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Energy Procedia 142 Energy Procedia 00(2017) (2017)3480–3485 000–000 www.elsevier.com/locate/procedia

9th International Conference on Applied Energy, ICAE2017, 21-24 August 2017, Cardiff, UK

A comparison of energy recovery from MSW through plasma The 15th International Symposium on District Heating and Cooling gasification and entrained flow gasification Assessing feasibility of1,using the heat2, demand-outdoor Luca Mazzoni1the , Manar Almazrouei Chaouki Ghenai and Isam Janajreh1* temperature function for a long-term district heat demand forecast Khalifa University of Science and Technology, Masdar Institute, Mechanical Engineering Dept. Abu Dhabi 54224, UAE 1

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Sustainable and Renewable Energy Engineering Dept, University of Sharjah, Sharjah 27272, UAE

I. Andrića,b,c*, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc a Abstract IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal b

Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France

c Département Systèmes between Énergétiques Environnement - conversion IMT Atlantique, 4 rue Alfred Kastler,recovery 44300 Nantes, This work presents a comparison twoetthermochemical pathways for energy fromFrance waste, entrained flow, and plasma gasification. The software Aspen Plus was used to build two equilibrium models to simulate the behavior of the two gasification processes when the feedstock consists of hand sorted municipal solid waste (MSW) with medium-high heating value. The impact of various level of air oxygen enrichment is investigated by comparing side-to-side the performance of the two Abstractin terms of syngas composition, syngas lower heating value and cold gas efficiency (CGE). As the results suggest, plasma processes gasification, with a maximum CGE of 74.8% can be considered a better option than entrained flow gasification with a CGE of District heating networks are commonly addressed in the literature as one of the most effective solutions for decreasing the 71.6%. greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat to the changed climate conditions ©sales. 2017 Due The Authors. Published by Elsevier Ltd. and building renovation policies, heat demand in the future could decrease, prolonging the investment return period. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. The main scope of this paper is to assess the feasibility of using the heat demand – outdoor temperature function for heat demand forecast. Plasma The district of Alvalade, located in Lisbon was used assolid a case district is consisted of 665 Keywords: gasification; entrained flow gasification; cold(Portugal), gas efficiency; municipal waste;study. Aspen The Plus modeling. buildings that vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were with results from a dynamic heat demand model, previously developed and validated by the authors. 1.compared Introduction The results showed that when only weather change is considered, the margin of error could be acceptable for some applications (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation One of the main challenges of today’s municipalities is to find an effective solution to mitigate the problem of solid scenarios, the error value increased up to 59.5% (depending on the weather and renovation scenarios combination considered). waste disposal. When looking at theonfield of within waste the management that energythatrecovery through The value of slope coefficient increased average range of 3.8% up deals to 8% with per decade, corresponds to the thermochemical conversion, several options are available. However, the most widespread pathway is incineration, decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weatherthat and isrenovation based on scenarios mass burning of waste Someincreased Europeanforcountries, Denmark,onand considered). Oninthegrate-fired other hand,furnaces function[1]. intercept 7.8-12.7%like per Sweden, decade (depending the the Netherlands have almost a zero to landfill strategy, with aparameters consistentfor presence of waste to energy, coupled scenarios). The valuesreached suggested couldwaste be used to modify the function the scenarios considered, and improve the accuracy of heat demand estimations.

© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * Corresponding author. Tel.: +97128109130; fax: +97128109901. Cooling. E-mail address: [email protected]

Keywords: Heat demand; Forecast; Climate change 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling.

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the scientific committee of the 9th International Conference on Applied Energy. 10.1016/j.egypro.2017.12.233

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Luca Mazzoni et al. / Energy Procedia 142 (2017) 3480–3485 Mazooni, Ghenai, and Janajreh / Energy Procedia 00 (2017) 000–000

