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Plasma Gasification for Energy Production

Paper Type: Free Assignment Study Level: University / Undergraduate
Wordcount: 4529 words Published: 28th Oct 2021

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1 Abstract

Plasma gasification can be a practicable technology for conversion of solid waste-to-energy. Some challenges are associated with current technology which need to be overcome for plasma gasification to be successfully commercialised. Also there are some knowledge gaps due to which no proper process model is available for plasma gasification. However when compared to other conventional thermochemical process like gasification, pyrolysis and incineration, plasma gasification was proven better. Also the process is found to be environment friendly as it uses plasma for its process and the produced syngas was of good quality compared to that produced by conventional thermochemical process. CRL(Community Readiness Level) of plasma gasification is on moderate level. Many alternatives are suggested for overcoming the challenges and increasing the CRL associated with plasma gasification.

Keywords: Plasma Gasification, CRL(Community Readiness Level),Challenges and Waste-toenergy.

Table of Contents

1 Abstract

2 Introduction

3 Plamsa Gasification

4 Operating and Technological Parameters

5 Plasma Gasification Current Status

6 Challenges Associated with Plasma Gasification

7 Alternatives For Overcoming the Challenges Assocaiated with Plasma Gasification

7.1 Alternative to Reduce Capital and Operation Cost

7.2 Alternative to Increase the Moderate Community Readiness Level

7.3 Developing a Better Process and Thermodynamical Model

7.4 Limited Commercialisation Technology

8 Conclusion

9 References

List Of tables

1 Comparison of plasma gasification with other thermo-chemical process

List of Figures

2.1 (a)Solid waste production in New-Zealand (b) Average composition of solid waste sent to landfills

3.1 Plasma gasification schematic for solid waste

5.1 (a)Technology Readiness Level (b) Community Readiness level scale

6.1 Challenges associated with plasma gasification

7.4.1 Steps for commercialisation of plasma gasification technology

2 Introduction

Nowadays, a huge amount of waste is generated out of which the solid waste is generated in large quantity.According to data 1.5 billion metric tons of solid waste is generated per annum world-wide and this number is going to increase to 2.5 billion metric tons by year 2025[1].For example the generation of solid waste in New-Zealand has increased to 3.2 million tons per year (Fig-2.1 shows the increase in solid waste in past years for New-Zealand)[2].

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Landfills and different recycling techniques are employed over time to treat solid wastes.However landfills creates problem of leaching and most importantly the cost of landfills are very high, also landfills causes huge loss of lands which can be used for agriculture and other economical purposes.Thus different techniques need to be adopted to convert these solid waste into valuable energy source.

Fig-2.1:(a)Solid waste production in New-Zealand (b) Average composition of solid waste sent to landfills.[1]

There are many techniques for solid waste treatment and disposal like biological method which includes composting, aerobic and anaerobic digestion, hydrothermal method which includes wet oxidation, thermal hydrolysis, liquefaction and carbonisation, thermochemical methods which includes techniques like gasification, incineration and pyrolysis[2].All these methods have their own advantages like generation of biogas, environmental friendly, reduces the volume of waste to be treated and also conversion rate is good.But their are some major problems with this methods which are requirement of large land areas, high operation costs, safety problems as some of them deals with high temperature and pressure, moreover biological methods are relatively slow process[4].

A new method is used or more precisely is under development for treating the solid waste problems called plasma gasification.In this technique we are going to use the fourth state of matter i.e plasma. Plasma gasification is relatively new method it requires high capital cost, highly energy intensive and also the operation cost is high[6]. But the advantages of using plasma gasification are that it can help in achieving zero-waste accumulation, produce renewable fuel and also protect environment.Most importantly plasma gasification reduces the volume of waste sent to landfills.

