Biomass utilization is becoming a subject of increasing interest as an alternative to clean fuel. A novel gasification process using highly preheated air gasifier using agricultural residue such as sugar bagasse, rice husks, and palm stem widely available in Tanzania is presented. The study examines, irreversibilities making the gasifier the least efficient unit in the gasification process employing a thermodynamic equilibrium model allowing predicting the main product gas composition CO, CO2, H2, and CH4. The derived model equations are computed using the MAPLE process simulation code in MATLAB. The gasification regime is investigated at temperatures ranging from 800 K to 1400 K and at equivalence ratio (ER) values between 0.3 and 0.4. The results obtained conform to the second law efficiency based on chemical exergy yielding maximum values for the types of biomass materials used. These results indicate that the application of preheated air has an effect on the increase of the chemical exergy efficiency of the product gas, hence reducing the level of irreversibility. Similarly, these results show that the combined efficiency based on physical and chemical exergy is low, suggesting that higher irreversibilities are encountered, since the exergy present in the form of physical exergy is utilized to heat the reactants. Such exergy losses can be minimized by altering the ratio of physical and chemical exergy in the syngas production.
The need for sustainable energy sources is growing as fossil energy sources are diminishing. Moreover, stringent environmental laws about the greenhouse gas emissions and increase in oil prices are prompting for the need to invest in the area of renewable energy sources. Biomass referring to all the forms of plant-based material that can be converted into usable energy is a renewable source of energy.
In Tanzania, biomass is in abundance and is a nonconventional source of energy. The main biomass sources available are in the form of wood sawdust and agricultural residues. The utilization of biomass in an efficient and sustainable manner will provide sufficient energy which can be utilized for electricity generation, engine applications, and so forth, through the deployment of gasification technology. Gasification is the process in which biomass is converted into clean and combustible gas in the presence of limited amount of air.
Thermoconversion process of biomass based on the gasification technology is the most convenient way for the utilization of biomass and is believed to be an efficient method for converting biomass materials into a useful energy source compared to other conversion processes. When compared to conventional combustion technologies, biomass gasification can offer a greater reduction potential on the formation of CO2 and
Maximizing the efficiency of the gasifier can be done through optimization of the operating parameters such as temperature (
The gasification of biomass results in the production of syngas having a wide range of heating values as a direct result of both the gasifier design used and the reactants chosen. In addition to syngas, other products include char and tars produced at varying degrees. Char chemically consists of devolatilized biomass and has a higher concentration of carbon and a lower concentration of hydrogen than that in biomass. The production of char in gas composition contributes significantly in the reduction of the overall efficiency of the gasification process.
The design study of a downdraft gasifier includes a gas cleaning system (cyclone and filter) and gas engine for power generation as presented in Figure
Block diagram of the HTAG biomass gasification system.
As the syngas exits the gasifier at higher temperature in the range of 1200°C, before use, the syngas is passed through a gas cleaning system comprising cyclones and filters. Here the clean syngas is also cooled further to about 25°C, being ready for combustion in a furnace, gas engine, or turbine depending on the desired thermal energy or for electricity production.
During gasification, as air is passed through the fuel bed, relatively discrete drying, pyrolysis, gasification, and oxidation (combustion) zones develop within the reactor. The fuel is dried and moisture removed in the drying zone. In the pyrolysis zone, fuel is converted to volatile compounds and char. The char is gasified in the presence of reactive agents such as carbon dioxide, steam, hydrogen, and oxygen in the gasification zone. Secondary reactions of primary gases and tars take place in the oxidation zone. Because the major product of biomass at temperatures below 600°C is char, biomass gasification requires high temperatures in order to gasify char. The location of these zones within a reactor depends on the relative movement of fuel and air, and the zones are differentiated by the variety of reactions and their temperatures. The depth and relative importance of each zone depend on the chemical composition of the fuel, its moisture content and particle size, the mass flow rate of the reactive agent, and the bed temperature.
