Simulation analysis of municipal solid waste pyrolysis and gasification based on Aspen plus

Abstract

To predict and analyze the municipal solid waste (MSW) pyrolysis and gasification process in an up-draft fixed bed more veritably and appropriately, numerical modeling based on Gibbs energy minimization was executed using the Aspen plus software. The RYield module was combined with the RGibbs module to describe the pyrolysis section, while the RGibbs module was used for the gasification section individually. The proposed model was used to forecast and analyze the target performance parameters including syngas composition, lower heating value (LHV) and carbon conversion rate under different conditions of the gasification temperatures, and ratios and types of gasifying agents. The results indicate that there is a good agreement between the experimental data and the simulated data obtained using this model. The predicted optimum gasification temperature is approximately 750°C, and the best ratio of water vapor as gasifying agent is around 0.4. The mixture of flue gas and water vapor has an economical and recycled prospect among four commonly used gasifying agents.

Keywords

municipal solid waste (MSW) / pyrolysis / gasification / Aspen plus / simulation

Introduction

The variable composition of municipal solid waste (MSW) leads to its extremely complex pyrolysis and gasification process, in which internal reactions occur and mutual influence exists [¹,²]. So the pyrolysis and gasification process of MSW is hard to be simulated, requiring complex mathematical models and iterative computations. The thermodynamic equilibrium model based on Gibbs free energy minimization is suitable for this kind of simulation calculation of complex equilibrium system, because it does not require the knowledge of specific chemical reactions [³]. Currently, many scholars are using the Aspen plus software to establish reaction process models and solve computational problems, most of whom are interested in combustion and gasification of coal and biomass [⁴-⁶].

Few people have paid attention to pyrolysis and gasification of MSW. Zheng [⁷] have used the Aspen plus software to establish MSW gasification and melting model to predict the composition of MSW gasification products, and to discuss the influence of air equivalence ratio, air preheating temperature, moisture content and other factors on the gasification and melting key temperature. Chen [⁸] have studied syngas composition, lower heating value (LHV) and carbon conversion rate under different conditions of the gasification temperatures, air equivalence ratio and moisture in MSW pyrolysis and gasification with a fixed bed. Begum et al. [⁹] have presented a numerical simulation model of a fixed bed gasifier, also using the Aspen plus software to analyze the gasification performance under varying operating parameters, such as air-fuel ratio, gasification temperature and moisture content of MSW.

These studies have aimed at revealing the mechanism characteristics of steady-state performance of MSW pyrolysis and gasification, where, the individual reaction stage has always been described by the built-in reactor or module in the Aspen plus software.

Many researchers have especially made great efforts to study the two key stages in the process, pyrolysis section and gasification section. In the simulation process, most researchers have applied the RYield module for the pyrolysis section. For example, Chen [⁸] and Begum et al. [⁹] have used it for MSW in fixed bed. Nikoo and Mahinpey [¹⁰] and Jin [⁶] have also applied this module for the feedstock decomposition stage of both MSW and biomass/water vapor gasification process in fluidized bed. For hydrogen production from biomass gasification in interconnected fluidized bed, Shen et al. [¹¹] have proposed the Ryield block of the Aspen plus corresponding to decomposer, too. The advantage of the Ryield module is that the process can be directly simulated by setting product yield while the relevant stoichiometry and kinetic parameters are not required. Therefore, this module is widely used for the pyrolysis section. But during the simulation process, the feedstock should be directly transformed to quantitative simple substances and ash, which can be regarded as pyrolysis products to be input in the software, based on the ultimate analysis, the proximate analysis, and the previous experiments. For MSW, these substances cannot roundly reflect the actual component distribution of the pyrolysis product. Furthermore, this module needs to set the component and the yield of the product based on the pyrolysis experimental data. Besides, the irregularity of MSW compositions has a great influence on the proportion of the composition of the pyrolysis product. Hereon, the applicability of only using the RYield module to describe the MSW pyrolysis section has been limited. While based on the chemical equilibrium and phase equilibrium limited by Gibbs free energy minimization, the RGibbs module can simulate decomposition, recombination and chemical reactions, and can predict the equilibrium composition of the produced syngas well [¹²], often used for the gasification section [⁶–⁸].

