Predictive modelling and simulation of integrated ...

Available online at www.sciencedirect.com

Energy Procedia 00 (2016) 000–000 www.elsevier.com/locate/procedia

The 8th International Conference on Applied Energy – ICAE2016

Predictive modelling and simulation of integrated pyrolysis and anaerobic digestion process Chaudhary Awais Salmana, Sebastian Schwedea, Eva Thorina, Jinyue Yana,b, a

School of Business, Society and Engineering, Mälardalen University, PO Box 883, SE-721 23 Västerås, Sweden b School of Chemical Science and Engineering, Royal Institute of Technology, SE 100 44 Stockholm, Sweden

Abstract Anaerobic co-digestion plant with biodegradable organic feedstock separated from municipal solid waste (MSW) have become a mature technology in past decade. The biogas produced can be upgraded to bio-methane or used in heat and power applications. However, not all the municipal waste fractions such as ligno-cellulose and green waste, are suitable for biodegradation. In this work, the non-biodegradable organic waste named as green waste is investigated as a potential substrate for a bio refinery concept based on combination of pyrolysis and anaerobic digestion. The main aim of the study was to evaluate whether or not the anaerobic digestion and pyrolysis process coupling could be beneficial from an energy and exergy point of view. The simulation results shows that the integration of pyrolysis process gives approximately 59% overall efficiency as compared to the 52% for anaerobic digestion stand-alone process. The results also revealed that the pyrolysis of green waste is more beneficial than green waste incineration for heat and power production. Keywords: Pyrolysis; Anaerobic digestion; heat Integration; combined heat and power; Process simulation; Performance analysis

1. Introduction The concept of integrating different biomass conversion technologies for numerous applications such as production of heat, power and/or different fuels is gaining much attention with possible benefits of high overall energy efficiency and production of fuels. Production of bio methane in anaerobic digestion processes is considered as a mature technology. However, more knowledge is still needed for the better utilization of different fractions of waste and to overcome inhibitions and process instability in order to increase the final methane yield. A large portion of municipal solid waste contains lingo-cellulose material, which is recalcitrant to bioconversion and requires pre-treatment in order to facilitate the biodegradation of lignin rich material[1]. Options for the utilization of non-biodegradable waste includes incineration for heat and power production or other thermochemical conversion to multiple products, such as pyrolysis or gasification [2], [3]. Integration and coupling of the biological anaerobic digestion and a thermochemical process such as pyrolysis offers an alternate approach with not only the possibility of high overall energy efficiency but also the possibility of using different pyrolysis products in the digester to enhance the bio methane production [4]. The state of the art pyrolysis technology converts the lingo-cellulose biomass in the absence of oxygen at atmospheric pressure with temperatures of approximately at 450-550 oC. Fast pyrolysis normally yields the mixture of products in all phases with approximately 75 % bio oil and water, 13 % non-condensable gases (mostly CO and CO2), and 12 % bio char. The final yield and properties of pyrolysis products depends on various factors such as feed composition, moisture content, heating rate and residence time [5]. Biochar obtained from pyrolysis added to the digester can increase the production of biogas from 5% - 31% as investigated and reported in [6-8]. It is found from reviewed literature that addition of biochar shortened the lag phase, minimized the ammonia inhibition by acting as adsorbent, and possessed the alkaline behaviour which can be used to increase the methane content in biogas by enabling the in situ upgrading of biogas [9]. So far the research for the integration of pyrolysis and anaerobic digestion is limited to experimental investigations in various ways and very little attention is given to the modelling and simulation of the whole pyrolysis and anaerobic digestion integration approach to check the process feasibility in terms of energy and

