Abstract
Colombian coffee industry produces about 0.6 million tons of husk (CH) per year which could serve as feedstock for thermal gasification to produce gaseous and liquid fuels. The current paper deals with: (i) CH adiabatic gasification modeling using air-steam blends for partial oxidation and (ii) experimental thermogravimetric analysis to determine the CH activation energy (). The Chemical Equilibrium with Applications Program (CEA), developed by NASA, was used to estimate the effect of equivalence ratio (ER) and steam to fuel ratio (S : F) on equilibrium temperature and gas composition of ~150 species. Also, an atom balance model was developed for comparison purposes. The results showed that increased ER and (S : F) ratios produce mixtures that are rich in H2 and CO2 but poor in CO. The value for the activation energy was estimated to be 221 kJ/kmol.
1. Introduction
Combusting fossil fuels produce green house emissions which cause a negative environmental impact. There are various fuel alternative technologies which can be developed in order to mitigate both the dependence on fossil fuels and the negative impact caused by its emissions. Biomass sources, for example, energy crops and wastes, can be used as feedstock in thermal processes such as direct combustion to produce heat, or gasification and pyrolysis to produce gaseous and liquid fuels. The coffee agriculture industry around the world produces a great amount of wastes, for example, only in Colombia about 13 million tons of coffee grain are produced per year, which result in 0.6 tons of coffee husk (see mass balances from coffee processing in Table 1) [1]. These residues can cause pollution of natural sources (land, water, and air) if treatment and storage systems are not correctly managed.
In thermochemical gasification processes biomass feedstock undergoes thermal degradation in an inert medium (pyrolysis, (1)) or partial oxidation in an oxidizing medium (gasification, (2)–(8)) to produce liquid or gaseous fuels, respectively [2]. The oxidizing source can be air, pure oxygen, or a mixture of those with steam. Also, pure steam is used in reforming processes in which the biomass is heated to strip the H2 from the H2O through the reaction C + H2O → CO + H2. Subsequently, the produced CO reacts with the remaining H2O (CO + H2O → CO2 + H2) to produce more H2 and CO2. The gas composition from partial oxidation of biomass depends upon the type of biomass and oxidizing source, as well as on the rate at which both biomass and oxidizer are simultaneously supplied to the gasifier. In general, gasification with air and pure oxygen produces mixtures rich in CO, whereas gasification with air-steam and pure steam produce gases high in H2 [3]. In biomass gasification many reactions occur simultaneously; however, the global process can be modeled using reactions (1) through (8) as shown below [4]:
Reaction (1) corresponds to any biomass pyrolysis, in which the biomass is heated to volatilize the volatile matter. Subsequently, the products released in pyrolysis (C and light gases) react with the oxidizer supplied and other gases generated to produce more products (reactions (2) to (8)). Reactions with are endothermic, and those with are exothermic. According to Annamalai and Puri, [5], reaction (2) dominates at low temperatures (below 800°K) while reaction (3) dominates at higher temperatures. In gasification processes, the reactions (4), (6), and (7) are important due to low oxygen and high steam contents in the oxidizing source.
In 2006, Xu et al. [6] reported experimental results on the effects that gasifier temperature, fuel particle size, steam/fuel ratio, residence time, and air supplied to the gasifier have on product gas compositions from gasification coffee grounds. The study was performed in a dual fluidized bed gasifier. In 2009, Gordillo and Annamalai [3] used chemical equilibrium and atom balance modeling to estimate the effect of modified equivalence ratio and S : F ratio on the composition of gases produced from air-steam gasification of dairy biomass (DB). In 2009, Velez et al. [7] studied the effect of the steam/fuel ratio on the production of CO, H2, and CO2 for fluidized bed cogasification of coal with coffee husk. In 2010, Lugano et al. [8] studied the effect of gasification temperature (700, 800, and 900°C) on coffee husk gasification rate under inert nitrogen conditions and oxygen concentrations ranging between 2% and 4%. Also, using the fitting kinetics analysis method for a single heating rate (210°C·min−1) in a furnace at 900°C and the coats approximation algorithm [9], and assuming a reaction model of first order, these researchers estimated the activation energy, E, and the pre-exponential factor, A, of the Arrhenius’s equation.