3 Plasma Gasification

Plasma is the fourth state of matter and by electrifying the gas it breaks the atoms and molecules into their constituent ions and electrons.Plasma gasification is an extreme thermal process in which the feed or waste is processed under a very high temperature i.e 2000-14,000 oC(typical temperature of plasma).[5]

Various stages are involved in plasma gasification like handling of waste, gasification of waste using plasma, cleaning of gas and different conversion unit process.[7] A typical plasma gasification schematic is shown below in fig-2,

Fig-3.1:Plasma gasification schematic for solid waste[1]

The solid waste is fed in the gasifier after proper treating of solid waste in the first stage as shown in the Fig-3.1. In the gasifier the feed is exposed to the heat of plasma along with the oxidant which converts the solid into syngas(mixture of CO and H2) and also slag is formed which is inert in nature.Different types of reaction taking place in gasifier are as follows,(All reaction Source [1])

Oxidation reactions(more commonly Combustion reaction)-Exothermic reaction

(ΔH= -1030 MJ/Kmol)

C + 0.5O2 = CO ; ΔH=-111 MJ/Kmol …(C partial oxidation) CO + 0.5O2 = CO2 ; ΔH=-283 MJ/Kmol …(CO oxidation)

C + O2 = CO2 ; ΔH=-394 MJ/Kmol …(C oxidation)

H2 + 0.5O2 = H2O ; ΔH=-242MJ/Kmol… (H2 Oxidation )

Gasification reactions with steam-Endothermic reaction (ΔH= +296 MJ/Kmol)

C + H2O = CO + H2 ; ΔH=+131 MJ/Kmol…(Water gas reaction)

CO + H2O = CO2 + H2 ; ΔH=-41 MJ/Kmol…(Water gas shift reaction) CH4 + H2O = CO + 3H2 ; ΔH=+206 MJ/Kmol…(Water gas reforming)

Gasification reactions with hydrogen-Exothermic reaction (ΔH= -302 MJ/Kmol)

C + 2H2 = CH4 + H2 ; ΔH=-75 MJ/Kmol…(Hydro gasification)

CO + 3H2 = CH4 + H2O; ΔH=-227 MJ/Kmol…(Methanation)

Gasification reactions with Carbon dioxide -Endothermic reaction (ΔH= +172 MJ/Kmol)

C + CO2 = 2CO ; ΔH=+172 MJ/Kmol …(Boudaurd reaction)

Decomposition of tars and hydrocarbons -Endothermic reaction

pCxHy(Tars) = qCnHm + rH2 …(Hydro gasification)

CnHm(Hydrocarbons) = nC + 0.5mH2 …(Carbonization)

Plasma gasification is achieved using different types of reactions like oxidation reaction with oxygen, reaction involving steam, carbon dioxide and hydrogen under gasification reactions and also decomposition reaction of tars and hydrocarbons.The ratio of oxygen to carbon is generally kept below one so as to create deficiency of oxygen.[2] This deficiency allows the partial oxidation to take place which generates heat as the process is exothermic in nature. The heat released favours the endothermic reactions in reactor.Furthermore during the plasma gasification hydrogen and nitrogen sulphides are formed avoiding the formation of nitrogen oxide i.e NOx and sulphur dioxide SO2, Nitrogen and Sulphur are present in the feedstock already.[8]

The output of plasma gasifier consist of raw syngas along with particulate matters, this gas is further treated i.e cooled and cleaned. Raw syngas is further converted to obtain hydrogen, alcohols and synthetic fuels.[2]

4 Operating and Technological parameters

Different gases like Argon, Nitrogen, Hydrogen, Carbon monoxide and Carbon dioxide are used to generate plasma required for plasma gasification.[9]

Gasifier configuration, feeding mechanism and accumulation & collection of syngas and slag are the design characteristics of a plasma gasification process.Just like a conventional gasifier have different types of gasification process, plasma gasification can also be classified into three main process i.e plasma moving bed gasifier, plasma fixed bed gasifier and plasma entrained bed gasifier.

In plasma entrained gasifier plasma gas is fed along with the feed and the output syngas is collected from the bottom of the gasifier.Whereas in a fixed bed gasifier the feed is taken on a fixed bed generally called grate, the plasma is fed from the top and the syngas generated is removed from the top of gasifier, leaving below the slag formed and is taken out from the bottom.For a moving bed gasifier plasma is injected along with the feed from the top and the snugs generated is take out from the bottom of gasifier.

