Research Paper
Characterization of Carbonaceous and Nitrogenous Species during the Reductive and Oxidative Combustion of Biomass: Case of Millet Stalks[S1]
Salifou Koucka Ouiminga1*, Thomas Rogaume2, Arsène H. Yonli1, Alfa Oumar Dissa1 andJean Koulidiati1
1Laboratory of Physics and Chemistry of Environment, University of Ouagadougou; 03 BP 7021 Ouaga 03, Burkina Faso Tel.: (226) 50 39 38 15; Fax: (226) 50 39 33 37
2PrimeInstitute, Dept. Fluid,Thermal, Combustion, (UPR 3346 CNRS), ENSMA – Teleport 2, 1 Avenue Clement ADER, BP40109, 86961 FUTUROSCOPE cedex, France
Tel.: (33) 549498290; Fax: 0549498291
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Abstract
Millet stalks are used as fuel in pottering crafting in Africa. A study was conducted to characterize the gaseous species produced during the reductive and oxidative combustion of millet stalks. The experiment was carried out on temperature ranging from 800-1000 oC, usingthe tubular furnace, coupled with two gas analyzers i.e. Analyzer Fourier Transforms Infrared (FTIR) analyzer and a non-dispersive infrared (NDIR) as experimental apparatus.The gaseous products analyzed are i.e. CO, CO2, CH4, C3H8, C2H4, C2H2, C6H6, HCN, N2O, NO, NO2 and NH3.The resultsshow that, in reducing conditions, the levels of carbon monoxide (CO), methane (CH4) and acetylene (C2H2) are increasing functions of temperature, while emissions of CO2 and benzene (C6H6) decreaseas the temperature increases. Emissions of other compounds were remained relatively stable. Reductive combustion produced various compounds like CO2, CO, N etc. while oxidative combustion produced compounds like light hydrocarbon, benzene etc. The over all efficiency of conversion was found about 15%.
Keywords:Combustion, Millet Stalks, Carbon, Nitrogen
1. Introduction
In Burkina Faso, agriculture is the most popular activity. It supports more than ten million people and is practiced by over 84% of the workforce. However, the rainy season lasts only four months a year, forcing farmers to engage in ancillary activities, such as pottery. This activity requires the use of furnaces, usually constructed of clay. These kilns have various geometries, classified the type I to type V(Daho, 2004).
The fuel is mainly composed of carbon, oxygen, hydrogen and nitrogen. The species analyzed are: CO, CO2, CH4, C3H8, C2H4, C2H2, C6H6, HCN, N2O, NO, NO2 and NH3. HCN and NH3 are important intermediates for the formation of fuel NO, (De Soete, 1974; 1989; Morley, 1976; 1980; Miller et al, 1983; 1984 and 1989), while the NO2 and N2O are involved in the reduction of NO (Miller et al, 1983; 1984; 1989;De Soete, 1989; Rogaume et al, 2003; 2004 and Richard et al, 2007). Similarly, hydrocarbons are the precursors to the formation of CO and CO2 and they intervene in the process of formation and reduction of NO (Dryer & Glassman, 1972 and Yetter et al, 1991a and 1991b).
2. Materials and Methods
2.1. Properties of Fuels
An elementary analysis of millet stalks was performed to accurately characterize our fuel and carry out material balances. To do this, samples of each component were analyzed for the determination of the major elements i.e. C, H, O, N, S, Cl, and H2O and ashes. These tests have been carried out by the analysis of the National Centre for Scientific Research (CNRS) with 5 different samples of each fuel. The values presented in Table 1 correspond to the average of rate from 5 analyses.
Table 1. Elementary Composition Millet StalksElements / Millet Stalks
C / 44.4%
H / 6.0%
N / 0.3%
S / 0.15 %
O / 43.8 %
Cl / 814 ppm
H2O / 4.4 %
Ashes / 0.8 %
The millet stalks being biomass predominantly composed of cellulose, it is interesting to characterize this fuel by comparing (Table 2), its elemental composition to that of fir wood, described by (Dumont & Gelus, 1982; Beaumont, 1985 and Rogaume et al, 2002). It appears that the elemental composition of millet stalks is very close to that of wood.
Table 2. Elementary Chemical Composition of the Wood and the Millet StalksComponents Composition (%) / Elements
C / H / N / S / O / Ashes
Dumont R. / 49.5 / 6 / 0.5 / - / 43 / 1
Beaumont O. / 50.9 / 5.76 / 0.2 / 0.04 / 42.1 / 1
Rogaume T. / 47.6 / 6.4 / 0.2 / 0.2 / 45.3 / -
Millet Stalks / 44.4 / 6.0 / 0.3 / 0.15 / 43.8 / 0.8
2.2. Description of Experimental Apparatus
The device used is the tubular reactor (Figure 1) instrumented to study the thermal degradation of solid materials in pyrolysis and combustion. This device was used to determinethe composition of the volatile gases generated by fast pyrolysis, for different temperatures.
