Syngas generation refers to the production of a synthesis gas that is mainly composed of CO and H2, in different proportions. Gasification is the referred technique to produce syngas. It can be used for different purposes, such as power and/or heat generation or for chemicals and fuels production. Syngas can be obtained from natural gas, coal, petroleum refinery fractions, biomass and organic wastes. Traditionally, natural gas and petroleum fractions have been the largest syngas sources worldwide because of the trade-off between costs and availability;

However, because of global economic, energetic and environmental contexts, coal and biomass/waste are of growing interest and use. 

Turning Municipal Solid Waste into Syngas

Faced with the costly problem of waste disposal and the need for more energy, a growing number of countries are turning to gasification, a time-tested and environmentally-sound way of converting the energy in municipal solid waste, MSW, into useful products such as electricity, fertilizers, transportation fuels and chemicals. On average, conventional waste-to-energy plants that use mass-burn incineration can convert one ton of MSW to about 550 kilowatt-hours of electricity.

With gasification technology, one ton of MSW can be used to produce up to 1,000 kilowatt-hours of electricity, a much more efficient and cleaner way to utilize this source of energy.

Gasification can help the world both manage its waste and produce the energy and products needed to fuel economic growth.

Gasification Model Development

A number of processes modeling software package have become available to develop computational model of gasification process and to perform simulation and validation studies. Generally, researchers and professionals use Aspen Plus, Computational Fluid Dynamics (CFD, composed of GAMBIT and FLUENT), ChemCAD and MatLab software packages to develop and optimize their gasification models. Although CFD is powerful software, the programs have high computational requirements. On the other hand, Aspen Plus is one of the sophisticated processes modeling computer software packages which is familiar to many users and has proven its capacity for gasification model development and simulation. 

This article is referring to the work of Begum and co-authors who used Aspen Plus to develop and simulate a fixed bed gasification process for different feedstocks (MSWs, wood wastes, green wastes and coffee bean husks).

The simulations of the biomass gasification process are based on the mass-energy balance and chemical equilibrium for the overall process. 

To develop a model for a fixed bed gasifier, the following sequential steps should be included: 

1. stream class specification
2. property method selection
3. system component specification (from databank) and identifying conventional and non-conventional components,
4. defining the process flowsheet (using unit operation blocks and connecting material and energy streams)
5. specifying feed streams (flow rate, composition, and thermodynamic condition)
6. specifying unit operation blocks (thermodynamic condition, chemical reactions, etc.). 

To simulate reactors sections, it is possible for users to input their own models, using FORTRAN codes and reactions nested within the Aspen Plus input file, to simulate the operation of a fixed bed.

Assumptions to be made:

1. the model is steady state, kinetic free and isothermal; 
2. chemical reactions take place at an equilibrium state in the gasifier, and there is no pressure loss; 
3. all elements except sulfur contact at uniformly and take part in the chemical reaction; 
4. all gases are ideal gases, including H2, CO, CO2, steam (H2O), N2 and CH4; 
5. char contains volatile matters composed of carbon, H2 and O2; 
6. tars are assumed as non-equilibrium products to reduce the hydrodynamic complexity.

Model and process description

A number of steps comprise the overall gasification process:

1. drying;
2. decomposition;
3. gasification;
4. combustion.

A process flowchart is shown in Figure 1. Feed is specified as a non-conventional component in Aspen Plus and defined in the simulation model by using the ultimate and proximate analysis.
 

Figure 1.

The characteristics of different feedstocks (MSWs, wood, green wastes and coffee bean husks) sourced from the literature (BEST Energies Australia Pty Ltd. Report [14], Wilson et al. [15], Naveed et al. [11] and Chen et al. [10]) are given in Table 1. The model is based on minimization of the Gibbs free energy at equilibrium. This simulation is developed under the assumption that the residence time is long enough to allow the chemical reactions to reach an equilibrium state.

Table 1.

Typical gasification operating parameters are given in Table 2.

Table 2.

Methods and steps for model development

Physical Property Method

The Redlich-Kwong-Soave cubic equation of state with Boston-Mathias alpha function (RKS-BM) can be used to estimate all physical properties of the conventional components in the gasification process. 

RKS-BM is recommended for gas-processing, refinery and petrochemical applications such as gas plants, crude towers and ethylene plants. Using RKS-BM, reasonable results can be expected at all temperatures and pressures. The RKS-BM property method is consistent in the critical region. The enthalpy and density model selected for both feed and ash are non-conventional components, HCOALGEN and DCOALIGT. In this example, feed was defined as non-conventional components from the perspectives of ultimate and proximate analysis as shown in Table 1. Ashes were also defined as a non-conventional component with an ash content set to 100%.

