Eminent depletion of fossil fuels and environmental pollution are the key forces driving the implementation cofiring of fossil fuels and biomass. Cogasification as a technology is known to have advantages of low cost, high energy recovery, and environmental friendliness. The performance/efficiency of this energy recovery process substantially depends on thermal properties of the fuel. This paper presents experimental study of thermal behavior of Kiwira coal waste/rice husks blends. Compositions of 0, 20, 40, 60, 80, and 100% weight percentage rice husk were studied using thermogravimetric analyzer at the heating rate of 10 K/min to 1273 K. Specifically, degradation rate, conversion rate, and kinetic parameters have been studied. Thermal stability of coal waste was found to be higher than that of rice husks. In addition, thermal stability of coal waste/rice husk blend was found to decrease with an increase of rice husks. In contrast, both the degradation and devolatilization rates increased with the amount of rice husk. On the other hand, the activation energy dramatically reduced from 131 kJ/mol at 0% rice husks to 75 kJ/mol at 100% rice husks. The reduction of activation energy is advantageous as it can be used to design efficient performance and cost effective cogasification process.
The ever increasing need for clean energy, environmental protection, and alternative use of fossil fuel has necessitated the recovery of energy from waste fossil energy resources. Efficient ways to recover damped coal waste are on record and range from circulating fluidized bed combustor to gasification and pyrolysis [
Tanzania has approximately 1.5 billion metric tons of proven coal [
Tanzania has a wide range of biomass including forestry and agricultural residue. Rice husk in Tanzania is not used efficiently and as such most of it is wasted. For example, Mhilu estimated 326,220 tons of rice husks are wasted annually compared to 10,400 tons of coffee husks [
Direct combustion of coal waste has a wide range of constraints from environmental pollution, low energy recovery, and high cost [
It has been shown that coal/coal waste-biomass blends not only reduce pollution especially carbon dioxide but also increase the recovery during gasification due to the catalytic nature of inorganic minerals in the biomass and reduction in operating temperature [
Biomass is a promising energy source due to its abundance [
Biomass and coal waste, however, have different chemical and physical properties, such as volatile matter, ash content, composition, density, and calorific value [
Earlier studies on thermal behavior of biomass and coal are on record. Bhagavatula et al. [
Thermal behavior of Tanzanian coal waste and biomass is not on record to date [
Coal waste samples were randomly sampled from Kiwira coal waste dump. Rice husk samples were randomly obtained from rice mill wastes in Dodoma.
The samples were ground to less than 2 mm in order to limit the effect of interparticle heat transfer [
Each sample was analyzed in triplicate and standard errors are calculated using (
Proximate analysis was done by standard method ASTM 3172 in the furnace. The calorific values were determined by ASTM D4809 standard method in a bomb calorimeter.
Determination of carbon, hydrogen, nitrogen, and sulfur was done by ASTM (E775, E777, and E778) standards methods. Oxygen was determined by difference, where the sum of ash, carbon, hydrogen, sulphur, and nitrogen was subtracted from 100% [
Thermogravimetric (TG) analysis is one of the thermal analysis techniques used to measure the mass change, thermal decomposition, and thermal stability of materials. Overall kinetics can be easily obtained by measuring the change in mass of a sample with time based on isothermal or nonisothermal thermogravimetric data [
Thermal stability of blends was studied under inert nitrogen condition using a simultaneous thermal gravimetric analyzer type NETZSCH STA PC Luxx TG. Nitrogen (99.95% purity) was used as the carrier gas controlled by gas flow meter at a flow rate of 60 mL/min and pressure of 0.5 bars to avoid unwanted oxidation. In the STA 409 PC Luxx TG, Preteus software was used to acquire, store, and analyze data in desktop computer.