2

Chaudhary Awais Salman/ Energy Procedia 00 (2016) 000–000

exergy efficiency. In this work the concept with integration of anaerobic digestion, pyrolysis and combined heat and power (CHP), where the biochar obtained from pyrolysis is added in the digester, is modelled and simulated to investigate the potential benefits in terms of bio methane increase and excess heat, power and fuel. 2. Methodology 2.1. Process overview and integration concept In this study the combustible waste named as green waste, which is not suitable for biodegradation in biogas plant, is used as the raw material to produce electricity, heat, and fuel in a pyrolysis process. The basic layout for the pyrolysis process is designed from the available data found in [12-14]. The conceptual process flow diagrams with integration of the anaerobic digestion and the pyrolysis processes are shown in Fig 1 and 2. The mass and energy balance for the process with the addition of 5% biochar, from the pyrolysis process, in the digester are simulated. The electricity and heat consumption for the biogas plant is taken from real plant data in 2009 [13][14]. Real biogas plant data has been used in this study for anaerobic digestion process. The anaerobic digestion plant description and feed data has been adapted from a wet anaerobic digestion biogas plant in Västerås, Sweden, utilizing source separated organic waste along with industrial grease, and liquid waste and ley crop silage as substrate to the plant produce approximately 15000 MWh biogas per year. A significant amount (about 25%-35% or 10000 tonnes per year in this biogas plant) of waste not suitable for digestion is sent for incineration for heat and power production, [13].

Fig 1.Pyrolysis process flow diagram. (adapted from [10], [11] )

2.2. Process modelling and simulation The green waste pyrolysis with steam recovery, and anaerobic digestion is simulated by using the simulation tool Aspen Plus® with steady state conditions [15]. The pyrolysis of green waste has been modelled on the basis of reaction kinetics adopted from [16] and simulations for the mass and energy balance has been performed. The process scheme for anaerobic digestion model is taken from Växtraft biogas plant and simulated with actual plant conditions. The amount of methane was estimated through the following expression [17]. 𝑀𝑒𝑡ℎ𝑎𝑛𝑒 𝑦𝑖𝑒𝑙𝑑 (

𝑚3 𝐶𝐻4 𝑘𝑔 𝑉𝑆

) = 0.415 𝑐𝑎𝑟𝑏𝑜ℎ𝑦𝑑𝑟𝑎𝑡𝑒𝑠 + 0.496 𝑃𝑟𝑜𝑡𝑒𝑖𝑛 + 1.014 𝐿𝑖𝑝𝑖𝑑𝑠

1


3

Where carbohydrates, protein and lipids are in percentage of source separated organic waste. The properties used for green and organic waste, key input and assumption data are described in Table 1 and Table 2. Three different cases has been simulated for the heat production to fulfil the pyrolysis heat requirements: Case I with combustion of syngas and char , Case II with combustion of bio oil and char and case III with direct combustion of green waste for heat and power production only. Sensitivity analysis has also been conducted to check the effect on thermodynamic performance by varying the pyrolysis plant capacity and by changing the initial moisture content of feed prior to pyrolysis process. Table 1. Feedstock properties for both processes.

Green waste [18], [19] Proximate analysis, % (dry) 8.18 Ash 76.11 Volatile Matter 15.71 Fixed carbon LHV MJ/kg 18.2 Organic waste [20],[21] Composition, % DM Carbohydrates 45.3 Proteins 12.5 Lipids 17.8 LHV MJ/kg 20

Ultimate analysis, % (dry) C 47.06 H 6.85 . O 37.08 N 0.83

Organic composition, % Cellulose 40.6 Hemi cellulose 16.89 Lignin 17.63

Moisture, % Dry matter TS, % VS (% TS)

75 25 66

Fig 2. Anaerobic digestion flow diagram (only grey area is simulated in the model) [13].

2.3. Pinch Analysis Though energy integration is vital for thermochemical processes to improve the overall efficiency, pyrolysis is an energy intensive process and requires certain amount of heat to fulfil the drying load and heat required for pyrolysis to run the stable process. The mass and energy balance calculations was performed taking into consideration the possibility of heat and power production using pinch analysis. Pinch analysis provide a systematic and rigorous approach for efficient energy utilization and integration [22]. Heat flow and temperature data were collected for streams that require heating, streams that require cooling, flue gases and utilities like water and steam. A composite curve representing heat flow and temperature was formed for all these streams to estimate the DTmin and pinch point for energy integration. DTmin is the minimum approach temperature in the heat

4


integration network and allows the determination of how close these hot and cold streams can be pinched without violating the law of thermodynamics. Processes and streams that required heating and cooling were identified within the model and FORTRAN subroutines were used to fulfil the heat demand of the process before utilizing the excess heat for heat and power generation. The amount of feed water in the steam turbine Rankin cycle is varied in the Aspen model in order to close the overall heat balance. 2.4. Thermodynamic performance Thermodynamic performance of the whole integrated process is estimated in terms of overall energy, exergy and chemical efficiencies defined by following equations [23]. 𝜂𝐿𝐻𝑉 =

