% soybean oil/water emulsion particle at various particle sizes at a fixed furnace temperature of 933 K

The wood systems burn much faster than the coal systems for similar parameters.

5.   Discussion and Conclusions

             Composite fuel combustion time is dominated by the solid combustion phase.  Addition of water to the liquid fuel to make an emulsion barely effects the overall combustion time.  Therefore the only adverse effect to emulsifying is the drop in the composite fuel energy value.  The benefits to emulsifying outweigh this issue.

Future work will involve diesel-water emulsions surrounding the solid fuel particle.

6.  References

[1]      Knickle, Harold N.  AIChE Meeting Abstract, May 2012

[2]    Knickle, Harold N. and Adam Duszkiewicz,  AIChE Meeting Abstract, May 2013

[3]      Turns, Stephen R. An introduction to combustion. Vol. 499. New York: McGraw-Hill, 1996.

[4]      Law, C.K., and Williams, F.A., ?Kinetics and Convection in the Combustion of Alkane Droplets,? Combustion and Flame, 19(3): 393-406 (1972).

[5]      Scribed Book Gunstone, Frank, ed. Vegetable oils in food technology: composition, properties and uses. Wiley. com, 2011..

[6]      Soybean chapter Pharos Hammond, Earl G., et al. "Soybean oil." Bailey's Industrial Oil and Fat Products (2005): 577-672.

[7]      Geankopolis, Christie J. Transport Processes and Separation Process Principles. Prentice Hall Professional Technical Reference, 2003.

[8]      Rossberg, Manfred, et al. "Chlorinated hydrocarbons. Ullmann's encyclopedia of industrial chemistry." (2003).

[9]      "Engineering ToolBox." Engineering ToolBox. N.p., n.d. Web. 5 Apr. 2013.

[10]  Allen J. Johnson & George H. Auth, Fuels and Combustion Handbook, First ed., McGraw-Hill, New York, 1951

[11]  "List and Values of Wood Fuel Parameters - Part 3." Woodenergy. Web. 22, 2011 http://www.woodenergy.ie/woodasafuel/listandvaluesofwoodfuelparameters-part3/,