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Biogas upgrading and liquefaction in an anaerobic digester plant
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73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018), 73rd Conference of the Italian Thermal Machines 12–14 September 2018, Engineering Pisa, Italy Association (ATI 2018), 12–14 September 2018, Pisa, Italy
Biogas upgrading and liquefaction in an anaerobic digester plant Biogas upgrading and aliquefaction in an anaerobic plant The 15th International Symposium anddigester Cooling a,b* on District Heating b a A. Bacciolia, L. Ferraria,b*, A. Marchionnib, U. Desideria A. Baccioli , L. Ferrari , A. Marchionni , U. Desideri
Assessing the feasibility of using the heat demand-outdoor temperature function for a long-term district heat demand forecast
a a
Department of Energy, Systems, Territory and Constructions Engineering, University of Pisa, Largo Lucio Lazzarino, Pisa 56122, Italy b National Council of Research of and ItalyConstructions CNR-ICCOMEngineering, – Via Madonna del Piano 10, 50019 Fiorentino (FI), Department of Energy, Systems, Territory University of Pisa, LargoSesto Lucio Lazzarino, PisaItaly 56122, Italy b National Council of Research of Italy CNR-ICCOM – Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy
Abstract a,b,c *, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc Abstract I. Andrić Thanks to the high energy density and to the extended range it can ensure, LNG is an attractive energy vector especially in a IN+ Center Innovation, Technology and Research - Instituto Superior Técnico, Av. Rovisco Pais 1,energy 1049-001 Portugal heavy-duty This fuel isand even moreextended interesting if it produced starting biogas because of vector theLisbon, reduced carbon Thanks totransportation. theforhigh energy density toPolicy the range can ensure, LNGfrom is an attractive especially in b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520biogas Limay, France footprint. Biogas production occurs in anaerobic digestion plants and various production strategies can be pursued to transform heavy-duty transportation. This fuel is even more interesting if produced starting from because of the reduced carbon c Département Systèmes et Environnement - IMT Atlantique, 4 rue Alfredtreatment Kastler, 44300 France has been biogas intoBiogas bio-LNG. In this studyÉnergétiques theanaerobic anaerobic digester plants of the municipal plant Viareggio footprint. production occurs in digestion and variouswastewater production strategies can Nantes, beofpursued to transform analyzed. The plant is equipped with a Capstone C600s co-generative micro gas turbine and with a boiler for the sludgehas heating. biogas into bio-LNG. In this study the anaerobic digester of the municipal wastewater treatment plant of Viareggio been Three different bio-LNG production been considered. micro In the gas firstturbine strategy baseline thesludge biogasheating. fueled analyzed. The plant is equipped with strategies a Capstonehave C600s co-generative and(the with a boilercase), for the boiler heat necessary for strategies the sludgehave heating. electric energy for wastewater treatment upgrading Three provides different the bio-LNG production beenThe considered. In therequired first strategy (the baseline case), and the for biogas fueled Abstract and liquefaction processes is bought the electric grid. the second gas turbine is operated thermal boiler provides the heat necessary for from the sludge heating. TheInelectric energystrategy, requiredthe formicro wastewater treatment and for in upgrading following mode and the electric energyfrom required by the plant is partly self-produced acquired from the grid. In and liquefaction processes is bought the electric grid. processes In the second strategy, the microand gaspartly turbine is operated in thermal District heating networks are energy commonly addressed in following theprocesses literature one self-produced of thetomost solutions decreasing the the third strategy, the micro-turbine is operated inbyelectric mode, attempting covereffective all the electric loadfor necessary to the following mode and the electric required the plant isaspartly and partly acquired from the grid. In greenhouse gasof emissions from theis building sector. These systems require high investments which are returned through the heat process part themicro-turbine heat necessary to the sludge heating. The plant wasattempting analyzed in by necessary considering the thirdand strategy, the operated in electric following mode, tosteady-state cover all theconditions electric load to the sales. Duepart tobythe changed climate conditions and building policies, demand inand thethefuture could decrease, heat requested thethe anaerobic digesters, the sludge efficiency curves ofrenovation the micro turbine and the boiler,conditions heat offprocess and of heat necessary to the heating. The plant was gas analyzed inheat steady-state by exchangers considering the prolonging the