A chemical equilibrium model for known outlet temperature of the products was carried out on a downdraft fixed bed gasifier. Biomass from oil palm kernel is used with proximate and ultimate analysis; the general gasification reaction takes into account the biomass moisture, the reaction products are formed by CH_{4}, H_{2}O, N_{2}, CO, CO_{2} and H_{2}. The model is described in detail and the equations are solved using the EES software. The model shows the results obtained for temperature ranges from 500 K to 1500 K and 1.0 atm. The model is validated by direct comparison of the gas composition with the results reported in the literature for similar conditions, obtaining favorable results.

Se llevó a cabo un modelo en equilibrio químico para temperatura de salida de productos conocida en un gasificador de lecho fijo de corriente descendente. Se utiliza biomasa de cuesco de palma de aceite con análisis elemental y próximo conocido. La reacción general de gasificación tiene en cuenta la humedad de la biomasa, los productos de la reacción están formados por CH_{4}, H_{2}O, N_{2}, CO, CO_{2} y H_{2}. Se describe detalladamente cómo se realiza el modelo, las ecuaciones son resueltas mediante el

Throughout human history, the use of fossil fuels has been the main driver of energy supply. However, the excessive use of these fuels has led to an increase in environmental pollution and a considerable decrease of them [

Biomass is found in large quantities, there are various types: municipal waste, agricultural waste, animal waste, energy crops, among others. Biomass can be used for direct burning, thus providing thermal energy in the required locations, however, direct combustion thereof causes environmental impacts due to polluting emissions such as CO_{2} or nitrous oxides, when the temperature of the products reaches relatively high values, these components are harmful to health and life on the planet. As an alternative to biomass combustion, gasification emerges as a promising technology, in which fuel gases with low calorific value are obtained for use in other applications. The advantage of gasification over combustion is the reduction of polluting emissions, since the process temperature does not reach values as high as those obtained in combustion, given that the amount of oxidizing agent is lower.

Gasification is the thermochemical process by which a carbon substrate is transformed by partial oxidation into a combustible gas containing, among other compounds, H_{2}, CO, CH_{4}, CO_{2}, and N_{2}, as well as various impurities or pollutants. The gas obtained allows its use in various current combustion machines, among which are internal combustion engines, turbines for electricity production, boilers, combustors, among others. It can also be used for the synthesis of higher value-added products [

Given the advantages of gasification, several authors have developed the process modeling in order to identify the characteristics of the process, while studying those variables that may affect it.

Several models have been developed in this topic; however, this work makes relevance on the models in chemical equilibrium, which allow predicting the greater composition that the process can reach under given operational conditions.

Jarungthammachote and Dutta [

A similar work was developed by Melgar et al. [_{4}, H_{2}O, N_{2}, CO, CO_{2}, H_{2}, SO_{2 }and O_{2}, the influence of the air-fuel ratio is investigated, as well as the moisture content on the process, assuming sufficient residence time to reach equilibrium and that all the carbon present in the biomass has been gasified, the model is validated with the data published by Jayah [

Khadse et al. [_{2}, H_{2} and CH_{4}. The model equations contain four atomic balances (C, O, H and N) and the relationships between the equations are solved using MATLAB. This model compares the gas concentration obtained for four types of biomass from India. A similar model was developed by Melgar et al [

The model developed with this work tries to predict with great similarity the gas concentrations of CH_{4}, H_{2}O, N_{2}, CO, CO_{2}, H_{2}, and will be a basis for future work in greater detail. The model is developed under the chemical equilibrium, the biomass used is oil palm kernel with established characteristics. The model determines the amount of air needed for the development of the process. For solving the equations, the EES software was used since the system is a non-linear system.

Initially, there is a brief description of the stages that occur in the gasification process in a fixed bed downdraft reactor, the considerations used for the calculation of the thermodynamic properties of the necessary components are shown, as well as the equations used; subsequently, the energy balance of the process is shown according to the first law of thermodynamics, followed by the parameters used to estimate the Gibbs energy of the compounds in the gasification process.

The

The

The reactions that the material to be gasified undergoes when it comes into contact with the oxidizing agent and the energy released determine the gasification zones.

