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Mathematical modeling of municipal solid waste plasma gasification in a fixed-bed melting reactor
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Energy and Furnace Technology.
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increasing yield of municipal solid waste (MSW) is one of the main by-products of modern society. Among various MSW treatment methods, plasma gasification in a fixed-bed melting reactor (PGM) is a new technology, which may provide an efficient and environmental friendly solution for problems related to MSW disposals. General objectives of this work are to develop mathematical models for the PGM process, and using these models to analyze the characteristics of this new technology.

In this thesis, both experimental measurement and numerical analysis are carried out to evaluate the performance of both air gasification and air&steam gasification in a PGM reactor. Furthermore, parameter studies were launched to investigate the effect of three main operation parameters: equivalence ratio (ER), steam feedstock mass ratio(S/F) and plasma energy ratio (PER). Based on the above analysis, the optimal suggestions aiming at providing highest syngas calorific value, as well as system energy efficiency, are given.

Six experimental tests were conducted in a demonstration reactor. These tests are classified into two groups: air gasification (case 1 and 2) and air&steam gasification (case 3 to 6). In all these cases, the plasma gasification and melting of MSW produced a   syngas with a lower heating value of 6.0-7.0 MJ/Nm3. By comparing the syngas yield and calorific value, the study found out that the steam and air mixture is a better gasification agent than pure air. It is also discovered that the operation parameters seriously influence the operation of the PGM process.

A zero-dimensional kinetic free model was built up to investigate the influence of operation parameters. The model was developed using the popular process simulation software Aspen Plus. In this model, the whole plasma gasification and melting process was divided into four layers: drying, pyrolysis, char combustion&gasificaiton, and plasma melting. Mass and energy balances were considered in all layers. It was proved that the model is able to give good agreement of the syngas yield and composition. This model was used to study the influence of ER, S/F and PER on average gasification temperature, syngas composition and syngas yield. It is pointed out that a common problem for the PGM air gasification is the incomplete char conversion due to low ER value. Both increasing plasma power and feeding steam is helpful for solving this problem. The syngas quality can also be improved by reasonably feeding high temperature steam into the reactor.  

In order to provide detailed information inside the reactor, a two-dimensional steady model was developed for the PGM process. The model used the Euler-Euler multiphase approach. The mass, momentum and energy balances of both gas and solid phases are considered in this model. The model described the complex chemical and physical processes such as drying, pyrolysis, homogeneous reactions, heterogeneous char reactions and melting of the inorganic components of MSW. The rates of chemical reactions are controlled by kinetic rates and physical transport theories. The model is capable of simulating the pressure fields, temperature fields, and velocity fields of both phase, as well as variations of gas and solid composition insider the reactor. This model was used to simulate both air gasification and air&steam gasification of MSW in the PGM reactor.

For PGM air gasification, simulated results showed that when ER varies from 0.043 to 0.077, both the syngas yield and cold gas efficiency demonstrated a trend of increasing. This is explained mainly by the increase of char conversion rate with ER. However, the increase of ER was restricted by peak temperature inside the fixed-bed reactor. Therefore, it is not suggested to use only air as gasification in the PGM process. The influence of plasma power is not obvious when PER varies from 0.098 to 0.138.

 The positive influences of steam addition on cold gas efficiency and syngas lower-heating-value are confirmed by the simulation results of PGM air&steam gasification. The main effect of steam addition is the rouse of water shift reaction, which largely accelerates the char conversion and final yields of hydrogen and carbon dioxide. The effect of steam injection is affected by steam feeding rate, air feeding rate and plasma power.

Based on the above modeling work, Interactions between operation parameters were discussed. Possible operation extents of operation parameters are delimitated. The optimal points aiming at obtaining maximum syngas LHV and system CGE are suggested.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology , 2011. , xiv, 87 p.
Keyword [en]
Mathematical modeling, plasma gasification, municipal solid waste, fixed-bed
National Category
Energy Systems
Identifiers
URN: urn:nbn:se:kth:diva-47451ISBN: 978-91-7501-141-7 (print)OAI: oai:DiVA.org:kth-47451DiVA: diva2:455377
Public defence
2011-11-25, Entreplan (D2), Lindstedtsvägen 5, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Available from: 2011-11-14 Created: 2011-11-09 Last updated: 2011-11-14Bibliographically approved
List of papers
1. Gasification of municipal solid waste in the Plasma Gasification Melting process
Open this publication in new window or tab >>Gasification of municipal solid waste in the Plasma Gasification Melting process
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2011 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 90, no 1, 106-112 p.Article in journal (Refereed) Published
Abstract [en]

new waste-disposal technology named Plasma Gasification Melting (PGM) was developed. A pilot PGM reactor was constructed in northern Israel. The reactor is an updraft moving-bed gasifier, with plasma torches placed next to air nozzles to heat the incoming air to 6000 °C. The inorganic substances of the feedstock are melted by the high-temperature air to form a vitrified slag in which undesirable materials such as heavy metals are trapped. The residual heat in the air supplies additional heat for the gasification process.