(𝑚.Δℎ)𝑚𝑒𝑡ℎ𝑎𝑛𝑒 +(𝑚.Δℎ)𝑏𝑖𝑜𝑜𝑖𝑙 +𝑃+ 𝑄𝑇ℎ𝑒𝑟𝑚𝑎𝑙

𝜂𝐸𝑥𝑒𝑟𝑔𝑦 =

(2)

(𝑚.Δℎ)𝐺𝑊,𝑑𝑎𝑓 +(𝑚.Δℎ)𝑀𝑆𝑊,𝑑𝑎𝑓

(m.Δ𝑘)𝑚𝑒𝑡ℎ𝑎𝑛𝑒 +(𝑚.Δ𝑘)𝑏𝑖𝑜𝑜𝑖𝑙 +𝑃+ 𝐸𝑞

𝜂𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 =

(𝑚.Δ𝑘)𝐺𝑊,𝑑𝑎𝑓 +(𝑚.Δ𝑘)𝑀𝑆𝑊,𝑑𝑎𝑓 𝐸𝑞 Δℎ𝑚𝑒𝑡ℎ𝑎𝑛𝑒 1 (𝑃+ ) 𝜂𝑁𝐺𝐶𝐶 Δ𝑘𝑚𝑒𝑡ℎ𝑎𝑛𝑒 𝜂𝐻𝑃

(𝑚.Δℎ)𝑚𝑒𝑡ℎ𝑎𝑛𝑒 +(𝑚.Δℎ)𝑏𝑖𝑜𝑜𝑖𝑙 +

(𝑚.Δℎ)𝐺𝑊,𝑑𝑎𝑓 +(𝑚.Δℎ)𝑀𝑆𝑊,𝑑𝑎𝑓

(3)

(4)

Δh and Δk are the lower heating and exergy values for specific components, m is mass flow rate of respective components with subscript daf shows the dry ash free feed, P is the electric power while Eq and QThermal are heat and its exergy value. The chemical exergy values for pyrolysis products and biomass can be found in [24]. Physical exergy value of thermal streams is calculated by using the enthalpy and entropy values generated by Aspen Plus model by following relation [24] : 𝐸𝑞 = (ℎ − ℎ𝑜 ) − 𝑇𝑜 (𝑠 − 𝑠𝑜 )

(5)

Where h and s denotes specific enthalpy and subscript 0 denotes the values at reference temperature (20 °C. The overall energy and exergy efficiencies gives the physical measure of energy conversion for whole or individual processes. Chemical efficiency is calculated to assess the value of products with the prospect of their use in alternate and competing processes for conversion to other energy products or services. To calculate the chemical efficiency the net electric power and cogenerated heat is substituted to equivalent amount of methane. Power generation is represented by natural gas combined cycle, and heat by electric heat pumps, with exergy efficiency of 𝜂𝐻𝑃 = 𝜂𝑁𝐺𝐶𝐶 = 55 % for both processes [23]. Table 2. Main inputs and assumptions used in the simulation [13], [10], [11].

Unit operations Green waste Grinding Drying Pyrolysis

Description The green waste for pyrolysis Particle size reduction to < 2mm Drying of green waste from 25% initial moisture to 10% moisture content Thermochemical conversion of green waste

Key inputs and assumptions 10000 tonnes per year Initial size of 150-200 mm Air drying at 200 °C

Cyclone Condensation

Separation of bio char from vapours Vapours condensation and bio oil recovery

Combustion and heat recovery

To fulfil the heat demand of process and steam generation for heat and electricity production

Anaerobic digestion Digester feed

Process conditions adaptation from Svensk Växtraft biogas plant [13] Source separated organic waste

25000 tonnes per year

Bio char addition

5% of TS in digester

5 % increase of bio methane in biogas

At 500 °C and 1 atm with N2 as Fluidizing gas 95% efficiency Cooling of vapours to 230 °C to recover heat and then rapid condensation Case I- Syngas and char combustion Case II- Bio oil and char combustion Case III- Combustion of all green waste