The first stage of the process corresponds to the drying stage, whose typical temperature is around 100 to 200 ºC. Conversion takes place due to heat transfer between the hot gases in the oxidation zone [

The reaction that occurs at the pyrolysis stage can be expressed as shown in

After the pyrolysis stage, the oxidation zone is presented, which provides the energy in the form of heat released by oxidizing the biomass, so that the drying, pyrolysis and other endothermic reactions develop during the reduction. In the latter, carbon dioxide and water are reduced to form carbon monoxide and hydrogen; more complex reactions also appear [

The biomass used in the process corresponds to oil palm kernel, the characteristics of it, are presented in

The analysis begins with the formulation of the general equation of biomass gasification (_{x}
_{y}
_{z} is formed from the coefficients

Where H, N, C, O are the percentages given by the ultimate analysis, M represents the molar mass of each element in ^{kg}/_{kmol}
_{1.366}
_{0.544}
_{0.014} is obtained.

From _{1}, x_{2}, x_{3,} x_{4} and x_{5} represent the number of moles of each component _{2} in the reaction.

The moles of water in the reagents can be estimated using the following equation:

From the proximate analysis the %moisture is obtained, when clearing from the previous equation the following is obtained:

The atomic balance for the reaction of

Carbon balance.

Hydrogen balance.

Oxygen balance.

Nitrogen in

Since it is necessary to solve the system of previous equations, it is necessary to specify three additional equations because there are six unknowns (_{1}, x_{2}, x_{3}, x_{4} y x_{5}). Two of these equations are provided by the following reactions according to what is stated by [

Boudouard reaction.

Water-gas reaction

Methane Reaction

The reactions (

The equilibrium constants have to be calculated for the reactions given by

Where _{13} and _{14} refer to the equilibrium constants of the reactions shown in

In this model, it is assumed that reagents enter at standard conditions (25ºC and 1.0 atm) and that the products come out at an assumed temperature T (773K) and 1.0 atm, in order to give a solution thereof.

The energy balance for a stationary flow system based on the first law of thermodynamics for a reacting system is given by:

For the case presented here, the above equation is expressed as:

Where _{R} and _{p} represent the moles of the reagents and the products respectively.

The enthalpy of formation for the known elements is shown in

The enthalpy of formation for the fuel is calculated according to that published by Souza Santos [

Where corresponds to the enthalpy of formation of the products under the premise of complete combustion (

From the balance of this equation, the values for _{th }= 1.0695,

For an initial estimate of the enthalpy of formation of the fuel, the value of the ^{kJ}/_{kmol}.

In this way, the enthalpy of formation of the fuel corresponds to:

Under the established by [

The calculated values as well as other data of interest are shown in

In this way, when expressing

According to

In order to do this, it is necessary to calculate the entropy at temperature T of the products and a To of the reagents. The entropy can be calculated by

Where

With these values, we proceed to find the Gibbs energy values (_{13 }and _{14}, thus completing the system of non-linear equations.

The mole fractions of the gases obtained through temperature variation and equivalence ratio (ER) are shown, as well as the calculation of the lower heat value (LHV) and its behavior with temperature increase.

The lower calorific value of the gas is estimated from the gas concentration, using

^{3}.

The model results have been compared with those obtained by Huan et al. [

A model in chemical equilibrium has been developed for the gasification of palm oil kernel by means of the software EES, this type of models allows obtaining the greater composition that could be obtained when the biomass is gasified, in this case the oxidizing agent used is air.

The maximum temperature for the gasification process in a fixed bed reactor is around 800 °C to 1500 ºC, in this work the maximum temperature of the reactor has been varied from 500 to 1500 K in order to observe its influence on the parameters of interest, the temperature has been the input value to the model for solving the system of non-linear equations obtained.

The lower calorific value for fixed bed gasification with oxidizing air ranges from 1.0 to 6.0 MJ/m^{3}; these values are obtained for temperatures from 900 to 1500 K.

The model adequately predicts the compositions of CO, H_{2}, CO_{2}, and N_{2} with values close to those reported in the literature, while the estimate of CH_{4} in some cases is overestimated and underestimated, something characteristic in equilibrium models, this is the main limitation of the proposed model, which must be considered and adjusted in future work.

Research article