Prediction of Syngas Composition During Gasification of Lignocellulosic Waste Mixtures

Abstract

Avoiding global dependence on fossil oils and improving the environmental impact of energy production are factors that drive research into renewable energies. Considering lignocellulosic biomass residues as a raw material for gasification, a thermochemical process that converts lignocellulosic resources into synthesis gas (H₂, CO, CH₄, and CO₂) is an alternative under study due to its low costs, high efficiency, and wide variety of applications. Fortunately, there are still areas for its improvement and technological development. For example, this can be achieved by gasification. Distinct types of lignocellulosic biomass, such as sugarcane bagasse, wheat straw, pine sawdust, or corn cob, differ in their physical, chemical, and morphological properties, which can affect the characteristics of the gasification process. This work uses the generalized stoichiometry and mass and atomic balances in the gasification reactor to predict the composition of syngas produced via the gasification of both individual substrates and mixtures. The results provide useful information for the design and operation of gasification reactors with an operating region between 2.0 bar and 4.5 bar and between 1023.15 K and 1223.15 K, particularly with regard to understanding the effects of distinct types of biomasses in terms of their humidity and molecular weight on the operation and performance of the process. One important conclusion reached after simulating the addition of more vapor is that the (H₂/CO) ratio cannot be increased indefinitely: it is limited by the thermodynamic equilibrium reached by the system.

1. Introduction: Biomass and Biorefineries

The substitution of materials and energy from fossil hydrocarbons has promoted the development of biorefineries based on different platforms, such as the gasification of biomass. Currently, this technology is considered as a reliable conversion technology, as it can produce relatively clean syngas for the generation of heat and power with low emissions of pollutants. Biomass gasification is a thermochemical conversion process that converts biomass into a mixture of gaseous components, including molecular hydrogen, carbon monoxide, carbon dioxide, and methane, as well as some amount of tar and char. The compositions of these major and minor products and the quality of the syngas produced via gasification depend on the biomass type, gasification agent, gasifier type, and operating parameters [1]. The set of reactions occurring mainly includes partial oxidization and reformation. This gasification is conducted in reactors that can receive as loads different lignocellulosic wastes (substrates) or mixtures of them, as well as a gasification agent such as water vapor. Gasification takes place at elevated temperatures and moderate pressures, using catalysts based on alkaline metals to promote the dehydrogenation of the substrates [2].

The approaches to the modeling and simulation of biomass gasification processes can be grouped into three different kinds of models: (a) kinetic models, (b) computational fluid dynamic (CFD) models, and (c) thermodynamic equilibrium models. The choice of model depends on the objective of the research: for example, reactor design and scale-up, or a gas particle flow or gas composition equilibrium approach. Although equilibrium models are the simplest to solve, they currently are not capable of adjusting the outlet composition of the gasification reactor. Kinetic models are complex and not easy to deal with, so their use is also limited [3]. CFD models are focused mainly on the hydrodynamics of the reactor and not really on the kinetics itself [4].

This work presents the theoretical background employed to predict the composition of syngas obtained from the gasification of singular lignocellulosic wastes and their mixtures. It starts by using empirical formulas to identify unitary cells of waste and the stoichiometry necessary for achieving mass balance in gasification reactors. The results provide the theoretical basis for developing a mathematical strategy useful for decision-making, which allows for estimating product stream composition and, consequently, important quality parameters such as the H₂/CO ratio and H₂ production as a function of the waste fed into the reactor. Thermodynamic feasibility is considered by fulfilling the equilibrium conditions of the reversible reactions that take place and evaluating the equilibrium constants as a function of temperature and pressure [1], thus bounding the actual operating region [5,6]. In the case of carbonaceous substrate gasification, such as lignocellulosic biomass, a water–gas shift reaction takes place as an intermediate step for hydrogen enrichment and CO reduction in the syngas. An important feature of this reaction is that it is governed by equilibrium [7]. Due to the presence of an alkaline metallic catalyst, commonly nickel, a water–gas shift reaction takes place, which is explicitly considered during the stoichiometric modeling of the lignocellulosic residue gasification process. One important aspect that should be taken into account is that the reactions proposed may or may not be the ones taking place inside the actual gasification reactor; however, since the process engineering concept establishes that the set of reactions to be used is the one useful for the modeling of chemical transformations, the set of reactions obtained can be considered as the minimum number of reactions necessary to model the stoichiometry of the reaction set [7].