The samples were dried at 100°C temperature for 24 h to remove moisture. 30 mg of the samples of particle size less than 2 mm were placed on a crucible and heated from 35 to 1000°C at constant heating rate of 10 K/min. The low heating rate was used in expectations of allowing the reactions to reach equilibrium [
Parameters that describe kinetics considered were activation energy and preexponential factor. Activation energy is defined as the height of energy barrier which has to be overcome by relative translation motion of the reactants for a reaction to occur [
Pyrolysis process of a solid can generally be described as
The degree of conversion
Rate of degradation of a material is expressed by a way of [
For pyrolysis and oxidation reactions under nonisothermal conditions, the heating rate plays a very important role in determining the kinetic parameters. Low heating rate means that a reaction is closer to equilibrium and vice versa [
Many authors have approximated the overall process as a first-order decomposition occurring uniformly throughout the coal and biomass particles [
When this method is used, (
DTG results showing reaction steps of rice husk.
Proximate and ultimate results are shown in Table
Proximate and ultimate results.
Sample | Kiwira coal waste | Rice husks | |
---|---|---|---|
Moisture content (%) | 3.26 ± 0.04 | 7.2 ± 0.02 | |
Proximate, |
VM | 16.84 ± 0.21 | 59.59 ± 0.43 |
FC | 19.23 ± 0.77 | 17.29 ± 0.45 | |
Ash | 63.93 ± 0.57 | 23.12 ± 0.06 | |
|
|||
Proximate, |
C | 19.68 ± 0.33 | 38.13 ± 0.12 |
H | 2.07 ± 0.03 | 4.59 ± 0.005 | |
O | 12.89 ± 0.36 | 33.10 ± 0.08 | |
Cl | NIL | 0.31 ± 0.002 | |
S | 1.00 ± 0.04 | 0.06 ± 0.003 | |
N | 0.43 ± 0.01 | 0.68 ± 0.032 | |
|
|||
HHV MJ/kg daf | 22.0 ± 0.35 | 19.6 ± 0.04 |
The TG weight loss curves of the blends in a nonisothermal heating at heating rate of 10 K/min are shown in Figure
TG analysis results of Kiwira coal waste/rice husk blends.
Weight loss profiles of blends are between the two profiles of coal waste and rice husk. The results showed that rice husk is more reactive than coal waste. This is in agreement with the work of Zakaria et al. [
The results also showed that, as rice husk content increased, temperature of pyrolysis decreased. For example, for pure coal waste, the pyrolysis temperature was about 760°C while that of 40% coal waste/rice husk blend and pure rice husk was about 690 and 650°C, respectively, as shown in Figures
DTG results of Kiwira coal waste.
DTG results of rice husk.
DTG profiles of Kiwira coal waste/rice husk blends.
Coal is considered as a complex polymer network consisting of aromatic clusters of aliphatic bridge [
Figures
Each sample showed a first peak which corresponds to moisture removal [
Table
Zones of reactions of blends.
Blend | Devolatilization | Char combustion | ||
---|---|---|---|---|
Temperature |
Maximum |
Temperature |
Maximum | |
Coal waste | 300–560 | 1.2 | 560–760 | 1.2 |
20 | 160–390 | 3.3 | 390–670 | 1.3 |
40 | 160–400 | 3.1 | 400–690 | 1.3 |
60 | 160–400 | 1.9 | 400–720 | 1.2 |
80 | 170–400 | 0.9 | 400–730 | 1.2 |
Rice husk | 160–380 | 4.8 | 400–650 | 1.4 |
Degradation rate increased with increase in rice husk. This was attributed to reactivity of rice husk (biomass). The presence of rice husks promotes the production of volatiles in coal waste/rice husk blends. This phenomenon was also reported by Haykiri-Acma and Yaman [
The temperature band width of reaction decreased with increase in rice husk due to the increase in volatile matter and decrease in fixed carbon leading to increased reactivity of the blend. The bond strength of coal waste can also be a reason for increased reaction temperature band width with increasing in coal waste. Coal has been reported to have a high bond energy of about 1000 kJ/mol [
Figure
Conversion rates of Kiwira coal waste/rice husk biomass blends.