The important chemical reactions that occur in the gasifier during biomass gasification are based on the following assumptions: biomass is represented by the general formula the reactions in the gasifier are at thermodynamic equilibrium (at atmospheric pressure = 1 bar), the reactions proceed adiabatically (heat losses neglected), ashes are considered to be negligible (very small amount 1%), the reactions of heat losses are neglected (Adiabatic process), no chars living with the exit of the gasifier products.
To model the gasification process, chemical reactions that occur in the gasifier are considered to be divided into several subprocesses that are represented by a block diagram shown in Figure
Block diagram for modeling HTAG gasification process in the gasifier.
The important reactions that occur during biomass conversion based on the above assumptions can be described using the global gasification equation, in which the amount of air to be used in the gasification process (
The values on the left-hand side of (
Similarly, the amount of air (
Major chemical gasification reactions.
Reaction no. | Reaction scheme | Chemical reaction |
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C + O2 → CO2 | Oxidation reactions |
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C + |
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CO + |
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C + CO2 |
Boudouard reaction |
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C + H2O |
Water-gas reaction |
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C + 2H2 |
Methanation reaction |
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CO + H2O |
Water-gas shift reaction |
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CH4 + H2O |
Methane reforming reaction |
Out of these, only four reactions are independent reactions, which are the oxidation (
In formulating the mathematical model describing the gasification process by performing mass balance in (
By applying the first law of thermodynamics, the equations that take into account of mole balance on carbon, hydrogen and oxygen, related with the number of atoms in the reactants and the product gas emanating from the global gasification reaction equation (
carbon balance:
hydrogen balance:
oxygen balance:
In the elemental balancing equations (
The value of
Compound |
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CO | −110.5 |
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CO2 | −393.5 |
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5.270 |
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H2O | −241.8 |
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0.0 | 2.868 |
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CH4 | −74.8 |
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The resulting eight nonlinear algebraic equations (
The proximate and ultimate analyses of the three types of biomass materials used in this study as initial inputs to the model are presented in Table
Characteristic values of selected tropical biomass materials.
Biomass type | Ultimate analysis (%), dry basis | Proximate analysis (%), dry basis | High heating value | ||||||
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C | H | O | N | Moisture | Volatile matter | Fixed carbon | Ash | HHV (kJ/kg) | |
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47.50 | 5.90 | 42.50 | 0.28 | 9.10 | 81.20 | 15.30 | 3.50 | 17,380 |
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35.60 | 4.50 | 33.40 | 0.19 | 8.80 | 59.20 | 14.60 | 26.20 | 13,240 |
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48.10 | 5.90 | 42.40 | 0.15 | 9.00 | 80.50 | 16.20 | 3.30 | 17,330 |
Biomass gasification is associated with the energy interchange processes during which the chemical energy of the biomass is converted into the chemical and thermal energy of the product gas (syngas). The energetic biomass conversion efficiency is dependent on the gasifier, whose performance is evaluated using the second law of thermodynamics applying the exergy analysis concept to model the energy interchange processes [
The total system exergy (
The exergy efficiency of a system can be expressed in terms of chemical exergy
Standard chemical exergy of some substances [
Substance |
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H2 | 238,490 |
CO | 275,430 |
CO2 | 20,140 |
H2O (g) | 11,710 |
CH4 | 836,510 |
N2 | 720 |
To have high system efficiency, it is desirable that the energy that could be converted into work but is wasted (irreversibility,
Flow chart for the computation of HTAG equilibrium model.
The evaluation of the effect of using preheated air on molar composition of the major components of the product gas: carbon monoxide (CO), carbon dioxide (CO2), hydrogen (H2), and methane (CH4) during the gasification of biomass was made using the equilibrium model based on (
For the purpose of this study, three different types of biomass materials were used as the inputs: rice husks, sugar cane baggase, and palm stem. Additionally, the gasification process was done under different equivalent ratio (ER) values of 0.3, 0.35, and 0.4. The model results obtained showing how the use of preheated air affected the gasification process, and the evolution of product gas molar fraction values for each biomass material used: (a) rice husks, (b) sugar cane bagasse, and (c) palm stem are shown in Figures
The effect of temperature on molar composition for CO, CH4, CO2, and H2 gases from the (a) rice husks, (b) sugar bagasse and (c) palm stem (equivalent ratio (ER) of 0.3).