A series of tests were conducted to study the performance of PGM gasification. The plasma power was varied from 2.88 to 3.12 MJ/kg of municipal solid waste (MSW), and the equivalence ratio (ER) was varied from 0.08 to 0.12. For air and steam gasification, the maximum steam/MSW mass ratio reached 0.33.

The composition of the syngas product was analyzed in all tests; the lower heating value (LHV) of the syngas varied from 6 to 7 MJ/Nm3. For air gasification, the syngas LHV decreased with increasing ER, whereas the gas yield and energy efficiency increased with ER. When high-temperature steam was fed into the reactor, the overall gas yield was increased significantly, and the syngas LHV also increased slightly. The positive effect may be attributed to the steam reforming of tar. In air and steam gasification, the influence of increased ER on syngas LHV was negative, while the effect of increased plasma power was positive. The maximum energy efficiency of the tests reached 58%. The main energy loss was due to the formation of tar.

National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-47491 (URN)10.1016/j.apenergy.2011.01.041 (DOI)000297426100017 ()2-s2.0-80055042180 (Scopus ID)
Note
QC 20111111Available from: 2011-11-11 Created: 2011-11-10 Last updated: 2017-12-08Bibliographically approved
2. Properties and optimizing of a plasma gasification & melting process of municipal solid waste
Open this publication in new window or tab >>Properties and optimizing of a plasma gasification & melting process of municipal solid waste
2010 (English)In: International Conference of Thermal Treatment Technology & Hazardous Waste Combustors, 2010, 296-316 p.Conference paper, Published paper (Refereed)
Abstract [en]

A new solid waste treatment method called Plasma Gasification & Melting (PGM) has been developed by Environmental Energy Resources Ltd. (EER). In this technology, high temperature plasma air and steam arc used to convert the waste into high-quality combustible syngas and vitreous benign slag. Due to the special features of the technology it is applicable for various stream of the solid waste field such as MSW, Medical Waste (MW) and Low Level Radioactive Waste (LLRW), where the technology was derived from. The aim of this study is to discuss the characteristics of this technology, and find out the optimal operation condition for a PGM plant. A simulation model of the PGM process was built up and validated by the test results of a PGM demonstration plant. The result shows that the syngas LCV of PGM is much higher than that of traditional gasification. For air gasification, there exists a lower limit of air/MSW mass ratio for 100% conversion of MSW. When the air/MSW mass ratio exceeds the limitation, the syngas LCV will descend by dilution of CO2 and N2. The tar yield will decrease, because of higher pyrolysis temperature. For air/steam gasification, high temperature steam as gasification agent can reduce the limitation of air/MSW mass ratio, so further enhance the syngas quality. The influence of plasma power will be more prominent for air/steam gasification than air gasification. Based on above discussion, an optimizing conception design aiming at producing syngas with high LCV and energy efficiency of a PGM process is suggested.

Keyword
Combustion, Combustors, Computer simulation, Energy efficiency, Gas generators, Gasification, Hazardous materials, Heat treatment, Industrial waste treatment, Optimization, Plasmas, Pyrolysis, Radioactive wastes, Slags, Solid wastes, Synthesis gas, Technology
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-47685 (URN)2-s2.0-77956313135 (Scopus ID)978-161738663-3 (ISBN)
Conference
International Conference on Thermal Treatment Technologies and Hazardous Waste Combustors 2010; San Francisco, CA; United States; 17 May 2010 through 20 May 2010
Note

QC 20111114

Available from: 2011-11-14 Created: 2011-11-11 Last updated: 2014-08-26Bibliographically approved
3. Eulerian Model for Municipal Solid Waste Gasification in a Fixed-Bed Plasma Gasification Melting Reactor
Open this publication in new window or tab >>Eulerian Model for Municipal Solid Waste Gasification in a Fixed-Bed Plasma Gasification Melting Reactor
2011 (English)In: Energy & Fuels, ISSN 0887-0624, E-ISSN 1520-5029, Vol. 25, no 9, 4129-4137 p.Article in journal (Refereed) Published
Abstract [en]