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with Denmark incinerating around 50% of its total waste [2]. Aside from incineration, pyrolysis and gasification are also applied in different configuration for waste to energy purposes [3-5]. Gasification is especially interesting, since the waste is converted into syngas (mainly CO and H2) which can then be burned in gas engines or gas turbines [2], yielding higher efficiency than incineration, or it can be converted to liquid fuels thus allowing a more flexible enduse [6].Among the different types of gasification reactors, entrained flow gasifiers, are commercially well established for coal and biomass gasification [7-10]. However, for waste to energy applications, various authors, identified plasma gasification as a superior technology [3, 11, 12]. Its distinctive features are the high-energy density and temperature of thermal plasma which yields fast reaction times and higher flexibility on the waste that it can process [12]. A commercial installation in Japan demonstrated some practical challenges and limitations of this technology [13]. Various modeling approaches can be adopted to study gasification processes. Among these, equilibrium modeling is well studied in the literature for its ability to capture the main trend in the gasifiers operation and syngas composition without being bounded to a specific design thus providing a preliminary indication of the process performance. Furthermore, its application to model entrained flow and plasma gasification is justified by their high operating temperature resulting in high reaction rates [14, 15]. Different types of entrained flow coal gasifiers are successfully modeled with equilibrium models using Aspen Plus with reported cold gas efficiencies ranging from 72% for a wet feed to 83% for a dry feed [16]. For plasma gasification of solid waste, various equilibrium models have been proposed [17-19], including one based on Aspen Plus which reported a plasma gasification efficiency of 69% [20]. A preliminary comparison between plasma and air gasification of different materials was proposed by [21]. However, no studies have compared entrained flow gasification with plasma gasification when MSW is considered as a feedstock. In this study, the two processes are compared by varying the amount of oxygen introduced into the reactor as it is one of the main parameter affecting the gasification efficiency and the syngas quality. 2. Methodology 2.1. Material characterization Typically, MSW exhibits a very high variability regarding composition, depending on the geographical location, and the local economy. This latter factor determines the amount of high heating value combustible leftovers such as plastic, paper, rubber, cloth, leather, and wood. However, a representative MSW composition in terms of proximate and ultimate analysis is taken from the literature [22] and reported in Table 1. The HHV of MSW is 17.57 MJ/kg [22] whereas the calculated lower heating value (LHV) is 16.42 MJ/kg. The considered MSW composition here considered agrees well with other literature data as well [4, 23]. Table 1. MSW proximate analysis as received (ar) and ultimate analysis on a dry and ash free basis (daf). Proximate analysis (ar)

Weight fraction (%)

Ultimate analysis (daf)

Moisture

7.56

Carbon

Weight fraction (%) 59.64

Volatile matter

53.61

Hydrogen

6.37

Fixed carbon

22.38

Nitrogen

1.50

Ash

16.45

Sulphur

0.37

Oxygen

32.12

2.2. Entrained flow gasification model An entrained flow gasification model was developed with Aspen Plus and is based on a representative Shell entrained flow gasifier operating at 40 bar [24]. It is assumed that the gasifier is adiabatic in addition to uniform temperature and perfect mixing inside the reactor. The adoption of equilibrium modeling is justified by the high operating temperature, which is set at 1,300C. As shown in Fig. 1 the waste feed is entered in a RYield reactor as stream 1 where the waste is modeled as a non-conventional solid where it is decomposed according to its ultimate analysis. The equilibrium is calculated using an RGibbs reactor which determines the product species concentration

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at the exit of the gasifier by using the Gibbs free energy minimization approach. The species considered are N2, O2, H2, CO, CH4, C2H2, CO2, H2O, HCL, H2S, COS, HCN, NH3, NO2, NO, S, SO2, SO3, Cl2, and solid carbon. For a fixed waste feed the amount of air, stream 6, and oxygen, stream 7, introduced in the reactor is calculated by a design specification to have the RGIBBS reactor operating at 1,300C. The cleaned syngas then exits the gasifier as stream 4 after separation, in block SEP1, of the inorganic fraction (stream 5).

Fig. 1. Aspen Plus flowsheet of the entrained flow gasifier’s model.

2.3. Plasma gasification model The Aspen Plus model developed for the plasma gasifier is based on [20]. As for the entrained flow gasifier, the waste is modeled as a non-conventional solid and is represented by stream 1 in Fig. 2. The heating of the waste as it enters the reactor is modeled with a couple of heat exchanger blocks, H1 and H2 and the heat stream HEAT2. The non-conventional solid material, stream 2 is then decomposed in the RYIELD reactor similarly as in the model for the entrained flow gasifier. Then stream 3 enters the separator block SEP1, where 85% of the moisture is diverted as stream 14. The decomposed solid waste is then sent as stream 4 into the HTR block, which is an RGibbs reactor simulating the high-temperature zone of the gasifier where the solid feed comes in direct contact with the thermal plasma, stream 13. The reactor assumes the same species considered for the RGIBBS reactor in the entrained gasification model. It is assumed that thermal plasma is generated with a DC non-transferred plasma torch modeled as a heat exchanger PLTORCH. This block heats up the plasma forming gas, stream 12, up to 4,000C. The plasma forming gas results from the mixing, through block MIX1 of air and oxygen, stream 10 and 11, respectively which are determined by a design specification which sets the temperature of the stream 5 to 2,500C. The inorganic fraction of the waste is separated from stream 6 as vitrified slag, through block SEP2. Then, stream 7 enters a second RGibbs reactor, LTR block, which models the low-temperature zone of the gasifier where the syngas formation is completed. The syngas exiting the LTR block as stream 8 proceed to the block MIX2 where it is mixed with the moisture, stream 14. The temperature of the syngas exiting the gasifier as stream 9 is between 1,250C and 1,300C.