The effect of temperature on molar composition for CO, CH4, CO2, and H2 gases from the (a) rice husks, (b) sugar bagasse, and (c) palm stem (equivalent ratio (ER) of 0.35).
The effect of temperature on molar composition for CO, CH4, CO2, and H2 gases from the (a) rice husks, (b) sugar bagasse, and (c) palm stem (equivalent ratio (ER) of 0.4).
Analyses of the results shown in Figure
The effect of preheated air temperature on product gas molar composition.
Biomass material | ER | Product gas molar composition (% Concentration) | |||||||
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H2 | CO | CH4 | CO2 | ||||||
800°K | 1400°K | 800°K | 1400°K | 800°K | 1400°K | 800°K | 1400°K | ||
Rice husks | 0.3 | 11.5 | 21.3 | 27.3 | 12.5 | 1.2 | 1.3 | 8.9 | 24.1 |
0.35 | 6.8 | 8.7 | 22.4 | 7.5 | 0.7 | 0.3 | 11.6 | 26.8 | |
0.4 | 4.2 | 5.02 | 16.5 | 3.9 | 0.26 | 0.1 | 15.7 | 28.4 | |
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Sugarcane baggase | 0.3 | 13.3 | 23.7 | 26.9 | 24.1 | 2.5 | 4.4 | 5.6 | 9.3 |
0.35 | 11.5 | 21.2 | 24.4 | 20.6 | 1.8 | 2.0 | 6.3 | 10.8 | |
0.4 | 9.9 | 18.9 | 21.9 | 17.4 | 1.4 | 1.6 | 7.1 | 12.1 | |
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Palm stem | 0.3 | 13.0 | 23.8 | 26.8 | 23.8 | 2.5 | 1.5 | 5.7 | 9.5 |
0.35 | 11.5 | 21.3 | 24.2 | 20.3 | 1.8 | 1.9 | 6.4 | 10.9 | |
0.4 | 9.8 | 18.9 | 21.8 | 17.2 | 1.4 | 0.9 | 7.2 | 12.3 |
The application of higher heating rates provided by the preheated air that increased from 800°K to 1400°K, allows the formation of a reasonable amount of H2, but such temperature increases resulted into a decrease in CO molar concentration. The reduction in CO production observed in this study may have occurred at the comparatively lower temperature at 850–900°C (1123–1173 K) for which the Boudouard reaction predominated the process. Similar results were also reported in the study in [
In this study,
These results indicate that the CO2 molar concentrations increases are higher in rice husks compared to relatively small increases in sugar cane baggase and palm stem biomass types. The molar concentration of CO2 in the two biomass materials was in a relatively same range under the similar temperature conditions and the same ER values.
For the ability of the gasifier to achieve high system efficiency, it is desirable that the energy that could be converted into work but is wasted (irreversibility,
The lower heating values of the biomass (LHVbiomass) at pressure of 1 bar and temperature condition of 25°C are calculated using (
The greater concentration of carbon monoxide and hydrogen that was established in these results contributes to the attained higher calorific values. The presence of methane in the gas has a tendency of increasing the calorific value. The increase of temperature for (a) rice husks does not; however, support and increase heating values for syngas obtained, while for (b) sugar cane bagasse and (c) palm stem, higher heating values depend more on the ER values.