In biorefineries whose platform is gasification, one of the quality specifications of syngas is the H₂/CO ratio. For example, if Fischer–Tropsch synthesis is among the downstream processes, a H₂/CO ratio of >2.5 is recommended; in the case of gas-to-liquid, H₂/CO > 1.8 is recommended; and in the case of lower ratios, the syngas can be used as fuel within the biorefinery [8]. This makes being able to estimate this ratio a priori of primary importance in determining the syngas’s intended use.

On the other hand, the most abundant source of hydrogen on Earth is water, which means that using it as a gasification agent increases the value of the H₂/CO ratio. Finally, taking advantage of the humidity that accompanies lignocellulosic waste also favors this ratio while avoiding the requirement for the expensive and energy-intensive pretreatment of drying it.

2. Methodology

2.1. Substrates and Their Characteristics

Due to the availability of their gasification experimental data, three substrates were chosen: cane bagasse, pine sawdust [9], and wheat straw [10]. Empirical formulas were developed based on the relative mass contents of the elements {C, H, O, N, S, and Ash}; then, by defining the degree of polymerization (DP) [11], the chain lengths of the organic molecules in each substrate were characterized. Additionally, the molecular weight of each substrate’s ashes (MW_ash) was estimated. The unitary cell compositions of the substrates considered in this work are shown in

Table 1. Substrates’ characterization.

Substrate	DP	Empiric Unitary Cell Formulae	${\mathbf{MW}}_{\mathbf{ash}} \mathbf{\left(}\mathbf{Da}\mathbf{\right)}\mathbf{/}\mathbf{\%}\mathcal{H}\mathcal{R}$
Cane bagasse	925	${\mathrm{C}}_{5947}{\mathrm{H}}_{7540}{\mathrm{O}}_{3980}{\mathrm{N}}_{37}{\mathrm{S}}_{23}{\mathrm{A}}_{\mathrm{C}\mathrm{B}}$	5993/50
Pine sawdust	1450	${\mathrm{C}}_{9824}{\mathrm{H}}_{14856}{\mathrm{O}}_{6263}{\mathrm{N}}_{135}{\mathrm{S}}_2{\mathrm{A}}_{\mathrm{P}\mathrm{S}}$	707/8.6
Wheat straw	2660	${\mathrm{C}}_{15814}{\mathrm{H}}_{18322}{\mathrm{O}}_{10988}{\mathrm{N}}_{188}{\mathrm{S}}_{27}{\mathrm{A}}_{\mathrm{W}\mathrm{S}}$	43,532/7.6

.

2.2. Generalized Stoichiometry of the Reacting System

The simulation of the reactor required the resolution of the mass balance, for which it was necessary to know the stoichiometric scheme with which the transformation of the substrate molecules into products would be monitored. Stoichiometric models [1] are developed on the assumption that certain chemical reactions reach equilibrium, and solving this type of model requires the calculation of the associated equilibrium constants for this purpose. After reviewing the literature, seven products were suggested, namely {CO, H₂, CO₂, H₂O, CH₄, N₂, and H₂S}. Using these products, the generalized stoichiometry technique, developed by Aris and Mah in 1963 [12], was used with the formula of the unit cell of each substrate. Details of this technique can be found, for example, in Cerro et al. [13] and in Reklaitis [14] Ch. 4.