return period. design behavior. Results showed that, according to the economic theand solution with and the high profitability canoffbe heat requested byinvestment the anaerobic digesters, the efficiency curves of reference the micro scenario, gas turbine the boiler, the heat exchangers Thewith main ofofthis paper tomicro assess theturbine feasibility of using the heatfollowing demand outdoor temperature function for heat demand that thescope boiler thatshowed with is the gas in thermal design behavior. Results that, according to theoperated economic reference scenario,–mode. the solution with the high profitability can be that with theThe boiler of that the micro gas turbine operated in thermal forecast. district of with Alvalade, located in Lisbon (Portugal), wasfollowing used as amode. case study. The district is consisted of 665 ©buildings 2018 Thethat Authors. by Elsevierperiod Ltd. and typology. Three weather scenarios (low, medium, high) and three district vary Published in both construction © 2018 2018 The Authors. Published by Elsevier Elsevier Ltd. © The Authors. by Ltd. This is an open accessPublished article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an and open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines compared with results from a dynamic heat demand model, previously validated of by theItalian authors. Selection and peer-review under responsibility of the scientific committeedeveloped of the 73rdand Conference the Thermal Machines Selection andAssociation peer-review under responsibility of the scientific committee of the 73rd Conference the Italianfor Thermal Machines Engineering 2018). The results showed that(ATI when only weather change is considered, the margin of error could beof acceptable some applications Engineering Association (ATI 2018). Engineering Association (ATI 2018). (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: Biogas, bio-LNG, Micro Gas Turbine, Operation scenarios, the error valueLiquefaction, increased up to 59.5% (depending on Mode the weather and renovation scenarios combination considered). Keywords: Biogas, bio-LNG, Liquefaction, Microon Gasaverage Turbine,within Operation The value of slope coefficient increased theMode range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Corresponding author. Tel.: +39 050 221 7132; fax: +39 050 221 7333. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * E-mail address: author. [email protected] Corresponding Tel.: +39 050 221 7132; fax: +39 050 221 7333. Cooling. E-mail address: [email protected] *
1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 1876-6102 © 2018 Thearticle Authors. Published by Elsevier Ltd. Selection under responsibility of the scientific of the 73rd Conference of the Italian Thermal Machines Engineering This is an and openpeer-review access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) Association 2018). under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines Engineering Selection and(ATI peer-review Association (ATI 2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018). 10.1016/j.egypro.2018.08.154
A. Baccioli et al. / Energy Procedia 148 (2018) 655–662 Author name / Energy Procedia 00 (2018) 000–000
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1. Introduction Biogas is an attractive fuel since it has a zero-carbon footprint and can be produced from waste organic materials such as manure, organic waste from agricultural activities, sewage from water treatment plant and municipal biowaste. Sewage treatment may represent a significant contribution to the increase of biogas production [1] especially if co-digested with municipal bio-waste to increase methane concentration of the resulting biogas [2-5]. Food waste is an optimum substrate to improve the digester yields of methane, due to its high biodegradability. The produced biogas is often used as fuel for a co-generator to produce electricity and heat for the digester. Internal combustion engines (ICE) or micro gas turbines (mGT) are usually employed [6]. Un interesting development could be the conversion of biogas in liquefied natural gas (LNG). Recently, the interest towards LNG as a fuel for heavy-duty terrestrial and marine transport has increased thanks to its lower carbon content and pollutants emission in comparison to diesel oil. The high energy density of LNG in comparison to compressed natural gas guarantees ranges similar to those achieved with diesel oil. As a result, several vehicle manufacturers began to produce LNG-fueled trucks and an increasing number projects has been financed by public funding to promote LNG use. To produce bio-LNG, the biogas from anaerobic digestion should be firstly depurated and upgraded to remove corrosive compounds (H2S, Siloxanes) and to increase the content of methane. Various upgrading techniques are currently available on the market which can provide a high-quality bio-methane, with a content of CH4 which varies from 95 to 97% [7]. The liquefaction process for small-scale system