The current paper deals with (i) CH adiabatic gasification modeling using air-steam blends for partial oxidation and (ii) pyrolysis kinetic model to determine, by thermogravimetric analysis (TGA), the CH activation energy (). The Chemical Equilibrium with Applications program (CEA), developed by NASA, was used to estimate the effect of both the equivalence ratio (ER) and steam to fuel ratio (S : F) on adiabatic temperature and gas composition of an unlimited number of species (~150), whereas atom balance was developed to estimate gas composition of a reduced number of species (CH4, CO2, CO, N2, and H2) and for comparison purposes. Thermogravimetric analysis (TGA) was carried out using N2 as carrier gas and under different heating rates (: 5, 10, 20, and 40°C/min), while the isoconversional method (i.e., free model) was used to estimated the activation energy ().
2. Adiabatic Gasification Modeling
The gas composition of the mixtures produced by gasification of biomass can be predicted using chemical equilibrium for a larger number of species or atom balance on components for a reduced number of species (CO2, CO, CH4, H2, and N2). In general, biomass gasification using air-steam mixtures as oxidizing source produces gas mixtures (dry basis) composed mostly by CO2, CO, CH4, H2, N2, and other species in trace amounts [3].
2.1. Atom Balance Model
Combustion processes can be classified as complete (stoichiometric), when the reactants undergo complete oxidation, or incomplete, when the reactants do not oxide totally. Equation (9) shows the stoichiometric reaction of any biomass with air as oxidizing source, while (10) presents the incomplete reaction of any biomass with air-steam as oxidizing source. In this last equation, only the most important products are shown
The equivalence ratio (ER) is a parameter which establishes the ratio between the stoichiometric oxygen and the actual oxygen supplied to the combustor. In the case of gasification with air-steam, the steam to fuel ratio (S : F) is also an important parameter since it determines the amount of steam supplied to the gasifier per fuel unit. Because both ER and S : F establish the ratio between the rate of biomass and oxidizer supplied simultaneously to the gasifier, they have a strong effect on the quality of gases produced. They can be defined as follows:
Adiabatic gasification implies equal reactant and product energy. Therefore, the total enthalpy of the reactants (HR), at inlet temperature (), equals the total enthalpy of the products (HP), at outlet temperature () (13) where and are the moles and total enthalpies of the reactants , at temperature (), and and are the moles and total enthalpies of the product , at . Using (11) through (13), along with atom balance on components (C, H, O, N, and S), the values of the (8) coefficients (, , , , , , , and ) in (10) can be estimated, as a function of the ER, S : F, and . The HHV or energy density of the gases produced can be calculated using (14) where and are mole fraction and gross heating value (kJ per SATP m3) on a dry basis of each fuel gas produced, respectively, = CO, CH4, H2, and so forth, and HHVgases is the energy density or HHV (kJ per standard ambient temperature and pressure (SATP) m3) of the product gases.
Although the energy density or HHV of the products gives information on the amount of energy per unit of gas produced, it does not provide information on the fraction of energy recuperated as fuel gases per each fuel unit gasified. The fraction of energy recuperated in air-steam gasification processes can be estimated using (15) where, and correspond to the moles of fuel and steam supplied, respectively, to the gasifier by each normal m3 of dry product gases, is the latent heat of steam, HHVFuel is the gross heat value (kJ/kmol of DAF fuel) of the fuel, and ECEGases is the energy conversion efficiency (ECE) or energy recovery.
2.2. Chemical Equilibrium Model
The Chemical Equilibrium with Applications program (CEA), developed by NASA, was used under adiabatic conditions to solve for about 150 species (including pure carbon) and adiabatic temperature. The CEA uses chemical equilibrium to estimate the species which are produced from a certain chemical reaction. If the reaction is adiabatic, the program requires as input data the reaction pressure and the composition and enthalpy of the reactants. In case of a nonadiabatic reaction, the input data required are the reactant composition and the temperature and pressure of the reaction. Atom and equilibrium models were developed under the conditions shown in Table 2.