Therefore, in order to predict and analyze the MSW pyrolysis and gasification process in an up-draft fixed bed more veritably and appropriately, a model is proposed in this paper by combining the RYield module and the RGibbs module to simulate the pyrolysis section, while the RGibbs module is used to describe the gasification section. The proposed model could forecast and analyze the target performance parameters including the syngas composition, the LHV and the carbon conversion rate under different conditions of reaction temperatures, ratios and types of gasifying agents. Moreover, the prospective numerical data could help the actual operation of MSW pyrolysis and gasification process.

Modelling approach

Simulation establishment

As shown in Fig. 1, the MSW pyrolysis and gasification process is divided into the combustor outside and the pyrolysis and gasification chamber inside. The MSW (stream ①) is fed into the inside pyrolysis and gasification chamber from the top. It, then, goes through the drying, the pyrolysis, and the gasification stage consequently. First, it is dried by the syngas (stream ②) generated from the pyrolysis layer and gasification layer. Then, it starts degradation due to low oxygenconcentration and high temperature. Finally, it reacts with the gasifying agent (stream ⑨). The generated syngas (stream ②) comes out from the left top of the inside chamber. Some of the syngas (stream ③) are reused to burn with combustion air (stream ④) in the combustor, providing heat to the inside chamber, while the rest of the syngas (stream ⑤) is stored for other uses. The exhaust flue gas (stream ⑥) partly (stream⑦) comes back and is reused as gasifying agent together with water vapor (stream ⑧), with the rest of the flue gas (stream ⑩) discharging into the follow-up heat recovery and or purification equipment. The ash (stream ) is removed from the bottom of the inside chamber.

The present model focuses on the reactions in the pyrolysis and gasification chamber. Wherein, the MSW experiences reactions as follows. After fed into the inside chamber, the feedstock would be dried first. Then it absorbs heat and turns into water vapor. Next, pyrolysis reactions occur quickly and release vast gaseous pyrolysis products (CO, H₂, CH₄, CO₂, H₂S, C_nH_m, etc.). The residual solid char (C) then reacts with the gasifying agent to generate gasification products (CO, H₂, etc.). Hence, the simulation model emphatically describes the three stages of drying, pyrolysis and gasification in the inside chamber.

Figure 2 shows the flowchart of the Aspen plus simulation of the MSW pyrolysis and gasification. This model includes 6 blocks of DRY, PYRO1, PYRO2, GASIFIER, SEPTOR1 and SEPTOR2, of which the built-in reactors or modules are Sep, RYeild, RGibbs, RGibbs, Sep, and Sep2, respectively.

1) The feedstock (MSW) goes into DRY to be dried first. Some of the evaporated water (STREAM1) is reused as the gasifying agent (H₂O).

2) Then, the dried DRYMSW enters PYRO1, in this RYield module, being converted into simple substances (D1-OUT) including C, H, O, N, S and ash, and specifies the yield distribution according to the MSW proximate analysis and ultimate analysis. These substances (D1-OUT) enter PYRO2 of the RGibbs module to participate in reactions producing pyrolysis products limited by the Gibbs free energy minimization principle. In short, the RYield and RGibbs modules are used for the pyrolysis section (PYRO1 and PYRO2) together.

3) The gasification stage is composed of high temperature reactions of pyrolysis products and gasifying agent. Therefore, the pyrolysis products (D2-OUT) enters the GASIFIER, the second RGibbs module, to react with the gasifying agent (H₂O or OTHER) to produce gasification products (PRODUCT). Finally, the gasification products (PRODUCT) are expressed by two steams of GAS and ASH by SEPTOR2 (Sep2 module).

In addition, the input gasifying agent (OTHER) could be different gasifying agents, such as flue gas, CO₂ or water vapor, etc.

Parameter settings

In Aspen plus, feedstock components are needed to be input into the model first. The built-up components are only divided into the conventional and the unconventional ones. The conventional components are limited, like C, H₂, O₂, N₂, S, CO, CO₂, CH₄, H₂O, C₂H₄, C₂H₆, H₂S, etc. Therefore, MSW and ASH have to be defined as nonconventional components. Since nonconventional components cannot directly participate in the reaction in the built-up modules, MSW are converted into its constituting components in the RYield module including C, H, O, N, S and ash.

Property calculations are also needed in the calculating process of all unit operation models. In software database, corresponding to different reaction conditions, there are different calculation methods, called constitutive equations. The Redlich-Kwong-Soave (RKS) cubic equation is selected to calculate the thermodynamic properties. This method is particularly suitable for high temperature reaction processes, such as hydrocarbon processing, supercritical extraction, and MSW pyrolysis and gasification etc.