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Parameters which are important in plasma gasification are the operating temperature(around 2000-14,000oC), plasma gas flow rates, oxidant and steam streams flow rates and residence time(generally around 30min to 3hr )[9].Gasification reactions(endothermic) are enhanced more by higher reaction temperatures also it reduces the tar content in syngas(less than 10mg/Nm3) and the volume of reactor required is also small[9].Gasification yield are increased by lower plasma gas flow rates.However shorter residence time is preferred in plasma gasification as it yields more stable and simple syngas composition.

Another important parameter in plasma gasification is the source of plasma gas.More H2 gas is generated for plasma gas containing N2 , plasma gas generated from steam generates both CO and H2 .Moreover using oxidising agent increases the carbon conversion i.e more syngas is generated.Increase of heating value from 9.3 to 10.2 MJ/m3 of product gas is found on addition of steam to a plasma gasifier along with syngas yield and char conversion.Some amount of liquid fuels are also produced on increasing the steam flow rates.To avoid the condensation of steam inside the plasma gasifier temperature of outgoing syngas should be more than boiling temperature of water.[9]

5 Plasma Gasification Current Status

Any new process or technology is rated in terms of Technology Readiness Level(TRL), TRL indicates the maturity of process or technology. TRL is estimated by the concept used for process or technology, capacity or efficiency of technology or process and requirements of other technologies and raw material during the working of technology or process.[10]

Fig-5.1:(a)Technology Readiness Level (b) Community Readiness level scale [10]

Fig-5.1 shows the sale of TRL and CRL(Community Readiness Level), it is clear from the fig that on the scale of TRL plasma gasification has achieved a level-8 technology and considered to be very efficient as per TRL [10].But when it comes to CRL the maturity levels are only upto level-5 or we can say that plasma gasification has been given moderate readiness level in CRL scale.CRL is also an important factor just like TRL because CRL indicates the readiness of society and politicians to accept the technology.

Now the reasons for such moderate CRL, society is not aware about the technology completely or aware partially, secondly many have a belief that using such technology which requires such high operating conditions can be very risky and hazardous(temperature of plasma).Additionally the technology is new with very less number of prototypes, no major government standards and suspicion effects on environments. [10] All these factors make plasma gasification a moderate level CRL technology and to achieve a high level CRL both society and government should be made aware of its benefits of treating waste and generating useful products like high quality syngas and other fuels.

Another important criteria for using plasma gasification is that it should be more efficient when compare to other thermo-chemical process like gasification, incineration and pyrolysis.Plasma gasification shows more efficiency in terms of cost, service life, environmental effects, energy comparison and processing capabilities.Table shows the comparison of plasma gasification with other thermo-chemical technologies. [10]

Table-1: Comparison of plasma gasification with other thermo-chemical process [10]

When compared to other thermochemical process plasma gasification shows higher net annual revenue potential, higher net energy production potential and most importantly less environmental impacts [10].From Table-1 we can see that the net annual revenue of plasma gasification is more compared to that of traditional gasification process.

Overall plasma gasification is rated better than other thermochemical process, but there are some problems observed with plasma gasification which are comparatively expensive technology, limited capability of handling wet feed waste, shorter service life of plant, proper sorting of feed need to be done before process starts and plasma gasification requires high level of automation.

6 Challenges associated with plasma gasification

Plasma gasification when compared to other thermochemical process has proven better in terms of environmental effects, product qualities, energy comparison with other process.But for successful implementation of plasma gasification on industrial scale their are some challenges associated with plasma gasification which are need to be solved[1]. The challenges associated are given in the Fig-6.1,

Fig-6.1:Challenges associated with plasma gasification[1]

When compared to other thermochemical process like gasification and pyrolysis, plasma gasification is relatively expensive technology (Table-1).Moreover plant capacity, regional characteristics, process parameters and composition of feed to be processed are the important factors for the economy of plasma gasification. Regional characteristics involves local government regulations and local technology cost.[1]

The cost of constructing a plasma gasification plant is higher compared too that of pyrolysis and gasification plant.Approximately USD 65- 200 is required for construction of plasma gasification plant of 500tons/day capacity, whereas an amount of USD 87 and USD 80 is only required for a pyrolysis and gasification plant of same capacity respectively[2]. Plasma gasification plants are expensive because it requires high level of automation, specialised materials of construction inorder to sustain such high plasma temperature, cost of generating plasma is also high as it requires plasma torch and plasma arc which are expensive too i.e approximately more than USD 50,000.[2]