is a technology which is recently available on the market but is characterized by an elevated specific energy consumption [8-10]. The production of bio-LNG highly increases the electric demand of the plant and opens the way to new opportunities in system management. In this study, the anaerobic digester of the town of Viareggio has been considered as a case study. The digester serves the wastewater treatment plant and has been recently updated to perform a co-digestion of organic municipal food. In the near future, the plant will be equipped with a Capstone C600s micro Gas-Turbine (mGT). The opportunity of installing a micro-scale liquefaction unit for bio-LNG production has been analyzed. Three different scenarios were considered to produce the electricity and the heat necessary for plant operation and sludge heating: 1) Biogas boiler provides the heat necessary to the process and electric energy is entirely bought from the national grid; 2) mGT operated according to an electric follow mode and waste heat used to cover part of the sludge heating needs 3) mGT operated according to a thermal follow mode and only part of the electricity need is covered by the cogeneration unit. The system has been analyzed from both a thermodynamic and an economic point of view. The plant performance was evaluated in four different climatic conditions of the year by considering components off-deign behavior. The configurations were compared in terms of profitability and potential reduction of CO2 emission for one year Nomenclature 𝑄𝑄̇ 𝑚𝑚̇𝑠𝑠 𝐶𝐶𝑝𝑝 𝑇𝑇𝑑𝑑 𝑇𝑇𝑖𝑖𝑖𝑖
thermal power [kW] sludge mass flow rate [kg/s] specific heat [kJ/kg/K] Digesters temperature [°C] Sludge inlet temperature [°C]
2. Case Study description
𝑈𝑈 𝐴𝐴𝑑𝑑 𝑇𝑇𝑎𝑎 𝐶𝐶 𝐴𝐴
Overall heat transfer coefficient [kW/m2/K] Digesters surface [m2] Ambient Temperature [°C] Cost [$] Capacity factor
A plant scheme with the mGTis reported in Figure 1. The plant is made up of two cylindrical anaerobic digesters with an operating volume of about 3300 m3 and 1300 m3 respectively. Before entering in the plant, the sewage is mixed with municipal bio-waste in the ratio of 4:1 and a final methane content in the biogas up to 65% is achieved. The mixture of bio-waste and sludge is diluted in water with a solid concentration of 7%. The total mass flow rate of the mixture entering in the plant is 10.8 t/h. The biogas capacity is about 276.6 kg/h [11, 12]. The digestate is currently
A. Baccioli et al. / Energy Procedia 148 (2018) 655–662 Author name / Energy Procedia 00 (2018) 000–000
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discharged and stored in a tank, but the opportunity of sludge regeneration was considered in this study. This solution can reduce the heat requested to increase the sludge temperature and keep the digester at 37°C [11-13].
Fig. 1. Scheme of the system with mGT.
3. Methodology The system has been modeled through equations in steady-state conditions, by considering, for each component the mass and energy conservation. The design ambient temperature of the system was 0°C and the design sludge temperature was 9.8°C, which are the lowest value recorded during winter condition. The system was simulated in off-design in four different days, representative of the average conditions of the various seasons in Viareggio. The average ambient and sludge temperatures, the system electric energy request (without considering upgrading and liquefaction) and the thermal power request (without considering the regenerator) are reported for each season in Table 1. Annual data are evaluated by multiplying the results of the average seasonal conditions for the number of the days of the season and summing the obtained values of each season. Table 1. Average ambient and sludge temperature in Viareggio and seasonal energy requests. Season
Air temperature [°C]
Sludge temperature [°C]
Electric Energy request [kWh/day]
Heat request [kWh/day]
Winter
7.3
10.3
4094
9069
Spring
13.2
14.6
4094
7578
Summer
22.9
24.8
4094
4393
Autumn
16.3
19.8
4094
5936
For each configuration, the avoided CO2 emissions and the profitability index (PI) were evaluated: the first index was calculated by considering the difference between the avoided emission due to the replacement of diesel oil with bio-LNG in heavy-duty vehicle and the CO2 emissions due to the production of the electric energy acquired from the grid. The emission of carbon dioxide in kg produced by the electric system was calculated by multiplying the required energy in kWh by the factor 0.325 [kg/kWh], as from [14], typical of Italian generation plants. The avoided emissions were evaluated by considering the equivalent emissions of the amount of diesel oil necessary to obtain the same heating value of bio-LNG. In other terms, bio-LNG heat content in kWh was multiplied for 0.27 [kg/kWh], according to the values reported in [15]. For the economic analysis, the profitability index was evaluated as the ratio between the actualized cash flows and the capital investment of the system.