The average values of 11 major Chinese cities in recent years are used as the characteristic data of MSW (Table 1). The main input parameters are listed in Table 2.

Gasification reactions input into the GASIFIER module are mainly

(1)\mathrm{C}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\rightleftarrows \mathrm{C}\mathrm{O+}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}+\mathrm{131}\hspace{0.17em}\mathrm{kJ}/\mathrm{mol},

(2)\mathrm{C}+{{\displaystyle \mathrm{C}\mathrm{O}}}_{\mathrm{2}}\rightleftarrows \mathrm{2}\mathrm{C}\mathrm{O}+\mathrm{172}\hspace{0.17em}\mathrm{kJ}/\mathrm{mol},

(3)\mathrm{C}+{{\displaystyle \mathrm{2}\mathrm{H}}}_{\mathrm{2}}\rightleftarrows {{\displaystyle \mathrm{CH}}}_{\mathrm{4}}-\mathrm{74.8}\hspace{0.17em}\mathrm{kJ}/\mathrm{mol},

(4)\mathrm{C}\mathrm{O}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\rightleftarrows {{\displaystyle \mathrm{H}}}_{\mathrm{2}}+{{\displaystyle \mathrm{C}\mathrm{O}}}_{\mathrm{2}}-\mathrm{41.2}\hspace{0.17em}\mathrm{kJ}/\mathrm{mol},

(5){{\displaystyle \mathrm{CH}}}_{\mathrm{4}}+{{\displaystyle \mathrm{H}}}_{\mathrm{2}}\mathrm{O}\rightleftarrows \mathrm{CO+3}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}+\mathrm{206}\hspace{0.17em}\mathrm{kJ}/\mathrm{mol},

(6){{\displaystyle \mathrm{CH}}}_{\mathrm{4}}+{{\displaystyle \mathrm{2}\mathrm{H}}}_{\mathrm{2}}\mathrm{O}\rightleftarrows {{\displaystyle \mathrm{CO}}}_{\mathrm{4}}+\mathrm{4}{{\displaystyle \mathrm{H}}}_{\mathrm{2}}+\mathrm{165}\hspace{0.17em}\mathrm{kJ}/\mathrm{mol}\mathrm{.}

Assumptions

All reactors are at steady-state operation in which the pressure is uniform and there is no heat loss.

The reaction takes place instantaneously and achieves an equilibrium state quickly. The temperature of gas and solid phases is non-gradient at the same time.

The MSW particles are of uniform size and temperature. The ash does not participate in any reaction. It is discharged as a part of the slag.

The main elements contained in the MSW are C, H, O, N, S, in which H, O, N, S are completely converted to gas phase. Most of the C is transformed into gas phase, and the rest of the C is transformed into residual carbon as the slag to be discharged.

All gaseous components are ideal gas. The influence of tar is not considered.

Model validation

The experimental data of the pyrolysis section are taken from Ref. [¹³], and the details of the experiment are described as follows. The MSW which has been screened and dehydration preliminary is chosen as the feedstock in this experiment. The MSW is pulverized to a particle size less than 1 cm before it is fed into the pyrolysis chamber, and the temperature in the chamber is increased to 800°C immediately. Then, the proportion of the collected combustible gas is analyzed by using the Austenite gas analyzer. The experimental data of the gasification section are taken from Ref. [¹⁴] in which the MSW sample is a mixture of five different components, that is, kitchen garbage, paper, textile, wood, and plastic. First, the MSW is dried at 105°C for 4 h and shredded into particles in sizes of approximately 5 mm. Then, it is mixed adequately before it is fed into the pyrolysis and the gasification chamber which is heated at a controlled temperature of 900°C. Water vapor is chosen as the gasifying agent, and dolomite as the catalyst. The comparative results of simulated and experimental data are listed in Fig. 3. The acceptable results illustrate the reliability and feasibility of the proposed model.

Results and discussion

First, the related terms used in the discussion are explained as follows:

The LHV (kJ/m³) of syngas is described by Eq. (7).