Model sequence

A number of Aspen Plus units were used to develop the model. The main processes were simulated by three reactors in Aspen plus: RStoic, RYield and RGibbs. 
The gasification process begins with the decomposition (pyrolysis) region and continues with the combustion region. The relevant reactions are the following:

1. C + O2 = CO2 (carbon combustion)  
2. C + 0.5O2 = CO (carbon combustion)  
3. C + CO2 = 2CO (Boudouard)  
4. C + H2O = CO + H2 (water-gas) 
5. CO + H2O = CO2 + H2 (CO shift) 
6. C + 2H2 = CH4 (methanation) 
7. H2 + 0.5O2 = H2O (H2 combustion) 
8.  CH4 + H2O = CO + 3H2  
9. CH4 + 2H2O = CO2 + 4H2  

Major gasification reactions are water gas, Boudouard, shift conversion and methanation. In accordance with the Boudouard reaction in Equation (3), at low temperatures both unburnt carbon and CH4 are present in the syngas. as the gasifier temperature increases the mole fraction of CO increases and that of CO2 decreases. Water gas reaction in Equation (4) suggests that high temperature increases the production of both CO and H2. According to the methanation reaction in Equation (6) the mole fraction of CH4 in syngas decreases and that of H2 increases with the increase in temperature. At higher temperatures yield of H2 and CO starts reducing. This is also attributed to the water gas reaction in Equation (4).

Drying

The purpose of this region is to reduce the moisture content of the feedstock. The Aspen Plus stoichiometric reactor, RStoic (model ID: DRIER), can be used to simulate the evaporation of moisture. The drying operation was controlled by writing a FORTRAN statement in the calculator block. RStoic converts a part of feed to form water which requires the extent of reaction known as:

10. Feed → 0.0555084H2O 

The yield of gaseous water is determined by the water content in the proximate analysis of particular feedstock. In case of model validation, the moisture content of MSW is 12%; therefore, the mass yield of gaseous water is set as 12%, based on the assumption that the physically bound water is vaporized completely in this process. The mass yield of dried MSW is correspondingly equal to 100% − 12% = 88%. In this step, the moisture of each feedstock is partially evaporated and then separated using a separator model, Sep2 (model ID: SEP1) through split fractionation of the components. The dried feedstock is placed into the next region for decomposition after being separated from the evaporated moisture. The evaporated moisture was drained out from the process. The produced heat of reaction associated with the drier (model ID: Q-DRIER) was passed by a heat stream into the RYield reactor where decomposition occurs.

Decomposition

Decomposition is one of the main steps of the gasification process where each feedstock is decomposed into its elements. The Aspen Plus yield reactor, RYield (model ID: DECMPOSE), was used to simulate the decomposition of the feed. The yield reactor converts non-conventional feed into conventional components by using a FORTRAN statement. In this step, feed is converted into its components including carbon, O2, N2, H2, sulphur and ash by specifying the yield distribution according to the feedstock’s ultimate analysis. The yield distribution of feed into its components was specified by a FORTRAN statement in the calculator block. The decomposed elements mixed with air at an Aspen MIXER block are ready for gasification.

Gasification

The RGibbs reactor is a rigorous reactor for multiphase chemical equilibrium based on Gibbs free energy minimisation. RGibbs was used to simulate the gasification of biomass. The Gibbs free energy of the biomass cannot be calculated because it is a non-conventional component. Therefore, before feeding the biomass into the RGibbs block it was decomposed into its elements (C, H, O, N and S, etc.) using the RYield reactor. The reactor calculates the syngas composition by minimising the Gibbs free energy and assumes complete chemical equilibrium. The heat of reaction associated with the decomposition (Q-PYROL) of feed was passed by a heat stream into the RGibbs reactor where gasification occurs. The decomposed feed and air enter into the RGibbs reactor where partial oxidation and gasification reactions occur. Carbon partly constitutes the gas phase, which takes part in devolatisation, and the remaining carbon comprises part of the solid phase. A very minimum heat (model ID: Q-GASIF) produced at gasification escapes from the process through a heat stream. A separator model, Sep2 (model ID: SEP2) was used to separate ash from the gas mixture using split fractionation of the components.

Combustion

To complete the gasification process, another RGibbs reactor was used in the combustion section with minimum air mixing. This combustion process is also based on the principle of minimization of Gibbs free energy. To identify the syngas components from by-products, a separator model, Sep2 (model ID: SEP3), was used.

Model Validation

To help you check the model validity, obtained results are shown in Table 3.

Table 3.

The simulation was done for syngas composition, such as H2, CO, CO2, CH4 and N2 using the experimental condition for both MSWs and food wastes. 

After is developed, this model can be used to explore the influence of key variables, such as:

• Effect of air-to-fuel ratio
• Effect of gasifier temperature
• Variation in feedstock

For more details, please refer to the reference list.

References:
Performance Analysis of an Integrated Fixed Bed Gasifier Model for Different Biomass Feedstocks, Sharmina Begum 1, Mohammad G. Rasul 1, Delwar Akbar 2 and Naveed Ramzan 3, 2013

Syngas from Waste, Emerging Technologies, Luis Puigjaner Editor, 2011