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3.1. Digesters The heat load necessary to keep the digesters at the temperature of 37°C was evaluated considering the convective losses through the wall and the energy necessary to warm up the sludge entering in the plant (or after the regenerator if this component is present):
Q = ms C p (Td − Tin ) + U Ad (Td − Ta )
(1)
The first term represents the heat necessary to warm up the sludge and the second term is the convective heat losses to the ambient. The first term of this equation has the highest impact on the thermal energy need, especially when the sludge regeneration is not adopted, due to the high sludge mass flowrate and to the low temperature of the sludge (especially during winter). 3.2. Boiler The boiler was designed to provide the heat necessary to the two digesters. The off-design behavior was simulated by considering a typical efficiency curve from the literature. The costs of the device were evaluated from manufacturers catalogues [16]. 3.3. mGT The mGT is a Capstone C600s micro-turbine, composed by three 200 kW modules. The turbine efficiency curve as a function of the load and the ambient temperature is reported in Figure 2, as well as the variation of the exhaust gas mass flow rate and temperature. The cost of the whole turbine was provided by the plant manager and was equal to 1.1 million of Euro. The total cost was divided by three to obtain the order of magnitude of the cost of the single 200 kW module.
Fig. 2. Capstone C600s m-GT Efficiency curve (left) and efficiency and power variation with the temperature (right).
3.4. Heat Exchanger and sludge regenerator All the heat exchangers of the systems, including the sludge regenerator were designed and modeled in off-design conditions through a ε-NTU approach. The average heat exchanger coefficients were taken from [17] and were considered as constants also in off-design conditions. In the case of mGT, an iterative procedure was used to solve the water loop, satisfying the ε-NTU equations on both the heat exchangers (flu-gas/water and water/sludge) of the loop. The cost of the heat exchangers was evaluated using the procedure reported in [18].
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3.5. Upgrading and Liquefaction Systems Before the liquefaction process, biogas needs a depuration to remove pollutants and acid substances (H 2S and siloxanes) and an upgrading process to reduce the CO2 content and obtaining high purity biogas (methane content higher than 95%). Various upgrading and depuration techniques are nowadays commercially viable. Among the upgrading techniques, pressure swing adsorption is one of the most used process. This process requires only electrical energy to drive the compressor that increases the biogas pressure for the adsorption of the CO2. Regarding the liquefaction plant, several types of refrigeration cycles can be implemented, but the reverse Joule-Brayton cycle seems to be the most promising technology for small-scale systems [19]. Commercial units are currently available for this plant typology [20]. The specific consumption of the upgrading and liquefaction system was set equal to 0.7 kWh/Stm3 [19] and was retained constant also during the off-design characterization since the mass flow rate of biogas does not have large variation during operation respect to the design point. The cost of the system was extrapolated from a previous published work [18], and then scaled down through a power law in the form:
A = C Cref A ref
0.6
(2)
Where Cref is the reference cost of the plant with the capacity Aref from [18], and A is the actual capacity of the liquefaction system. The cost of the upgrading system was supposed to be of the same order of magnitude of the liquefaction system. 3.6. Wastewater treatment plant and sludge pre/post processing plant The wastewater treatment plant and the pre/post processing plant of the sludge are characterized by an electric consumption which can be considered constant in all the investigated ambient conditions and equal to 4.09 MWh/day. 4. Results The heat requested for the sludge heating and for keeping the internal temperature of the digesters decreased with the regenerator surface (Figure. 