3. Pyrolysis Kinetic Model Based on Thermogravimetric Analysis
Model-fee and model-fitting have been applied to estimate the kinetics parameters based on thermogravimetric analysis data. In this section a kinetic model based on the isoconversional method (model-free) proposed by Ozawa [10] is presented. This method requires carrying out a series of experiments at different heating rates and assumes basically that the reaction of any solid as shown in (16) is independent of temperature [11]. The reaction rate of solids is usually based on a single step reaction which can be expressed as where is the extent of conversion, is time, the pre-exponential factor of the Arrhenius’s equation, the activation energy, and a particular function, called the reaction model, which describes the dependence of the reaction rate on the degree of conversion . The isoconversional method assumes that , , and are independent of temperature and that and are independent of . Under no isothermal conditions, (16) can be expressed as follows: where is the heating rate and is the temperature. Integrating (17) and taking natural logarithm gives Using Doyle’s approximation algorithm [12], Equation (19) can be expressed as
Since , , and are assumed independent of , the activation energy, , can be estimated from the slope of the linear curves which result of plotting ln versus for a constant extent of conversion and the corresponding temperatures of the different heating rates [13].
4. Materials and Methods
Coffee husk samples were obtained from Colombian coffee industry and were characterized by ultimate and proximate analysis including heating value. Table 3 shows the results from those analyses (DAF basis). The empirical formula was derived using chemical composition and atom balance on compounds. The samples analyzed by thermogravimetric analysis were crushed in order to reduce the particle size to ≤425 μm.
The thermogravimetric analysis was carried out using a NETZSCH STA 409 PC Luxx calorimeter and the software NETZSCH Proteus for MS Windows and under the conditions listed in Table 4.
5. Results and Discussion
5.1. Atom Balance Model
This section discusses the effect of the operating parameters (ER and S : F), estimated by atom balance, on the production of CO, CO2, CH4, and H2. Others species such as N2 (present in large amounts) and H2S (and many others present in trace amounts) are not shown.
Figure 1 shows the effect of ER and S : F on the H2, CH4, CO, and CO2 production for a product temperature of 873 K. At constant ER, increasing S : F implies more steam moles in the oxidizer source per each mol of air entering the gasifier; hence, the gasification process occurs in an ambient rich in H2O, which favors the production of H2 and CO2 via the following reactions: C + H2O → CO + H2 and CO + H2O → CO2 + H2. More C and H atoms producing CO2 and H2 mean less C and H atoms available to produce CH4 and CO, which leads to decreased CO and CH4 production. From Figure 1, it is apparent that gasification with only air (S : F = 0) produces more CO and CH4 and less H2 than gasification with air-steam (S : F = 0.5).
5.2. Equilibrium Model
The effects of ER and S : F on adiabatic temperature and gas composition, estimated by equilibrium model, is presented in this section. Although, about 150 species were analyzed, only results on the more relevant species (H2, CO, CH4, and CO2) are presented here. With exception of N2, other species were in trace amounts. The effect of the ER on adiabatic temperature () is illustrated in Figure 2 for various S : F ratios. At constant S : F ratio, increase in ER results in decrease in the oxygen entering the gasifier; therefore, there are less O atoms available for the oxidation of C via the reactions (2) and (3) which are exothermic. Consequently, less heat is released, which leads to lower adiabatic temperatures. Furthermore, the results show that decreased S : F ratios increase the adiabatic temperature. In general, the maximum temperatures were attained for gasification with only air (S : F = 0) while the minimum temperatures were obtained at S : F = 0.8. At the effect of the ER on is negligible. This suggests that gasification of CH at tends to be pure pyrolysis.
Figures 3, 4, 5, 6 show the effect of ER, on CO, CO2, H2, and CH4 for various S : F. At constant S : F, higher ERs imply less oxygen entering the gasifier for the reaction C + (1/2) O2 → CO; thus, there are more C atoms available to react with the steam to produce H2 via the reaction C + H2 O→CO + H2. The CO produced by the reactions of C with steam and C with oxygen reacts with the remaining steam to produce more H2 and CO2 (shift reaction). More C atoms consumed by the shift reaction (CO + H2O → CO2 + H2) imply less C atoms leaving as CO.