Figure 1. Device of Tubular FurnaceThe central part of the diagram was consisted of the tubular kiln and the quartz reactor. The kiln is placed horizontally and measures88 cm long. It is connected to an automatic temperature control (heat resistance) with 1250°C like maximum instruction. The quartz reactor with a total length of 120cm and an inside diameter of 7 cm, is partly placed inside the kiln. A thermocouple of type K is placed at its centre in order to know the temperature. A basket sample holder (in quartz) is used for the introduction of the fuel inside the kiln.
2.3. Experimental Conditions
The millet stalks have been cut into thin sticks 1.5 cm long. Samples, with a mass of 500 mg, were then weighed with a balance of precision 10-2 mg. These sizes and masses have allowed to avoid the effect of temperature gradient at the degradation of the fuel. This physical phenomenon has influenced the mode of degradation. Indeed, the degradation of a "fuel thermally thick" would imply the presence of a temperature gradient, while that of a "fuel thermally thin" would be no temperature gradient since in this case the temperature is supposed to be uniformed throughout the solid.
The temperature of preheating of the kiln has been programmed to the desired value. The flow of gas was selected to have a fixed residence times. It was based on the residence time of hot gas and the local temperature of degradation. Once the set temperature was reached and stable, the oxidizing gas flow was set. The flow of oxidizing gas is performed by a digital control box, brand electronic controller, type 0154, connected to a mass flowmeter brand BROOKS, type 5850E, which allows a maximum flow of gas 30 NL.min-1 with accuracy to 10-1 NL.min-1.
3. Results and Discussion
3.1. Reductive Combustion
Figures 2 show the emissions of carbonaceous and nitrogenous species, in the reductive combustion with 10% oxygen, of millet stalks in our experimental conditions.
Figure 2.Rate of Carbon Oxides Analyzed in the Reductive Combustion of Millet Stalks as a Function of Temperature3.1.1. Carbon
The above results reveal that in reducing medium, millet stalks decompose mainly in carbon dioxide (CO2). This is partly due to the oxidation of carbon monoxide (CO) because CO2 is mainly produced by oxidation of CO (Dixon & Williams, 1977; Gardiner & Olson, 1980; Westbrook & Dryer, 1984; Glarborg et al, 1998; Faravelli et al, 2003 and El Bakali et al, 2004). CO is also produced in significant quantity but remains below the rate of CO2. CO, methane (CH4) and acetylene (C2H2) increases with temperature, while emissions of CO2 and benzene decreased.This phenomenon can be explained by the fact that the gas residence time is fixed, when the temperature increases, the flow of oxidizing gas (air + nitrogen) decreases and the amount of oxygen available for oxidation reactions decreases. The presence of hydrocarbons often inhibits the oxidation of CO to CO2. In the degradation of hydrocarbons, it is shown that CO is formed in considerable quantities but its conversion to CO2 is often delayed until all hydrocarbons and all hydrocarbon fragments have been consumed.Indeed, Westbrook & Dryer, 1984; Glarborg, 1998; Faravelli et al, 2003; El Bakali et al, 2004 and Dryer Glassman, 1978 show that the rate of oxidation of hydrocarbons is higher than that of CO. The emissions of other compounds are only slightly influenced by variations of temperature and remain relatively stable. Table 3 shows that in our experimental conditions, a conversion of about 65% of carbon in CO2, 7% in CO and 4% in C6H6 (800 to 900°C).For other hydrocarbons the conversion rates vary between 0 and 1%. Thus, in the species analyzed, more than three quarters (77%) of carbon of fuel is detected during combustion, in a reducing environment, of millet stalks.
4. Conclusion
Our experimental study aims to characterize carbonaceous and nitrogenous species during reductive combustion and oxidative combustion of millet stalks. This characterization is needed to understand better the mechanisms of formation of gaseous pollutants. It was carried out using the tubular furnace at temperatures ranging from 800 to 1000°C. Reductive combustion tests (with 10% of oxygen) and oxidative combustion tests (with 21% of oxygen) were made. The furnace is connected to two gas analyzers of gas: a Fourier Transform Infrared spectrometer (FTIR) and a Non-Dispersive Infrared Analyzer (NDIR).
Acknowledgement
We are thanks for Mr Smith to his efforts in completion of this work.
References
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Dixon,L.G., andWilliams,D.J. (1977)Comprehensive Chemical Kinetics. vol. 17. In: Branford, C.H.,and Tipper, C.F.H. eds.Amsterdam, Elsevier B.V., pp. 1-248.[S3]
Dumont, R.,andGelus, M. (1982)Preparation of firewood. Chemical Enhancement of Wood,Paris, Masson Publishing.[S4]
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