3). The amount of heat necessary to heat the sludge had a deep impact on all the considered strategies both on the system design and on the management, since it represents a constraint of the system. The heat was supposed to be produced from biogas, without any external contribution from fossil fuels. The maximum amount of bio-LNG was produced in the baseline case, i.e. with the boiler (Figure 4, left). With the mGT, both in electric and in thermal follow mode, the amount of bio-LNG produced was lower. With the boiler, the bio-LNG amount increased with the regenerator area: the reduction of the heat necessary to the bio-digester, led to a lower consumption of biogas from the boiler, which was therefore available for bio-LNG production. The same trend was found with the mGT in thermal follow mode. Since the turbine was controlled to keep the operating temperature inside the digester, a reduction of heat consumption led to a larger availability of biogas for upgrading and liquefaction. In the case of mGT in electric follow mode the bio-LNG production was constant, since the consumption of the turbine did not depend on the regenerator size. It is also worth noting that in the case of electric follow mode, a minimum regenerator area of about 6 m2 was necessary to obtain enough heat for the sludge. The electric consumption (Figure 4 right) was obviously maximum in the baseline case. With the mGT operating in electric follow mode the electric consumption was zero, whereas in thermal follow mode the amount of electricity bought from the grid sharply increased with the regenerator area, due to the larger amount of bio-LNG produced and to the consequent increase of electric consumption.
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Fig. 3. Annual (left) and seasonal (right) thermal power requested by the Anaerobic Digesters.
Fig. 4. Average daily produced bio-LNG (left) and acquired electric energy (right).
From the trend of bio-LNG production and electric energy purchasing, it is hard to identify the optimal strategy, since if, from one hand, the baseline case provided the highest bio-LNG production, from the other this is the strategy which requires the highest electric energy purchase. Similarly, the mGT in electric follow mode did not require any electric energy purchasing but provided less bio-LNG than that produced in all the other strategies. The evaluation of the avoided CO2 emissions (Figure 5) may provide an indication about the effectiveness of the various strategies. As stated above, the avoided CO2 emissions consider the CO2 emission for energy production and the avoided emission due to the replacement of diesel oil with bio-LNG for final users (heavy-duty vehicles). The trend of the CO2 avoided emission was the same both for the baseline case and for the mGT in thermal follow mode. The sludge regenerator has a benefic effect since avoided CO2 emissions increases with the size of this component. This trend highlights that the largest savings in CO2 emissions were obtained with large bio-LNG production, despite the highest amount of purchased electric energy. This is due to the greatest impact on CO2 emissions of the replacement of diesel oil with bio-LNG rather than of the electric energy production. For this reason, in the case of mGT operating in electric follow mode, although no electric energy purchasing was necessary, the CO2 avoided emission were lower than in the other cases. Since in this case the production of bio-LNG did not depend on the regenerator, the CO2 avoided emissions were constant with the regenerator area. The seasonal trend of bio-LNG production, acquired electric energy and CO2 avoided emission are reported in Figure 5 left in the case of a 40 m2 regenerator. The avoided emissions strongly depend on the heat requested by the system, and on the turbine efficiency. With a 40 m2 regenerator, the heat request is very low and therefore just one mGT module was operating. During winter the efficiency of the mGT was high, but the produced bio-LNG was lower with the mGT than with the boiler. In spring and autumn, the efficiency of the mGT decreases but the largest amount of bio-LNG lead to higher avoided emissions. In summer, the efficiency of the mGT was very low (the mGT module operated at very low load) and the amount of bio-LNG produced was not compensated by the produced electric energy in terms of CO2 emissions.