Devolatilization rate increased with increase in rice husk content. It is known that volatile matter leads to production of tar which is not needed in the syngas [
High conversion rate of devolatilization occurred at around 320°C while for char degradation it occurs at 500°C. High reaction rate of devolatilization with increase in rice husk content can be explained by devolatilization behavior of most biomass fuels. Biomass contains reactive components responsible for initial steps of devolatilization. Final tail of devolatilization, which is the decomposition of lignin and mainly produces char, is suggested to be caused by the less reactive structure of the remaining solid after main pyrolysis [
The kinetic properties, activation energy and preexponential factor, have been calculated using (
Kinetic properties of Kiwira coal waste/rice husk blends.
Blend |
Degradation step | |||
---|---|---|---|---|
Volatilization | Char combustion | |||
|
|
|
|
|
100 | 51.34 ± 0.75 | 347 ± 7.0 | 131.02 ± 1.6 | 7.7 ± 0.2 |
80 | 58.89 ± 0.44 | 2.8 ± 0.12 |
83.35 ± 0.27 | 3.9 ± 0.9 |
60 | 59.43 ± 0.19 | 3.8 ± 0.45 |
81.09 ± 0.25 | 3.4 ± 1 |
40 | 60.60 ± 0.20 | 3.9 ± 0.12 |
78.63 ± 0.67 | 5.4 ± 2.8 |
20 | 63.70 ± 0.9 | 8.8 ± 0.7 |
76.51 ± 0.7 | 6.6 ± 1.2 |
0 | 84.9 ± 0.5 | 1.5 ± 0.2 |
75.14 ± 0.92 | 1.2 ± 0.75 |
The activation energy for devolatilization was found to increase with increase in rice husk. The results indicated that activation increased from 51 to 85 for 100% coal to 0% coal, respectively. This was due to the increase in volatile matters.
In char combustion step, the activation energy was observed to increase with increase in coal waste, ranging from 131 to 75 kJ/mol for 100% coal to 0% coal, respectively. This was attributed to the high content of fixed carbon in coal waste than that in rice husk. Smaller values of average activation energy mean a more reactive solid, while larger values mean a less reactive solid [
Overall activation energy at char combustion stage decreased with increase in rice husk. This is attributed to weak bonds in rice husk than that in coal waste [
Thermogravimetric analysis has been performed on Kiwira coal waste/rice husk blends aiming at establishing data for cogasification for syngas production. The kinetic parameters have been calculated using multistep first-order reaction at 10 K/min heating rate. The following information has been obtained which is essential to design cogasification process. Thermal stability of coal waste is high and decreases with increase in rice husk. Blending of coal waste and rice husk may reduce thermal stability of coal waste and thus offer designing economic and environmental friendly thermochemical recovery method. Increase in degradation rate with increases in rice husk shows the reactivity of rice husk. This also favors thermochemical process to recover energy from coal waste. Activation energy in char pyrolysis zone has decreased with increase in rice husk: 131–75 kJ/mol. This is associated with decrease in the fixed carbon of blend with increase in rice husk. The overall activation energy of pyrolysis of blends has decreased with increase in rice husk, 131–85 kJ/mole. Decrease in activation indicates that operating temperature also decreases. This shows that gasification of blends occurs at low temperature than is coal waste alone. This is advantageous to reduce pollutants production that depends on high temperature, such as NO Cogasification to recover energy from coal waste is a breakthrough technology favoured by decreasing operating temperature with blending technique.
The study has shown that, using blending technique, thermal stability and activation energy properties of coal waste/rice husk blends have been reduced by increasing rice husk. Thermochemical energy recovery process can be undertaken at low temperature compared to coal waste alone. The use of low temperature process minimizes construction material cost and reduces pollutants formation. With these data obtained it is expected that cogasification of coal waste and rice husk is less costly and releases less pollutants when compared to coal waste gasification alone.
Kiwira coal waste
Rice husk
Thermogravimetric
Differential thermogravimetric
Nelson Mandela African Institution of Science and Technology
Commission for Science and Technology.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors sincerely appreciate the support provided by NM-AIST and COSTECH. Appreciation is also extended to the Administration of Kiwira Coal Mine for providing access in obtaining the samples. Sincere thanks are also extended to the University of Dar es Salaam for allowing the access of its laboratories and providing necessary support.