As working on plasma is not so common, so finding experts to work on plasma gasification is difficult.Moreover if the automation knowledge is not sufficient then it can create temperature fluctuations which can cause thermal shocks, affect the refractory linings of vessels and overall reduce the service life[10].Refractory linings are also affected if proper sorting of waste is not done in the initial stages as it releases highly reactive chlorine gas. [8]

Other challenges associated are maintenance of plasma source. The electrodes used in plasma source discharges frequently, so a periodic replacement is done.Also plasma sources uses expensive DC supply to run and some parts of plasma source requires to be changed within 30 days .Plasma energy breaks molecular bonds to separate complex molecules into individual gaseous phase, it is used to heat, melt and vaporise waste for molecular dissociation, all this requires large amount of DC power supply increasing the load on power grid system indirectly adding the overall cost of process.Moreover plasma gasification process being highly energy extensive process is responsible for greenhouse gas emissions and according to "Paris Agreement" these emissions should be zero by 2070.[8]

Waste sorting in plasma gasification is also one of the major concerns.Syngas production and materials used for refractory linings both are adversely affected by improper sorting of waste. [2].Syngas yield is also affected if the waste is not dried properly and the wet waste requires more energy compare to dry waste. Waste sorting removes inorganic , glass components and metal components, as this components affects the quality of syngas produced and fuels synthesised.Waste sorting is a key part of any thermochemical plant but for plasma gasification it is still under development, however sorting is achieved properly for pilot scale run of plasma gasification.[1]

All the factors like waste sorting, DC power supply, automations, cost have made plasma gasification difficult for industrial scale application.Only five locations are available worldwide where plasma gasification is accepted commercially, North America and European countries are still not able to use plasma gasification technology because of process safety concerns and high labour requirements.Another important issue faced in successful implications of plasma gasification is that their is lack of thermodynamical and fundamental data. Thermodynamics is an important concept for designing of process and its optimisation.[10] So in-order for successful commercialisation of plasma gasification all the factors need to be overcome, more specifically for implementation on industrial scale thermodynamics data need to be collected on priority basis.

7 Alternatives for overcoming the challenges associated with Plasma Gasification

Plasma gasification is very fruitful technology as it gives high quality of syngas and fuels compared to other thermochemical technologies.But high operating cost, moderate community readiness level, high energy extensive processing and other challenges seen in above section-6 makes plasma gasification difficult for commercialisation. So their are some alternatives for coping up the challenges.

7.1 Alternatives to reduce high capital and operation cost

Plasma gasification process plant, technologies and equipment need to be very sophisticated and effective when compared to other thermochemical technologies, so reducing the initial capital cost seems difficult for plasma gasification.[5]

The process can be made more cost-effective by generating revenue in different steps of process.The slag generated at the bottom of plasma gasifier reactors can be collected and used for construction purposes as it mixes easily with cement.[5] Almost pure syngas is generated through plasma gasification so high quality of gases like H2, CO, fuels and other valuable chemical compounds can be separated form syngas. Transportation cost of solid waste can be reduced by setting up plant near the place where solid waste is collected.

7.2 Alternatives to increase the moderate community readiness level

Plasma gasification needs to achieve higher CRL(community readiness level) to be perceived as a viable technology for generation of syngas by both society and end-users[1]. A proper model explaining the benefits of plasma gasification compared to other thermochemical process needs to be prepared. Also sorting of solid waste is quite complex process as it requires more mechanical work to separate out less desirable solids for feed processing like metals and glass materials, as metals and glass reduces the overall heating value of feed. So in-order to get more sorted or high heating value feed, proper recycling of waste and materials by city residents need to be done.[1]

7.3 Developing a better process and thermodynamical model

Plasma gasification process doesn't have proper understanding of thermodynamics due to which optimisation and a better process model is not available.A process model dealing with thermodynamics data, effect of mass transfer on process and chemical kinetics need to prepared.