It was found that, for the proposed substrates, it was sufficient to solve the progress of three reactions, all of them restricted by equilibrium: water–gas shift (1), wet methane reforming (2), and dry methane reforming (3).

$$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2$$

(1)

$$\mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\leftrightarrow {\mathrm{H}}_2\mathrm{O}+{\mathrm{C}\mathrm{H}}_4$$

(2)

$$2\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\leftrightarrow \mathrm{C}{\mathrm{O}}_2+{\mathrm{C}\mathrm{H}}_4$$

(3)

In this work, the way in which the feasible operation region was constrained was by ensuring that the reactions that reached equilibrium ((1)–(3)) satisfied this restriction. Since the equilibrium constants are a function of temperature and pressure, by satisfying this restriction, realistic values of the H₂/CO ratio achievable at each operating condition were obtained, since this ratio cannot be increased infinitely during the gasification reactions.

Next, experimental data available in the literature were simulated, adjusting the gasifier outlet composition of each individual substrate. To do this, two other reactions were added to fine-tune the composition within the feasible operating region, the same adjustment as was used when simulating the gasification of their mixtures. These two reactions were the wet reforming of the substrate (4) and the dry reforming of the substrate (5); the case of sugar cane is shown.

$${\mathrm{C}}_{5947}{\mathrm{H}}_{7540}{\mathrm{O}}_{3980}{\mathrm{N}}_{37}{\mathrm{S}}_{23}{\mathrm{A}}_{\mathrm{C}\mathrm{B}}+1967{\mathrm{H}}_2\mathrm{O}\to 5947\mathrm{C}\mathrm{O}+5714{\mathrm{H}}_2+23{\mathrm{H}}_2\mathrm{S}+{\displaystyle \frac{37}{2}}{\mathrm{N}}_2+{\mathrm{A}}_{\mathrm{C}\mathrm{B}}$$

(4)

$${\mathrm{C}}_{5947}{\mathrm{H}}_{7540}{\mathrm{O}}_{3980}{\mathrm{N}}_{37}{\mathrm{S}}_{23}{\mathrm{A}}_{\mathrm{C}\mathrm{B}}+1967\mathrm{C}{\mathrm{O}}_2\to 7914\mathrm{C}\mathrm{O}+3752{\mathrm{H}}_2+23{\mathrm{H}}_2\mathrm{S}+{\displaystyle \frac{37}{2}}{\mathrm{N}}_2+{\mathrm{A}}_{\mathrm{C}\mathrm{B}}$$

(5)

The reaction advances were solved assuming the conversion of the substrate up to approximately 76 wt. %, except for the ashes; the rest was distributed as tar and char, which were treated as side by-products. To date, modeling the tar and char products is still an unsolved problem; however, it has been proposed that their presence does not affect the equilibrium of reactions [5,6].

There was a point of interest in the way the reactions were developed. There were two kinds of water in the system. The first one was the reactant, which arrived with the substrates or could be supplied independently. The second was the water that was produced (rather than supplied) via the water–gas shift reaction. However, to satisfy the equilibrium conditions, these water molecules must have been equivalent. In the case of carbon dioxide, the reactant used in the dry reforming reactions was the one produced via water–gas shift. All partial pressure played a role in determining the global equilibrium in the reacting mixture.

Data on the composition of the reactor effluent were available for the gasification of sugar cane bagasse, pine sawdust [9], and wheat straw [10] as individual substrates. The adjustment of the advances of the proposed stoichiometric scheme, constrained to the equilibrium conditions described above, led to very good agreement with the reported data (Figure 1).

2.3. Mass Balances in the Gasification Reactor

Once the set of possible stoichiometric equations for the system were generated, the degrees of freedom of the mass balance subproblem in the reactor were analyzed to identify the possible restrictions of each proposed reaction and the independent data that must be known to solve this problem. The objective was to predict the reactor effluent compositions for different dual mixtures of one mol of total substrate at three different proportions. Because each mixture is different from any other, and there are an infinite number of possible dual mixtures, the parameters adjusted for individual substrates were used in the prediction of the gasification of the mixtures. Kangas et al. [8] and Raheem et al. [15] proposed operating in the pressure region of 2.0 bar to 4.5 bar and the temperature region of 1123 K to 1223 K. The propriety of this operating region was confirmed by the evaluation by Ortiz-Sánchez et al. [5] and Shafiq et al. [6] of the equilibrium conditions after the gasification reactions took place. These energy balance constraints were used to evaluate the equilibrium constants that restrain the operation region. Then, the advances of the three reactions that reach equilibrium were solved by means of numerical methods involving nonlinear algebraic equations. Finally, by substituting these three advances in the mass balance, the composition of the reactor effluent could be known. The accuracy of the solution was indicated by the matching of atomic balances.