(7)\mathrm{L}\mathrm{H}\mathrm{V}={{\displaystyle q}}_{\mathrm{C}\mathrm{O}}{{\displaystyle V}}_{\mathrm{C}\mathrm{O}}+{{\displaystyle q}}_{{{\displaystyle \mathrm{H}}}_{\mathrm{2}}}{{\displaystyle V}}_{{{\displaystyle \mathrm{H}}}_{\mathrm{2}}}+{{\displaystyle q}}_{{{\displaystyle \mathrm{C}\mathrm{H}}}_{\mathrm{4}}}{{\displaystyle V}}_{{{\displaystyle \mathrm{C}\mathrm{H}}}_{\mathrm{4}}}+{{\displaystyle q}}_{{{\displaystyle \mathrm{C}}}_{\mathrm{2}}{{\displaystyle \mathrm{H}}}_{\mathrm{4}}}{{\displaystyle V}}_{{{\displaystyle \mathrm{C}}}_{\mathrm{2}}{{\displaystyle \mathrm{H}}}_{\mathrm{4}}}+{{\displaystyle q}}_{{{\displaystyle \mathrm{C}}}_{\mathrm{2}}{{\displaystyle \mathrm{H}}}_{\mathrm{6}}}{{\displaystyle V}}_{{{\displaystyle \mathrm{C}}}_{\mathrm{2}}{{\displaystyle \mathrm{H}}}_{\mathrm{6}}},

where q (kJ/m³) is the heating value of each component and V (%) is the volume fraction of each component.

The carbon conversion rate can reflect the gasification reaction extent. The definition of the carbon conversion rate is expressed as

(8)\mathrm{Carbon}\hspace{0.17em}\mathrm{conversion}\hspace{0.17em}\mathrm{rate=}{\displaystyle \frac{{{\displaystyle m}}_{\mathrm{1}}}{{{\displaystyle m}}_{\mathrm{2}}}}\times \mathrm{100}\%,

where m₁ (kg) is the carbon content in syngas and m₂ (kg) is the carbon content in the MSW.

In practical application, the moisture contents of the MSW are quite different over time and sites. The ratio of gasifying agent (\alpha) is defined as

(9)\alpha ={\displaystyle \frac{{{\displaystyle m}}_{\mathrm{q}}}{{{\displaystyle m}}_{\mathrm{gl}}}},

where m_q (kg/h) is the amount of gasifying agent and m_gl (kg/h) is the amount of dry MSW.

Effect of gasification temperature on the simulation results

Assuming that the gasifying agent is water vapor with a ratio of 0.4, and the gasification temperature varies from 550°C to 1000°C, as shown in Fig. 4, the proportion of H₂ and CO in syngas gradually increases with the increase in gasification temperature, while the proportion of CH₄, H₂O and CO₂ decreases, and the N₂ component remains almost unchanged. When the gasification temperature reaches 800°C, each component becomes stable. The impact of temperature on the chemical reaction equilibrium follows the Le Chatelier’s principle. That is to say, the increase in temperature could promote the endothermic reaction and prevent the exothermic reaction. H₂ and CO are mainly produced by gasification reactions (1) and (2) which are both the endothermic reactions. Meanwhile, H₂O and CO₂ are the reactants of reactions (1) and (2), so their proportions decrease. CH₄ is mainly produced by gasification reactions (3) and (5) which are both exothermic reactions. Therefore, the proportion of CH₄ decreases. As N₂ does not participate in the reactions, its proportion remains constant.

Figure 5 displays the effect of gasification temperature on LHV and carbon conversion rate. As can be seen in Fig. 5, LHV increases at 600°C, which means that the gasification reaction starts to occur. Temperature increasing can promote the generation of gasification products (CO and H₂) of reactions (1) and (2) [¹⁵], which directly decides the LHV of the syngas. When the temperature reaches 800°C, the gasification reaction tends to be completed, and the LHV becomes stable at about 2700 kcal/m³. After that, the temperature has a little effect on the LHV. On the other hand, the carbon conversion rate increases gradually and becomes stable, close to 99%, at about 800°C.

In summary, when the gasification temperature reaches 800 °C, the ratio of the syngas component, the LHV, and the carbon conversion rate remains almost stable. Considering the economic benefits, the gasification temperature should be set at about 750°C in industrial applications.

Effect of ratio of gasifying agent on simulation results

In the simulation, it is assumed that the gasification temperature is 800°C, the gasifying agent is water vapour whose ratio changes from 0.1 to 1. As shown in Fig. 6, with the gasifying agent ratio increasing, the yield of H₂ increases gradually. CO increases quickly until the ratio of the gasifying agent is 0.4. Then it increases slowly and starts to decrease when the ratio of the gasifying agent is 0.5. CO₂ and H₂O increases when the ratio of the gasifying agent is higher than 0.4, too. However, N₂ and CH₄ stay stable from the beginning to the end. The injection of water vapor mainly influences the water gas reaction (reaction (1)). Therefore, the yields of CO and H₂ increases greatly, and other compositions change slightly. When the ratio of gasifying agent is higher than 0.4, excess water vapor reacts with CO to generate CO₂ and H₂ (reaction (1)). Therefore, the yield of CO₂ increases significantly and CO decreases.