5

3. Results 3.1. Base case simulation results The results for the simulation of both processes are shown in the Table 3 and 4. The fast pyrolysis at 500 °C, resulted in 51% bio oil, 17.6% bio char and 15% syngas. A large amount of heat is required to run the pyrolysis process as 0.219 MWh/MWh dry biomass (1.8 MJ/kg of feed), which is in accordance to results found in [25]. Air drying at 200 °C requires 0.043 MWh/MWh dry biomass. Regarding the effect of bio char addition on the methane production and digestion process, the simulation results shows an approximately 7% increase in efficiency of anaerobic digestion – pyrolysis combined process by assuming 5% increase in methane content. Different cases are also simulated to estimate the utilization of the various pyrolysis products (Table 4). Case: I by combusting syngas to fulfil the heat demand of process, case II: by combusting bio oil for heat demand and case III tells the utilization of green waste if it would not be subjected to pyrolysis and used in waste incineration based CHP process as it is. All three cases has been simulated and all scenarios shows high overall efficiency as compared to standalone anaerobic digestion process efficiency i.e. 52%. Pyrolysis with syngas and char combustion gives the maximum overall efficiency while with bio oil combustion more heat and electricity can be produced with the availability of syngas as a fuel which can be further upgraded to methane in final mix. High chemical efficiency, i.e. 66%, is achieved for the overall process which indicates the amount of fuel that can be utilised for different processing technologies other than electricity and district heat. Table 3. Simulation results for individual cases.

Input : Anaerobic digestion Organic waste Collection & transportation Heat demand Electrical demand

MWh 95.12 (25000 tpa*) 3.35 4.06 4.8

Output Methane

56

Anaerobic digestion standalone efficiency, %

Input: Pyrolysis Green waste Power required for grinding Heat required for drying Heat for pyrolysis Bio oil Syngas Char

MWh 103 (10000 tpa*) 4.4 6.7 16.2 50.7 26 11

52

*tpa: tonnes per year Table 4: Simulation results for all the integrated cases with 5% biochar addition to anaerobic digestion

Output, MWh Methane Power Heat Bio oil Syngas Thermodynamic performance Energy efficiency, % Exergy efficiency, % Chemical efficiency, %

Case I Syngas combustion 60 3.3 8.6 50.7 -

Case II Biooil combustion 60 7 18 26

Case III CHP 13 37 -

59 57 66

56 44 76

49 36 -

3.2. Pinch analysis and optimum heat integration The energy flows throughout the process without any steam generation are shown as composite curves in Fig 3 (a). The analysis indicates that the pyrolysis process can be pinched at 530 °C and the heat for drying and endothermal pyrolysis process must be supplied above this temperature. The hot flue gases are used to provide the heat for the pyrolysis reactor. The air for the dryer is heated from the cooling of hot vapours leaving the reactor. A significant amount of sensible heat is available below pinch point, from which 74 % of heat is recovered as electricity and heat as shown in Fig 3 (b). It is important to note that after extracting the remaining heat from the flue gases the pinch point is shifted to 92 °C, the temperature at which district heat is supplied to community. From the results it can be concluded that the base case integration design has significant potential for utility reduction and heat integration. The heat integration can also be improved by introducing another low pressure (LP)

6


turbine and extracting the steam from the steam turbine while reheating the steam before entering into the LP turbine. 3.3. Sensitivity analysis Sensitivity analysis has also been performed to check the effect of various parameters on efficiencies. The result of the sensitivity analysis is shown in figure 3. With the increase of plant scale the efficiencies of the integrated process increases, but the effect on chemical efficiency is more prominent. Which shows the possibility to produce more pyrolysis products (bio oil and syngas) that can be utilized via various pathways as shown in Fig 4(a). The efficiency declines with increased moisture content of the feedstock after drying. Also, the chemical efficiency here is affected the most because of the increased water content in the bio oil.

Fig 3. (a) Composite curve of integrated process. (b) Grand composite curve with steam turbine inclusion.

The high water content results in reduction of heating value of the bio oil. Increasing feed stock moisture content also results in increasing the heat load of the pyrolysis reactor, which eventually effects the final efficiency of the process Fig 4(b).