The CO and H2 curves show a peak with the ER (Figures 3 and 5). The CO mole fraction increases with increased ER until beyond which it starts to decrease. The lowest value of CO is reached at ER = 1 (stoichiometric reaction) while the maximum (~16%) is attained at and S : F = 0.30 (Figure 3). The concentration of H2 also shows an inflection point at . At , increased ER increases very strongly the concentration of H2 but at ER > 3.2, and the effect of the ER on the fraction of H2 is rather weak (Figure 5). Although, the effect of S : F on the H2 concentration is insignificant the results suggest that the steam-to-air ratio entering the gasifier affects the H2/CO ratio leaving the gasifier. At constant S : F, increasing the ER (decreased oxygen supplied through air) increases steam-to-air ratios. From these results, it is evident that higher S : F ratios increase the H2/CO ratio. At , the CO2 decreases with increased ER and S : F ratios whereas at increasing both ER and S : F produces mixtures rich in CO2. This is because of the higher steam concentration in the reactor, which favors the reaction of CO with steam (shift reaction) to produce more H2 and CO2. As shown in Figure 6, more available H atoms in the gasifier lead to CH4-rich concentrations. From Figure 6, it is evident that at , the effect of the S : F ratio on the concentration of CH4 is practically negligible and that the production of CH4 is only possible at . In general, these results show that at (increased oxygen through air), the concentration of CO and H2 increases and that the concentration of CO2 and adiabatic temperature decrease with increased ER (Figure 3 through 6), indicating that the heterogeneous C + H2O → CO +H2 reaction (which is endothermic, kJ kg−1 of C) is more important than the homogeneous CO + H2O → H2 + CO2 reaction, which is a slightly exothermic ( kJ kg−1 of CO). At (less O2 supply), the shift reaction begins to be important because the H2O concentration in the reactants is much higher. Hence, CO production starts to decrease whereas the production of H2 increases.
The molar fraction of carbon estimated with equilibrium model is presented in Figure 7 as a function of ER and various S : F. It is evident that the production of carbon (C) is possible only at . This suggests that at those ER the oxygen supplied is not enough to burn completely the carbon atoms through the reactions (2) and (3). On the other hand, at constant S : F and , increasing ER increases carbon production, because of the less oxygen supplied for each kg of fuel gasified. Also, the results show that at constant ER, increased S : F ratios produce lower carbon indicating that the more H2O in the reactant react with char. In general, the results on C production indicate that at the pyrolysis tend to be important.
The results from chemical equilibrium modeling and atom modeling are compared in Table 5 showing that the gas composition (except CH4 and CO), predicted with atom-balance and equilibrium model are almost similar. The difference in CH4 and CO is due to the fact that the equilibrium model includes a larger number of species (~150) compared to atom balance that includes the production of only 6 species (CO, CO2, CH4, H2, N2, and H2S). The lower amount of species estimated with atom balance is due to the lower number of available equations. Also, due to the low number of species, atom balance supposes that all the fixed carbon (FC) contained in CH and all H atoms contained in both CH and oxidizer reacts completely, through the reactions (2), (3), (4), (6), and (8), to produce the secondary products (CO, CH4, CO2, and H2) shown in the global reaction (10). Thus, there is no presence of C and H2O in the products. However, equilibrium model includes in the products pure carbon (Figure 7) and H2O.
5.3. HHV of Gases
Table 6 presents the energy density of the gases at and 0.3 < S : F < 0.8. At , increasing S : F decreases the HHV. In contrast, at , the HHV of gaseous fuel increases with increased S : F ratios. As discussed earlier, increased S : F decreases the production of CO (Figure 3); thus, the HHV tend to decrease. Although, at , increasing S : F decrease CO, the gas HHV increases due to more production of CH4 (Figure 6) which has a higher HHV (~38000 kJ/SATP) as compared to that of the CO (~11000 kJ/SATP m3). In general, at constant S : F, increasing the ER tends to increase the HHV. The HHV for the selected operating conditions varied from 2643 to 5037 kJ/SATP m3. The highest HHV (5037 kJ/SATP m3) was achieved for a and S : F = 0.80, whereas the lowest HHV (2643 kJ/SATP m3) was attained at and S : F = 0.8. From Table 6, it is evident that the effect of ER ratio on gas HHV is stronger than that of the S : F.