The effect of the ratio of gasifying agent on LHV is shown in Fig. 7. When the ratio of water vapor is 0.1, the H₂ volume concentration (v_c) has the highest value of 63.46%, with CO occupying 29.53%, and CH₄ only 1.71%. With the ratio of gasifying agent increasing to 0.4, the volume concentration of CO increases and H₂ and CH₄ decreases relatively (v_c (%): H₂, 57.79%; CO, 35.17%; CH₄, 1.42%). Meanwhile, The LHVs of CO and H₂ are comparetively close, much less than that of CH₄. Therefore, the LHV of the syngas is apparently stable. When the ratio of gasifying agent is higher than 0.4, the LHV of syngas begins to decline quickly. Because reaction (4) donates much here, that is, the water vapor reacts with CO to generate CO₂ and H₂, incombustible component concentrations of CO₂ and excess water vapor in syngas increase drastically, resulting in a very low value of the whole LHV of syngas.

The carbon conversion rate versus the ratio of gasifying agent is also illustrated in Fig. 7. The carbon conversion rate increases with the ratio of gasifying agent increasing to about 0.5, then stays stable and close to 100%. This means that the gasification reaction already executes completely.

In conclusion, the extensive gasifying agent not only increases the operation load of related equipment, but also affects the quality of syngas. Therefore, the preferred optimum ratio of gasifying agent is 0.4 in designing the system scheme.

Effect of types of gasifying agent on simulation results

The types of gasifying agent have a significant influence on syngas characteristics. Therefore, four different frequently-used gasifying agents are studied by using the proposed model and Aspen plus. In Case 1, water vapour is used. In Case 2, CO₂ is used. In Case 3, water vapor and CO₂ (weight percentage: 45, 55 respectively) are used. In Case 4, water vapor and flue gas are used, in which the water vapor concentration is 20%, and flue gas (weight percentage: CO₂, 30%; N₂, 70%) is 80%. The ratio of gasifying agent in each simulation mode is 0.4, and gasification temperature is 800°C.

The simulation results of LHV and the total heat involved in syngas are tabulated in Table 3. The syngas from Case 1 has the highest total heat, that from Case 3 takes the second place, and that from Case 3 ranks the third. This indicates that the water vapor used as the gasifying agent has a better gasification result than CO₂. While the LHV of syngas from the above three cases are almost the same. Therefore, water vapor is chosen to mix with smoke as gasifying agent in Case 4. It is worth mentioning that the total heat of syngas in Case 4 is almost equal to that in Case 2, with the LHV slightly lower than that of other cases. In industrial applications, using water vapor as gasifying agent would lead to a waste of resource, using CO₂ as gasifying agent is costly, while reusing flue gas not only makes full use of waste combustion products in a greater extent, but also reduces environmental emissions.

Therefore, using mixture of flue gas and water vapor as gasifying agent is suggested as an economical and green method.

Conclusions

In order to analyze and predict the MSW pyrolysis and gasification process in an up-draft fixed bed, a model has been established in which the RYield and RGibbs modules are used for the pyrolysis section, with RGibbs module for the gasification section.

Assuming that the gasifying agent is water vapor with a ratio of 0.4, the predicted optimum gasification temperature is about 750 °C, following the thermodynamic limit.

If the gasifying agent is water vapor, considering both the LHV of syngas and carbon conversion rate, the best ratio is around 0.4. The carbon conversion rate would reach 92.80%.

When gasification temperature is 800°C and the ratio of the gasifying agent is 0.4, four different frequently-used gasifying agent are studied (water vapor, CO₂, mixture of water vapor and CO₂ (weight percentage: 45%, 55%), and mixture of water vapor and flue gas (weight percentage: 20%, 80%)). Reusing flue gas and water vapor as gasifying agent is an economical and green method.

In future study, this model can be applied to industrial practice, and it will be used to predict the optimum gasification temperature, the ratio and type of gasifying agent. At the same time, it will be used as guidelines for follow-up studies and will provide a reliable experiment prediction.

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