Fig 4. Effect on efficiency by changing the (a) pyrolysis plant scale. (b) Feedstock moisture content after drying.

4. Discussion The approach to integrate pyrolysis and anaerobic digestion is focused on the possible utilization of pyrolysis products in anaerobic digestion to increase the methane yield and/or productivity. The addition of biochar as a potential for enhancing the biomethane production is included in the simulations in this study. In a recent study by Hübner et al [4] they investigated the effect on biomethane production by digesting the aqueous fraction of bio oil obtained from pyrolysis. They found that pyrolysis conditions and organic fractions of bio oil have a large impact on overall bio methane production. Another possibility of increasing the biomethane content in the final mix of gases from the anaerobic digestion and pyrolysis process is bioconversion of CO and hydrogen to methane in the anaerobic digester. However, the process also has some constraints like solubility of CO in water, which limits the conversion, and the presence of CO also favours the growth of acetogenic and hydrogenic bacteria over methanogenic ones, which ultimately effects the final products. Also, previous studies shows the feasibility of biomethanation under suitable technical configurations. [26], [27]. 5. Conclusion The study shows the preliminary energy evaluation of a bio refinery system to convert different fractions of organic waste to power, heat, and fuel. The conversion system includes the combination of the biological processanaerobic digestion and the thermochemical process-pyrolysis. The results show a net positive energy balance with excess heat and power production. Moreover, the proposed concept also offers flexible options in terms of fuel as liquid or gaseous fuels.


7

Acknowledgements The project has been performed as a co-production study within the framework Future Energy Track 1, Renewable energy technologies, and especially the area of “New materials for bioenergy utilization with a focus on concepts and systems that use waste from human activities”. Most significantly the Knowledge Foundation in Sweden (KKS), Vafab Miljö AB and Eskilstuna Energi och Miljö should be thanked for their funding and knowledge contributions. References [1] [2] [3] [4]

[5] [6] [7] [8] [9]

[10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]