The lower heating (calorific) value of the product gas (LHVgas) was calculated based on the main composition of the product gas (carbon monoxide and hydrogen) obtained under similar temperature and pressure conditions for all biomass materials used and then by summing the product of mole fraction
The effects of preheated air temperature on lower heating value (LHVgas) and cold gas efficiency
Product gas heating values (kJ/kg) and cold gas efficiency |
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Biomass material |
Rice husks | Sugarcane baggase | Palm stem | ||||
800°K | 1400°K | 800°K | 1400°K | 800°K | 1400°K | ||
ER | 0.3 | 4278 | 2402 | 5343 | 5482 | 5309 | 5446 |
0.35 | 3185 | 1466 | 4535 | 4606 | 4505 | 4574 | |
0.4 | 2129 | 782 | 3857 | 3876 | 3833 | 3850 | |
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0.3 | 39.1 | 69.5 | 63.1 | 64.7 | 62.9 | 64.6 |
0.35 | 28.2 | 56.8 | 58.8 | 59.7 | 58.7 | 59.5 | |
0.4 | 15.2 | 41.4 | 54.5 | 54.8 | 54.3 | 54.0 |
These results obtained on the basis of the first law of thermodynamics have a similar efficiency decrease as the ER is increased from 0.3 to 0.4. The effect of temperature did not show a dramatic increase on the cold gas efficiency (also known as the first law of thermodynamics efficiency). For the types of biomass materials used, the maximum cold efficiency achieved ranges from 39.1 to 69.5% for rice husks as the gasification temperature increases from 800°K to 1400°K, from 63.1 to 64.7% for sugarcane baggase, and from 62.9 64.6% for palm stem using ER of 0.3.
The amount of the energy that could be converted into work but is wasted (irreversibility,
The effect of temperature and equivalence ratios on second law efficiency based on chemical exergy for (a) Rice husks, (b) Sugar bagasse, and (c) Palm stem.
Figures
Analyses of the results shown in Figure
The effects of preheated air temperature on (a) chemical, (b) chemical, and physical exergy efficiencies.
Biomass material | ER | Chemical exergy efficiency |
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and irreversibility ( | |||||
for the biomass materials | |||||
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800°K | 1400°K | 800°K | 1400°K | ||
0.3 | 73.8 | 83.5 | 26.2 | 16.5 | |
Rice husks | 0.35 | 79.6 | 89.0 | 20.4 | 11.0 |
0.4 | 85.4 | 93.2 | 14.6 | 6.8 | |
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0.3 | 79.2 | 79.8 | 20.8 | 20.2 | |
Sugarcane baggase | 0.35 | 82.3 | 81.9 | 17.1 | 17.7 |
0.4 | 84.3 | 84.6 | 15.7 | 15.3 | |
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0.3 | 81.9 | 82.4 | 19.1 | 18.6 | |
Palm stem | 0.35 | 84.2 | 84.6 | 15.8 | 15.4 |
0.4 | 86.3 | 86.6 | 13.7 | 13.4 |
Biomass material | ER | Chemical and physical exergy efficiency | |||
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the biomass materials | |||||
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800°K | 900°K | 1400°K | 900°K | ||
Rice husks | 0.3 | 62.9 | 64.8 | 46.3 | 25.2 |
0.35 | 68.6 | 70.9 | 51.5 | 29.1 | |
0.4 | 74.4 | 76.9 | 55.6 | 23.1 | |
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Sugarcane baggase | 0.3 | 71.7 | 72.1 | 54.2 | 27.9 |
0.35 | 74.4 | 74.7 | 56.7 | 25.3 | |
0.4 | 76.7 | 77.0 | 58.9 | 23.0 | |
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Palm stem | 0.3 | 75.3 | 75.7 | 59.9 | 24.3 |
0.35 | 77.7 | 77.9 | 62.1 | 22.1 | |
0.4 | 79.7 | 79.9 | 64.0 | 21.1 |
For the case of combined chemical and physical exergy efficiencies, these results are presented in bar charts provided in Figures
The effect of temperature and equivalence ratios on second law efficiency based on chemical and physical exergy for (a) rice husks, (b) sugar bagasse, and (c) palm stem.