Fig. 2. Aspen Plus flowsheet of the plasma gasifier’s model.

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The model has been validated using the experimental data from a recent study on the plasma gasification of refused derived fuel [25]. The syngas composition of the experimental study is compared with the one predicted by the developed model in Table 2. The syngas composition predicted by the model agrees well with the experimental one, apart from an under-prediction of the methane content, which is typical for equilibrium-based models. Table 2. Syngas composition resulting from refused derived fuel plasma gasification. Comparison between simulation and experimental. Mole fraction (dry vol%)

CO

H2

CO2

CH4

Current work

32.4

60.6

7.0

0.0

Agon et al. [25]

33.0

58.4

4.2

4.4

2.4. Oxygen ratio In this work, the amount of oxygen introduced in the entrained flow gasifier and the plasma forming gas is varied to determine its effect on the process efficiency. Thus, for convenience, the performance metrics discussed hereafter are examined for different oxygen ratios which are calculated according to Eq. 1. ω=

ṁO2 ṁO2 + ṁair

(1)

Where ṁO2 and ṁair are the mass flow rate of oxygen and air respectively.

3. Results and discussion

The two gasification processes are compared by looking at the syngas composition, syngas lower heating value (LHV) and cold gas efficiency (CGE). The syngas composition in terms of CO, H2, N2, CO2, and H2O for the entrained flow and plasma gasification as the oxygen ratio is varied is depicted in Fig. 3. It is worth noting that in the case of entrained flow gasification ω need to be at least 21% to reach the specified temperature of 1,300C in the reactor.

Fig. 3. Syngas composition comparison between entrained flow (solid lines) and plasma gasification (dashed lines).

For both gasification processes when the oxygen ratio is increased from zero to one an increasing trend is evident in the mole fractions of the combustible species H2 and CO as a result of a less diluted syngas, since the N2 mole fraction concurrently decreases. Overall, it is evident that plasma gasification produces a syngas with sensibly higher


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H2 and CO mole fractions compared to entrained flow gasification. When the oxygen ratio is one, the syngas composition in the case of plasma gasification is 51% vol. CO and 29% vol. H2 while that of entrained flow gasification is 43% vol. CO and 22% vol. H2. On the other hand, the products of combustion reactions like CO2 and H2O are higher in the case of entrained flow gasification as one would expect from an allothermal process such as entrained flow gasification. Also, N2 is higher in the case of entrained flow gasification due to a higher air requirement to reach the specified temperature in the reactor. The syngas composition directly affects its lower heating value (LHV) which is shown in Fig.4 (a) for both gasification processes as the oxygen ratio is varied from zero to one. It is evident that plasma gasification yields a syngas with a higher LHV, ranging from 7.6 MJ/kg to 10.7 MJ/kg than that of entrained flow gasification which ranges from 3.5 MJ/kg to 7.8 MJ/kg.The process efficiency was also assessed using the CGE defined as the ratio between the power output and the power input to gasifier as per Eq. 2. CGE =

ṁsyngas ∙ LHVsyngas ṁfeed ∙ LHVfeed + Ẇtorch

(2)

Where ṁsyngas and ṁfeed are the mass flow rate of the syngas and the waste feed respectively, while LHVsyngas and LHVfeed are the lower heating value of the syngas and the waste feed respectively. In the case of plasma gasification, the power required to produce the thermal plasma is also taken into consideration with the term Ẇtorch . As can be seen in Fig. 4 (b) the CGE of plasma gasification is higher than that of entrained flow gasification. It ranges from 65.9% to 74.8% for the former process and from 60.4% to 71.6% for the latter one.

Fig. 4. Comparison between entrained flow and plasma gasification for syngas LHV (a) and CGE (b).

4. Conclusion In this paper, we compared entrained flow and plasma gasification when MSW is used as a feedstock, and different levels of oxygen enrichment of air are considered. It was evident that plasma gasification performed better than entrained flow gasification, yielding a higher calorific syngas with higher mole fractions of CO and H 2. Overall, the CGE was also higher in the case of plasma gasification thus meaning that the energy required to create the thermal plasma did not negatively affect the overall process efficiency. For both processes, it is evident that increasing the oxygen ratio introduced in the reactor is highly beneficial increasing significantly all performance metrics. Acknowledgements The authors acknowledge the partial support of the Takeer Research Center (TRC) and Tadweer in Abu Dhabi.

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