Table 7 presents the energy conversion efficiency (ECE) estimated with equilibrium model for the range of operating parameters studied ( and 0.3 < S : F < 0.8). Even though at the energy density of the gases increases with increased ER, ECE decreases with increased ER. This is because under those operating conditions () there is more production of carbon and the gasification tends to be near pyrolysis that produces lesser amount of combustible gases and more combustible loss through char. On the other hand, at , increased ER tends to increase the ECE. For the range of the operating conditions studied, ECE ranged from 0.56 to 0.81; the remaining fraction corresponds to the energy returned in char and sensible heat of gases leaving the gasifier.
The results on gas composition and ECE show that the highest productions of H2 (23%) and CO (~15%) and the highest ECE (81%) are achieved at and S : F = 0.3, which suggests that those are the best operating conditions.
5.4. Kinetics Model
This section presents results obtained from the kinetic analysis. Figure 8 illustrates the thermogravimetric analysis (TGA) of the coffee husk pyrolysis for four different heating rates (5, 10, 20, and 30°C/min). Also, in Figure 8, the different conversion degrees (: 20, 30, 40, and 50%) used to estimate the activation energy (E) are pointed out.
The mass released between 300°K and 400°K corresponds to the moisture content (~7.5%) in CH. On the other hand, the results from Figure 8 indicate that most of the volatile matter (VM) content in CH is volatilized between ~500 K and 700 K (higher slope of the curves). After ~700 K, the mass tends to remain constant, indicating that most of this mass corresponds to char (fixed carbon and ash contents in HC biomass). In general, the results from TGA show that the volatilization of VM is very important at 500 K < < 700. Conversely, at K the rate of VM released is negligible, which indicates that the CH pyolysis process occurs at temperatures ranging between 500 and 700 K.
Figure 9 shows the plots of versus 10−3 for conversion degrees () of 20, 30, 40, and 50% and the corresponding heating rates () of 5, 10, 20, and 40°C·min−1. The activation energy was estimated from the slopes of linear curves which match the experimental results.
Table 8 illustrates the slopes and the activation energies of the thermal decomposition kinetics of CH for different conversion degrees. Also, the arithmetic average of all conversion degree studied and the standard deviation are presented.
The average activation energy discussed here for the CH pyrolysis (211 kJ/kmol) is higher than those presented by Lugano et al. [8] for CH (161 kJ/kmol) and Sanchez et al. [15] for combustion of biowastes (143 kJ/kmol), but it is a little lower than that presented by Garcia-Pèrez et al. [16] for sugarcane bagasse pyrolysis (235 kJ/kmol). The difference between the value of the activation energy presented here and that presented by Lugano et al. [8] is due to the use of different methods to estimate E. In this study, the isoconversional (free) method was used for different heating rates (5, 10, 20, and 30°C/min), while Lugano et al. [8] used the fitting method for a single heating rate (210°C/min). Also, the fitting method, for a single heating rate, requires assuming previously a particular function (), which depends on the reaction’s mechanism, in order to estimate the activation energy, whereas the isoconversional method (i.e., free-model) does not require assuming previously any particular function to estimate the activation energy.
6. Conclusions
The results suggest that gasification of CH with air-steam could produce gaseous combustibles with H2 concentrations from 0 to ~25%, CO from 0 to ~16%, and CH4 from 0 to ~7%.
Equilibrium temperature decreases with increased ER until . At , the effect of ER on equilibrium temperature is negligible, indicating that under these operating conditions the process tend to be near pyrolysis.
Increased ER increases both the production of CO until after which it starts to decrease and the production of H2 until after which it tend to be constant, indicating that at ER > 3.2 the effect of the ER on the fraction of H2 is rather weak.
In general, increasing S : F ratio tends to produce richer mixtures in CO2 and CH4 but poorer in CO. On the other hand, the effect of the S : F on the H2 production is negligible.
The activation energy of the CH pyrolysis (211 kJ/kmol) is higher than those presented by [8, 15] for CH and combustion of biowastes, respectively, but it is a little lower than that presented by [16] for sugarcane bagasse pyrolysis (235 kJ/kmol). The difference between the activation energy presented here and that presented by [8] is due to the use of different method to estimate it. According to [11], the kinetics parameters of the Arrhenius’s equation, estimated using the fitting method, are different to these estimated using the free method.