Examination of the results shown in Figure
The results of the combined chemical and physical exergy efficiencies were observed to fall as the gasification temperature is increased from 800°K to 900°K and then start to fall as the temperature is increased to 1400°K. The same trend was also observed as the ER values were increased from 0.3 to 0.4. These results suggest that for the combined chemical and physical exergy efficiencies, the optimum efficiency value can only be reachs at a temperature of 900 K, which are the low irreversibility values (degree of irreversibility (
The validity of the formulated equilibrium model has been established by making comparison between the results of gaseous compositions predicted in this study and those available from the literature [
Comparison of gaseous composition-predicted values with the literature values.
Ultimate analysis |
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ER | CO % mole concentration | CO2 % mole concentration | H2 % mole concentration | Reference | |||||
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C | H | O | Predicted | Literature | Predicted | Literature | Predicted | Literature | |||
0.30 | 27.90 | 23.40 | 6.70 | 13.20 | 20.10 | 12.50 | |||||
47.3 | 5.8 | 45.0 | 1073 | 0.35 | 24.20 | 24.90 | 8.20 | 12.50 | 17.50 | 13.00 | [ |
0.40 | 21.04 | 24.20 | 9.50 | 12.80 | 15.20 | 13.80 | |||||
50.0 | 6.0 | 44.0 | 1073 | 0.40 | 22.03 | 22.50 | 8.70 | 9.20 | 15.70 | 16.80 | [ |
51.0 | 6.8 | 39.2 | 1073 | 0.40 | 20.20 | 24.50 | 7.90 | 6.50 | 15.90 | 15.00 | [ |
The utilization of biomass can provide sufficient energy which can be used for electricity generation using internal combustion engines. The gasification process in which biomass is converted into clean and combustible gas can be studied using thermodynamic equilibrium model allowing the predicting of the main product gas compositions CO, CO2, H2, and CH4 which is an important step in modeling the gasification process. The model assumes that the principle reactions are at thermodynamic equilibrium. The model equations containing four molar balances (C, O, H, and N) and three equilibrium relations are computed using the MAPLE process simulation code in MATLAB. The three types of biomass materials from Tanzania are used for prediction of equilibrium gas compositions. These three samples are compared with results based on percentage mole concentration values that compare favorably well with results obtained from previous studies.
The results for rice husks sugar, sugarcane bagasse, and palm stem indicate that the concentration of H2 has maximum values of about 21.3% to 23.8% with a temperature of 1400°K and an equivalence ratio of 0.3, while the formation of CO is maximum at values of about 26.8% to 27.3% at 800°K with the same equivalence ratio. While this is the case for H2 and CO, maximum CO2 concentration is about 12.1% to 28.4% captured at a temperature of 1400°K with an equivalence ratio of 0.4.
The cold gas efficiency was established to be dependent on the equivalence ratio (ER), which depicted a decreasing trend as the ER was increased. Such reduction is understood to be due to the diversion of energy availability in unconverted solid carbon as well as decreased quality of gas flow, which in turn is caused by decreasing air introduced in large ERs. The loss of cold gas efficiency is also due to the diluting effect of nitrogen content of the oxidant (using air as a gasifying agent). The maximum syngas heating values that were achieved for the biomass materials used were between 4278 and 5482 kJ/kg when the ER value was 0.3.
The high temperatures and equivalent ratio increase favoured the second law efficiency based on chemical exergy with maximum values for thermodynamic efficiency for the types of biomass materials used being between 84.6% and 93.2%. The increase in ER value raises the exergetic efficiency based on chemical and physical exergy. The maximum value for thermodynamic gasification efficiency based on chemical exergy for these biomasses ranges from 79.8% to 93.2%.
The modeling studies conducted gave results that indicate that the application of preheated air has an effect on the increase of the chemical exergy efficiency of the product gas, hence reducing the level of irreversibility. These results show that the efficiency combined based on physical and chemical exergy is low, which suggests that higher irreversibilities are encountered for this case, as some of the exergy present in form of physical exergy is utilized to heat reactents. Such exergy losses (irreversibilities) can be minimized by altering the ratio of physical and chemical exergy in the syngas production.