Y. Cao and A. Pawłowski, “Sewage sludge-to-energy approaches based on anaerobic digestion and pyrolysis: Brief overview and energy efficiency assessment,” Renew. Sustain. Energy Rev., vol. 16, no. 3, pp. 1657–1665, 2012. D. Fabbri and C. Torri, “Linking pyrolysis and anaerobic digestion (Py-AD) for the conversion of lignocellulosic biomass,” Curr. Opin. Biotechnol., vol. 38, no. March, pp. 167–173, 2016. T. Hübner and J. Mumme, “Integration of pyrolysis and anaerobic digestion - Use of aqueous liquor from digestate pyrolysis for biogas production,” Bioresour. Technol., vol. 183, pp. 86–92, 2015. F. Monlau, M. Francavilla, C. Sambusiti, N. Antoniou, A. Solhy, A. Libutti, A. Zabaniotou, A. Barakat, and M. Monteleone, “Toward a functional integration of anaerobic digestion and pyrolysis for a sustainable resource management. Comparison between solid-digestate and its derived pyrochar as soil amendment,” Appl. Energy, vol. 169, pp. 652–662, 2016. A. V. Bridgwater, “Review of fast pyrolysis of biomass and product upgrading,” Biomass and Bioenergy, vol. 38, pp. 68–94, 2012. D. Meyer-Kohlstock, T. Haupt, E. Heldt, N. Heldt, and E. Kraft, “Biochar as additive in biogas-production from bio-waste,” Energies, vol. 9, no. 4, 2016. J. Mumme, F. Srocke, K. Heeg, and M. Werner, “Use of biochars in anaerobic digestion,” Bioresour. Technol., vol. 164, pp. 189– 197, 2014. J. Cai, P. He, Y. Wang, L. Shao, and F. Lu , “Effects and optimization of the use of biochar in anaerobic digestion of food wastes,” Waste Manag. Res., vol. 34, no. 5, pp. 409–416, 2016. Y. Shen, J. L. Linville, M. Urgun-Demirtas, R. P. Schoene, and S. W. Snyder, “Producing pipeline-quality biomethane via anaerobic digestion of sludge amended with corn stover biochar with in-situ CO2 removal,” Appl. Energy, vol. 158, pp. 300–309, 2015. M. Ringer, V. Putsche, and J. Scahill, “Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis,” Nrel/Tp-510-37779, no. November, pp. 1–93, 2006. M. M. Wright, J. a. Satrio, R. C. Brown, D. E. Daugaard, and D. D. Hsu, “Techno-economic analysis of biomass fast pyrolysis to transportation fuels,” Fuel, vol. 89, no. November, pp. S2–S10, 2010. M. B. Shemfe, S. Gu, and P. Ranganathan, “Techno-economic performance analysis of biofuel production and miniature electric power generation from biomass fast pyrolysis and bio-oil upgrading,” Fuel, vol. 143, pp. 361–372, 2015. S. R. Esteves, “Västerås ( Växtkraft ) Biogas Plant CASE STUDY – SOURCE SEGREGATED BIOWASTES,” 2007. H. L. Li, J. Lindmark, E. Nordlander, E. Thorin, E. Dahlquist, and L. Zhao, “Using the Solid Digestate from a Wet Anaerobic Digestion Process as an Energy Resource,” Energy Technol., vol. 1, no. 1, pp. 94–101, 2013. “Aspentech.” [Online]. Available: http://www.aspentech.com/. [Accessed: 01-Jan-2016]. E. Ranzi, M. Corbetta, F. Manenti, and S. Pierucci, “Kinetic modeling of the thermal degradation and combustion of biomass,” Chem. Eng. Sci., vol. 110, pp. 2–12, 2014. C. Zhao, H. Yan, Y. Liu, Y. Huang, R. Zhang, C. Chen, and G. Liu, “Bio-energy conversion performance, biodegradability, and kinetic analysis of different fruit residues during discontinuous anaerobic digestion.,” Waste Manag., vol. 52, pp. 295–301, 2016. V. P. Bhange, S. P. M. Prince, A. N. Vaidya, and A. R. Chokhandre, “Green Waste As a Resource for Value Added Product Generation : A Review,” vol. 4, no. 1, pp. 22–33, 2012. Z. Yao, X. Ma, and Y. Lin, “Effects of hydrothermal treatment temperature and residence time on characteristics and combustion behaviors of green waste,” Appl. Therm. Eng., vol. 104, pp. 678–686, 2016. T. Freidank, S. Drescher-hartung, A. Behnsen, J. Lindmark, E. Thorin, and P. Klintenberg, “MIDTERM OUTPUT REPORT – PILOT B IN SWEDEN,” 2014. U. Sonesson and H. Jönsson, “Urban biodegradable waste amount and compostiion - Case study Uppsala.” 1996. N. R. Canada, Pinch Analysis: For the Efficient Use. 2003. M. Gassner and F. Maréchal, “Thermo-economic optimisation of the polygeneration of synthetic natural gas (SNG), power and heat from lignocellulosic biomass by gasification and methanation,” Energy Environ. Sci., vol. 5, no. 2, p. 5768, 2012. J. F. Peters, F. Petrakopoulou, and J. Dufour, “Exergy analysis of synthetic biofuel production via fast pyrolysis and hydroupgrading,” Energy, vol. 79, no. C, pp. 325–336, 2015. D. E. Daugaard and R. C. Brown, “Enthalpy for pyrolysis for several types of biomass,” Energy and Fuels, vol. 17, no. 4, pp. 934– 939, 2003. S. R. Guiot, R. Cimpoia, and G. Carayon, “Potential of wastewater-treating anaerobic granules for biomethanation of synthesis

8

[27]

Chaudhary Awais Salman/ Energy Procedia 00 (2016) 000–000 gas,” Environ. Sci. Technol., vol. 45, no. 5, pp. 2006–2012, 2011. G. Luo, W. Wang, and I. Angelidaki, “Anaerobic digestion for simultaneous sewage sludge treatment and CO biomethanation: process performance and microbial ecology.,” Environ. Sci. Technol., vol. 47, pp. 10685–93, 2013.

Chaudhary Awais Salman is a doctoral student in Mälardalen university, Sweden. He holds double master degrees in Renewable energy and Energy innovation from Royal Institute of Technology, Sweden and Universitat Politècnica de Catalunya, Spain. His research interests includes modelling and simulation of thermo and bio chemical conversion of biomass such as pyrolysis, gasification and anaerobic digestion with the main focus on ploygeneration.