3. Results and Discussion

The compositions of the reactor effluent with different dual mixtures of one mole of total substrate at three different proportions were simulated. Because each mixture is different to any other, and there are an infinite number of possible dual mixtures, there are no experimental results available for the mixtures below. Consequently, the parameters adjusted for individual substrates were used in the prediction of the composition of the reactor effluent.

Cane bagasse was used as the ‘pivot’ waste, because of its high content of water. Two more substrates, pine sawdust and wheat straw, were mixed with the cane bagasse in order to assess the effect of using a dry substrate on the product distribution at the gasification reactor outlet. The results can be used to predict the behavior of the gasification reactor for each mixture of substrates, based on the individual (and validated) results for the gasification of each one individually.

3.1. Sugar Cane Bagasse and Pine Sawdust

The first mixture to be analyzed was sugar cane bagasse and pine sawdust, in three different proportions—8:2, 7:3, and 6:4. These proportions were selected because the incorporation of more pine sawdust prevents the gasification reactions. An atomic account of the first mixture, with the proportion 8:2, is given in

Table 2. Atoms in the mixture sugar cane-pine sawdust, with proportion 8:2.

	C	H	O	N	S
Atoms	6722	15,793	7831	57	19

.

The production of molecular hydrogen at the proportion 8:2 (Figure 2a) exhibits two noticeable maxima, one at 2.5 bar and the other at 3.8 bar, both at 1223.15 K. Therefore, it is possible to infer that there should be points close to the optimum yield of hydrogen. On the other hand, the minimum values are reached at the maximum operating pressure, which means that the pressure inversely affects the hydrogen production.

The production of CO at the proportion 8:2 (Figure 2b) follows a similar trend to that exhibited by molecular hydrogen, showing two possible relative maximum points of production, at 2.0 bar and 1223.15 K and 3.5 bar and 1223.15 K. Both gasses are the principal components of the syngas, and therefore it is possible to infer that syngas production is favored at low pressures and the maximum operating temperature.

The H₂/CO ratio at the proportion 8:2 (Figure 2c) follows a different pattern, which is important given the effect this ratio has on the potential uses of this syngas. In the simulated cases, this ratio remains mostly unchanged, oscillating between values of 0.85 and unity. This result agrees with experimental observations made previously [6,13].

The water available in the reactor effluent at the proportion 8:2 exhibits slight changes (Figure 2d), exhibiting its relative maximum at 4.5 bar and 1223.15 K. Although the water profile oscillates, it is possible to infer that its content in the reactor effluent is improved by increasing the pressure, and the same behaviors were observed previously in the context of CLC production, taking water–gas shift as one of the main reactions; further, in gasification, water shift was proposed as one of the main schematic reactions [16].

The production of carbon dioxide at the proportion 8:2 (Figure 2e) exhibits an oscillating behavior, with its relative maximum value achieved at 4.5 bar and 1173.15 K, and two relative minima reached at 2.0 bar and 1123.15 K and 4.5 bar and 1223.15 K. Therefore, its production is favored at high pressure and intermediate temperature and prevented at high pressure and either low or elevated temperature.

Finally, the production of methane at the proportion 8:2 (Figure 2f) exhibits medium oscillations, reaching its relative maximum at 4.5 bar, 1173.15 K, and its relative minimum at 4.5 bar, 1223.15 K. It also shows an incline with the relative maximum at the intermediate value.

When the proportion is changed to 7:3, the global behavior will be similar, with changes in some variables. The atomic account of the second mixture, with a proportion of 7:3, is given in

Table 3. Atoms in the mixture sugar cane-pine sawdust, with proportion 7:3.

	C	H	O	N	S
Atoms	7110	15,924	7760	66	17

.