Chemical kinetics data helps in designing of an environment friendly process i.e it will help in designing reactors configuration their size , performances, operating conditions required for fast conversion of feed to product. Also kinetics data will help in reducing the waste products generated during reactions.[10] Process models for plasma gasification can be prepared using different process simulators and numerical modelling tools. But using available process simulators is challenging because it do not includes all the data related to feed(Mixed Solid Waste). So to develop a proper process and thermodynamical model components which are unavailable can be treated as pseudo components and their basic properties can be used like viscosity, thermal conductivity, heat capacity and density.[10]

This process models will help in proper process optimisation and integrations.Also more case studies and research work on plasma gasification will help in developing better process models, Ultimately all this process models and accurate thermodynamics data will help in increasing both TRL and CRL.[1]Thus making commercialisation of plasma gasification more easy.

7.4 Limited commercialisation technology

For plasma gasification to be successfully commercialised their are five important project aspects which are need to be assessed properly(mentioned in Fig-7.4.1).[12]

1. Using literature choosing a proper plasma gasification technology from the available technologies.

2. Gross economical benefit by analysing selected plasma gasification technology.

3. Analysing TRL of selected plasma gasification technology.

4. Calculating economy of selected process for future economic benefits and risk.

5. And finally evaluating community readiness level(CRL).

Fig-7.4.1: Steps for commercialisation of plasma gasification technology [12]

8 Conclusion

Plasma gasification is a practicable technology, however there are few challenges associated with it which need to overcome for its commercialisation successfully. Challenges like the operational and capital cost, no proper thermodynamic data available for the process, optimisation problems and low CRL(Community Readiness Level). Despite being having some problems plasma gasification have many benefits i.e low environmental emissions, no leachable ash is produced compared to other thermochemical process, produces good quality of syngas, conventional gasification syngas have chances of having some trace amount of methane gas whereas in plasma gasification no methane is found in syngas composition, moreover the energy production potential of plasma gasification is more than other thermochemical process.

So, overall plasma gasification is an effective technology for converting waste into energy.There are some problems with plasma gasification which need to be removed for its successful commercialisation.

9 References

1. Munir MT, I.Mardon, S. Al-Zuhair, A.Shawabkeh, N.U. Saqib.Plasma gasification of municipal solid waste for waste-to-value processing.University of Auckland, American university & Rotorua Lakes Council 2019.

2. Munir MT, Mansouri SS, Udugama IA, Baroutian S, Gernaey KV, Young BR. Resource recovery from organic solid waste using hydrothermal processing: opportunities and challenges. Renew Sustain Energy Rev 2018.

3. Ministry for the Environment,Environmental report card: solid waste composition.Wellington: Ministry for the Environment 2009.

4. Anthraper D, McLaren J, Baroutian S, Munir MT, Young BR. Hydrothermal deconstruction of municipal solid waste for solid reduction and value production. J Clean Prod 2018.

5. Tanigaki N, Manako K, Osada M.Co-gasification of municipal solid waste and material recovery in a large-scale gasification and melting system. Waste Management 2012.

6. Lupa CJ, Wylie SR, Shaw A, Al-Shamma'a A, Sweetman AJ, Herbert BM. Experimental analysis of biomass pyrolysis using microwave-induced plasma. Fuel Process Technol 2012.

7. Arena U. Process and technological aspects of municipal solid waste gasification. A review. Waste Manage 2012.

8. Anyaoku CC, Baroutian S. Decentralized anaerobic digestion systems for increased utilization of biogas from municipal solid waste. Renew Sustain Energy Rev 2018.

9. Callegari A, Bolognesi S, Cecconet D, Capodaglio AG. Production technologies, current role, and future prospects of biofuels feedstocks: a state-of-the-art review. Crit Rev Environ Sci Technol 2019.

10. Kumar A, Samadder SR. A review on technological options of waste to energy for effective management of municipal solid waste. Waste Manag 2017.

11. Kadir SASA, Yin C-Y, Sulaiman MR, Chen X, El-Harbawi M. Incineration of municipal solid waste in Malaysia: salient issues, policies and waste-to-energy initiatives. Renew Sustain Energy Rev 2013.

12. McKay G. Dioxin characterisation, formation and minimisation during municipal solid waste (MSW) incineration. Chem Eng J 2002.


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