Plasma gasification melting (PGM) is a promising waste-to-energy process, which provides many features superior to those of conventional gasification. In this work, a steady Euler Euler multiphase model is developed to predict the performance of municipal solid waste (MSW) gasification inside a PGM reactor. The model considers the main chemical and physical processes, such as drying, pyrolysis, homogeneous reactions, heterogeneous char reactions, and melting of the inorganic components of MSW. The model is validated by one experimental test of a pilot reactor. The characteristics of PGM gasification, such as temperature distribution, syngas composition, tar yield, and energy conversion ratio (ECR, chemical energy of the gas phase divided by the total energy input), at the proposed condition are discussed. A total of nine cases are used to investigate the effects of the equivalence ratio (ER) and plasma power with a fixed flow rate of MSW. It is found that the ER has a positive effect on the cold gas efficiency of PGM gasification. However, the increase of the ER is restricted by the peak temperature. The influence of the plasma power then is not obvious for PGM gasification.

Keyword
MSW GASIFICATION, FLUIDIZED-BEDS, PACKED-BEDS, PYROLYSIS, BIOMASS, INCINERATION, COMBUSTION, FLOW, AIR
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-42373 (URN)10.1021/ef200383j (DOI)000294874800032 ()2-s2.0-80052911052 (Scopus ID)
Note
QC 20111010Available from: 2011-10-10 Created: 2011-10-10 Last updated: 2017-12-08Bibliographically approved
4. Modeling of steam plasma gasification for municipal solid waste
Open this publication in new window or tab >>Modeling of steam plasma gasification for municipal solid waste
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2013 (English)In: Fuel processing technology, ISSN 0378-3820, E-ISSN 1873-7188, Vol. 106, 546-554 p.Article in journal (Refereed) Published
Abstract [en]

Plasma gasification melting (PGM) is a promising gasification technology aiming at providing sustainable disposal for various wastes. In this work, an Euler-Euler multiphase model was developed to study the characteristics of air and steam gasification of municipal solid waste in a PGM reactor. The model is validated by measurement data from a demonstration PGM reactor. With this model, three groups of simulations were performed to study the influences of operating conditions. It is confirmed that injection of high temperature steam is important for increasing the cold gas efficiency and syngas lower-heating-value. The effect of steam injection is affected by steam feeding rate, air feeding rate and plasma power. Based on the simulated results, an optimal condition is suggested for air and steam gasification in the PGM reactor.

Keyword
Gasification, Melting, Modeling, MSW, Plasma
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-47787 (URN)10.1016/j.fuproc.2012.09.026 (DOI)000314191800072 ()2-s2.0-84870496293 (Scopus ID)
Note

QC 20130215. Updated from submitted to published.

Available from: 2011-11-14 Created: 2011-11-14 Last updated: 2017-12-08Bibliographically approved
5. Performance analysis of municipal solid waste gasification with steam in a Plasma Gasification Melting reactor
Open this publication in new window or tab >>Performance analysis of municipal solid waste gasification with steam in a Plasma Gasification Melting reactor
Show others...
2012 (English)In: Applied Energy, ISSN 0306-2619, E-ISSN 1872-9118, Vol. 98, 219-229 p.Article in journal (Refereed) Published
Abstract [en]

Plasma Gasification Melting (PGM) is a novel gasification technology which offers a promising treatment of low-heating-value fuels like municipal solid waste (MSW), medical waste (MW) and other types of waste. By considering the differences in pyrolysis characteristics between cellulosic fractions and plastics in MSW, a semi-empirical model was developed to predict the performance of the PGM process. The measured results of MSW air and steam gasification in a PGM demo-reactor are demonstrated and compared with the model predicted results. Then, the effects of dimensionless operation parameters (ER. PER, and SAMR) are discussed. It was found that all three numbers have positive effects on system cold gas efficiency (CGE). The reasons can be attributed to promoted tar cracking by enhanced heat supply. The effects of PER and ASME on syngas LHV are also positive. The influence of ER on syngas pyrolysis can be divided into two parts. When 0.04 < ER < 0.065, the effect of ER is on LHV positive; when 0.065 < ER < 0.08, the effect of ER is positive. This phenomenon was explained by two contradictory effects of ER. It is also found that interactions exist between operation parameters. For example, increasing PER narrows the possible range of ER while increasing SAMR broadens possible ER range. Detail extents for those operation parameters are demonstrated and discussed in this paper. Finally, the optimal point aiming at obtaining maximum syngas LHV and system CGE are given.

Keyword
Plasma, Gasification, MSW, Simulation, Optimizing, Steam
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-47793 (URN)10.1016/j.apenergy.2012.03.028 (DOI)000306889200023 ()2-s2.0-84862237883 (Scopus ID)
Note

QC 20120921. Updated from submitted to published.

Available from: 2011-11-14 Created: 2011-11-14 Last updated: 2017-12-08Bibliographically approved

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