The first change is that the H₂/CO ratio exhibits some decrease, reaching a relative maximum close to one at 2.0 bar and 1223.15 K, but with a smaller minimum value than in the case of the proportion 7:3. However, the productivities of hydrogen and carbon monoxide at the proportion 8:2 are similar to those reached at the proportion 7:3. Hence, the H₂/CO ratio is not a good quality criteria, as has been mentioned in the literature [17].

As regards the water in the reactor effluent at the proportion 7:3 (Figure 3d), there is a decrease in all its values, and its relative minimum is observed at intermediate temperatures. This situation changes the shapes of the profiles to hill-like ones.

The level of carbon dioxide produced remains at similar values for the proportion 7:3 (Figure 3e) in comparison with 8:2 (Figure 2e); however, the shape of the profile changes, reaching its relative maximum at 2.5 bar and 1123.15 K and its relative minimum at 4.5 bar and 1223.15 K.

The production of methane changes shape at the proportion 7:3, increasing its content in the reactor effluent and reaching two relative maximum values at 2.0 bar and 1173.15 K and 4.5 bar and 1173.15 K. Further, in the case of the relative minima, it is possible to observe oscillatory behavior, with the minima reached at 2.5 bar and 1123.15 K and 4.5 bar and 1223.15 K.

The atomic account of the third mixture, with the proportion 6:4, is given in

Table 4. Atoms in the mixture sugar cane-pine sawdust, with proportion 6:4.

	C	H	O	N	S
Atoms	7498	16,054	7687	76	15

.

The production of hydrogen (Figure 4a) decreases with respect to the previous proportions and exhibits an oscillating behavior. Its relative maximum is reached at 2.0 bar and 1223.15 K, like in the previous two cases, but now there are two relative minima at 4.5 bar and 1123.15 K and 4.5 bar and 1223.15 K. Therefore, the conclusion is that lower pressure favors hydrogen production. The production of CO (Figure 4b) remains similar to that in the other two cases, as does its relative maximum and relative minimum. Due to the changes in hydrogen production, the profiles of the H₂/CO ratio (Figure 4c) change to a hill-like shape, reaching the relative maximum at 2.0 bar and 1223.15 K and showing two relative minima, both close to 0.65, at 4.5 bar and 1123.15 K and 4.5 bar and 1223.15 K. Again, although the levels of production of hydrogen and CO are similar to those in the previous cases, this ratio changes significantly, which is not a particularly good quality parameter to use to classify the syngas. In comparing the mixtures containing the three proportions of sugar cane bagasse and pine sawdust, it can be noted that the levels of production of hydrogen and carbon monoxide remained similar in the three cases; however, the H₂/CO ratio changed its behavior noticeably. The amounts of water (Figure 4d), carbon dioxide (Figure 4e) and methane (Figure 4f) in the reactor effluents decreased in the sequence 8:2, 7:3, and 6:4; this situation is a consequence of the equilibrium conditions and difficult to associate with any single influence.

3.2. Sugar Cane Bagasse and Wheat Straw

The second mixture to be analyzed comprised sugar cane bagasse and wheat straw, in the same three proportions (8:2, 7:3, and 6:4). The operating conditions for the gasification reactor here were the same as in the last case, so it is possible to compare the reactor effluents directly. An atomic account of the fourth mixture, with the proportion 8:2, is given in

Table 5. Atoms in the mixture sugar cane-wheat straw, with proportion 8:2.

	C	H	O	N	S
Atoms	7920	16,744	8906	67	24

.

The production of molecular hydrogen at the proportion 8:2 (Figure 5a) exhibits two noticeable maxima, one at 2.5 bar and the other at 3.8 bar, both at 1223.15 K. Again, it is possible to infer that there are points that should be close to the optimum yield of hydrogen. The relative minimum values are reached at the maximum operating pressure, which means that the pressure inversely affects the hydrogen production. The production of carbon monoxide at the proportion 8:2 (Figure 5b) still shows slight changes in all operating regions, reaching two relative maxima at 2.0 bar and 1123 K and 4.5 bar and 1223 K. This level of production reaches its relative minimum at 4.5 bar and 1123 K. Although these values are stable, they are larger than in the case of the other mixture.

The H₂/CO ratio values exhibited are small at the proportion 8:2 (Figure 5c), reaching two relative maxima of about 0.84 at 2.0 bar and 1223 K and 4.5 bar and 1173 K. However, for this mixture, the level of hydrogen production is similar to that yielded by the mixture of sugar cane bagasse and pine sawdust, but the level of production of carbon monoxide is higher.

The amount of water in the gasification reactor outlet at the proportion 8:2 (Figure 5d) reaches its relative maximum at 4.5 bar and 1223 K and its relative minimum at 2.0 bar and 1223 K. Its shape is hill-like and oscillating, while the relative changes are sensitive.

The production of carbon dioxide at the proportion 8:2 (Figure 5e) exhibits an oscillating behavior, with its relative maximum value reached at 4.5 bar and 1173.15 K and its relative minimum reached at 4.5 bar and 1223.15 K. Therefore, its production is favored at high pressures and intermediate temperatures and prevented at high pressures and elevated temperatures.

The production of methane at the proportion 8:2 (Figure 5f) exhibits medium oscillations, reaching its relative maximum at 4.5 bar and 1173.15 K and its relative minimum at 4.5 bar and 1223.15 K. Therefore, high pressures and intermediate temperatures favor the production of methane.

The fifth mixture, with the proportion 7:3, exhibits a somewhat different and oscillating behavior, and the atomic account of this mixture is given in

Table 6. Atoms in the mixture sugar cane-wheat straw, with proportion 7:3.

	C	H	O	N	S
Atoms	8907	17,350	9371	82	24

.

The productivities of hydrogen (Figure 6a) and carbon monoxide (Figure 6b) exhibit hill-like shapes, with their relative maximum showing a plateau between 2.0 bar and 2.5 bar and at a temperature of 1223.15 K. Both exhibit two relative minima at 4.5 bar and 1123.15 K and 4.5 bar and 1223.15 K. The amounts produced are the highest for both gasses analyzed in this work. In fact, the profiles of both production curves are remarkably similar.

As a consequence of the similarity mentioned above, the H₂/CO ratio (Figure 6c) continues to show slight changes in all the operating regions, exhibiting values as low as 0.73 at 2.0 bar and 1223.15 K and 0.54 at 4.5 bar and 1123.15 K. Again, this ratio is not a suitable quality index to indicate the availability of hydrogen.

The amount of water in the reactor effluent (Figure 6d) exhibits its relative maximum at 4.5 bar and 1223.15 K and its relative minimum at 2.0 bar and 1173 K. Its behavior is consistent, without profound changes in the operating zone, as a consequence of the equilibrium reached by the reactions.

The production of carbon dioxide (Figure 6e) remains consistent without profound changes in the operating zone, reaching its relative maximum at 4.5 bar and 1123.15 K and its relative minimum at 2.0 bar and 1173.15 K.

Finally, the production of methane (Figure 6f) exhibits slight changes in the operating region, reaching two relative maxima at 4.5 bar and 1123.15 K and 4.5 bar and 1223 K, and reaching its relative minimum at 2.0 bar and 1173.15 K.

For the last simulation, the atomic account of the sixth mixture, the proportion 6:4, is given in

Table 7. Atoms in the mixture sugar cane-wheat straw, with proportion 6:4.

	C	H	O	N	S
Atoms	9894	17,956	9836	97	25

.

The levels of hydrogen production for this mixture oscillate (Figure 7a), reaching three relative maxima at 2.0 bar and 1223.15 K, 2.5 bar and 1223.15 K, and 3.5 bar and 1223.15 K. The CO production (Figure 7b) exhibits slight changes in the operating region, reaching relative maxima at 2.0 bar and 1223.15 K and 4.0 bar and 1223.15 K. It also exhibits sensitive changes at elevated temperatures, reaching its minimum at 4.5 bar and 1123.15 K.

The H₂/CO ratio (Figure 7c) exhibits very low values, reaching its maximum of 0.6 at 4.0 bar, 1173.15 K, and its minimum of 0.5 at 4.0 bar, 1173.15 K. Again, these values do not reflect the amount of hydrogen in the reactor effluent.

The water in the reactor effluent (Figure 7d) exhibits an oscillating behavior, reaching two relative maxima at 4.5 bar and 1123.15 K and 4.5 bar and 1173.15 K. The production of carbon dioxide (Figure 7e) is very flat, reaching its maximum at 4.0 bar and 1226.15 K and its minimum at 2.0 bar and 1223.15 K. Finally, the production of methane (Figure 7f) exhibits very low changes, reaching its relative maximum as a plateau between 2.0 bar and 1223.15 K and 2.5 bar and 1223 K; its minimum is reached at 2.0 bar and 1223.15 K.

The amounts of gasses produced changed according to the proportions of the sugar cane bagasse–wheat straw mixture. The productions of hydrogen and carbon monoxide changed minutely for the mixture with a proportion of 7:3, exhibiting their maximum values. As regards the other mixtures, the production of hydrogen was slower for the mixture with a proportion of 6:4, and the lowest level of carbon monoxide production was obtained for the mixture 8:2. As regards the H₂/CO ratio, the observed values decreased from mixture to mixture, even when the levels of production of the gasses were very different, indicating again that this ratio does not closely reflect the characteristics of the syngas. The amount of water in the reactor effluent decreased from simulation to simulation, as a consequence of the equilibrium reached by the reactions. The production of carbon dioxide exhibited its maximum level for the mixture with the proportion 8:2, its minimum in the mixture with the proportion 7:3, and an intermediate value in the mixture with a proportion of 6:4: again, all as consequences of the equilibrium. The production of methane increased slowly from the first to the second mixture but increased very notably for the third mixture.

Comparing the production profiles between the mixtures of pine sawdust with sugar cane bagasse and wheat straw with sugar cane bagasse, we found that all the values were higher for the mixtures with wheat straw. This was expected, given the greater amounts of carbon and hydrogen atoms supplied by each mixture; however, it is not easy to predict the final distribution of these atoms in the products of the gasification reactions. Therefore, this methodology of using empiric stoichiometry to reflect the mass balances in the gasification reactor, constrained by the equilibrium conditions for the chemical reactions, contributes to the prediction of production profiles during the gasification of mixtures of lignocellulosic waste, giving results similar to the experimental results. Simultaneously, substrates typically considered as waste are here valued as useful, contributing to the circular economy in Mexico, as suggested by Nizami et al. (2018) [17].

4. Conclusions

By applying the methodology of generalized stoichiometry, we identified three reversible reactions as useful in restraining the actual operating region of gasification reactors. By including a couple of additional reactions, as well as the wet and dry reforming of the lignocellulosic waste, it was possible to adjust the experimental results given by the gasification of single substrates, and, with the same relative ratio of wet and dry reforming for the substrates, it was possible to predict the composition of the gasification reactor effluent using dual mixtures. This methodology can be applied to any lignocellulosic substrate and may be extended to mixtures of multiple residues. The main steps are to represent the wastes in terms of their unitary cells and to develop the stoichiometry for the wet and dry reforming reactions. The other three reactions are useful in constraining the operating region. It is important to note that the H₂/CO ratio cannot be increased without limit because of the constraints of thermodynamic equilibrium. This ratio is also not as useful as the quality parameter of the syngas, because it does not reflect the availability of hydrogen. Therefore, the gasification of lignocellulosic waste with different degrees of humidity, or even including more water as the gasification agent, encounters a physicochemical limit in the context of syngas production and composition, which can be considered as one of the equilibrium conditions for the chemical reactions. The main point to emphasize is that the capacity to model stoichiometry contributes to predicting feasible operating regions and possible results when these kinds of substrates are gasified. This effort is still limited to the available data and the possibility of performing controlled experiments analyzing both feedstock substrates and gasification products, not only with lignocellulosic-based biomass but also with other kinds as well.