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These analyses did not univocally lead to the definition of an optimal bio-LNG production strategy and therefore an economic analysis was performed. The profitability indexes of the various cases as a function of the bio-LNG and electricity cost were estimated (Figure 6). The analysis was reported for the baseline case and for the mGT operating in thermal follow mode. The profitability indexes for mGT operating in electric follow mode were not reported since the values were largely lower than those obtained in the other cases. The highest values of the profitability indexes were obtained in the case of large regenerator area, which maximized bio-LNG production. From the analysis of the maps of Figure 6, it is apparent that with both the boiler and the mGT, the profitability index is positive in large ranges of electricity costs and bio-LNG sale price. The boiler was more convenient in the case of high bio-LNG prices and low electric energy costs (light blue area of Figure 7), whereas the mGT resulted more convenient at high electric energy costs and low bio-LNG prices (red area of Figure 7). The boiler became competitive in comparison to the mGT operated in thermal follow mode if the selling price of bio-LNG was higher than 0.63 $/kg and for electricity costs lower than 0.12 $/kWh.
Fig. 5. Average daily CO2 avoided emissions
. Fig. 6. Profitability index as a function of the electricity cost and of bio-LNG selling price for the baseline case (left) and for mGT operating in thermal follow mode (right)
Fig. 7. Convenience regions of mGT and of boiler as a function of the electricity cost and of bio-LNG selling price.
5. Conclusion In this study, a preliminary analysis on the possible operation modes of an anaerobic digestion plant to produce bio-LNG was investigated. The digestion plant of Viareggio was considered as a reference. Three different operation modes have been considered:
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1) a biogas fueled boiler provides the heat necessary to the digesters and the entire electric need is acquired from the grid; 2) a mGT operating in electric follow mode satisfies the electric energy need and part of the thermal one; 3) a mGT operating in thermal follow mode covers the entire thermal need and part of the electric one. The system was analyzed in off-design conditions over one year by considering as reference four characteristic days, one for each season. The bio-LNG production strongly depends on the operation mode and in the first and third case also on the size of the sludge regenerator: bio-LNG production largely improves with the size of this device. The potential avoided CO2 emissions are maximum (and practically the same) in the case of using the boiler or the mGT operating in thermal follow mode. The economic analysis shows that mGT is more suitable than the boiler for high electric energy costs, whereas the boiler could be convenient only if the bio-LNG selling price was higher than 0.63 $/kg and the cost of electric energy lower than 0.12 $/kWh. Acknowledgements The research is part of the project BIO2ENERGY funded by MIUR-Regione Toscana DGRT 1208/2012 and MIUR-MISE-Regione Toscana DGRT 758/2013 PAR FAS 2007-2013- Linea d'Azione 1.1 in sub-programme FARFAS 2014. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
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73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018), 73rd Conference of the Italian Thermal Machines 12–14 September 2018, Engineering Pisa, Italy Association (ATI 2018), 12–14 September 2018, Pisa, Italy
Biogas upgrading and liquefaction in an anaerobic digester plant Biogas upgrading and aliquefaction in an anaerobic plant The 15th International Symposium anddigester Cooling a,b* on District Heating b a A. Bacciolia, L. Ferraria,b*, A. Marchionnib, U. Desideria A. Baccioli , L. Ferrari , A. Marchionni , U. Desideri
Assessing the feasibility of using the heat demand-outdoor temperature function for a long-term district heat demand forecast
a a
Department of Energy, Systems, Territory and Constructions Engineering, University of Pisa, Largo Lucio Lazzarino, Pisa 56122, Italy b National Council of Research of and ItalyConstructions CNR-ICCOMEngineering, – Via Madonna del Piano 10, 50019 Fiorentino (FI), Department of Energy, Systems, Territory University of Pisa, LargoSesto Lucio Lazzarino, PisaItaly 56122, Italy b National Council of Research of Italy CNR-ICCOM – Via Madonna del Piano 10, 50019 Sesto Fiorentino (FI), Italy
Abstract a,b,c *, A. Pinaa, P. Ferrãoa, J. Fournierb., B. Lacarrièrec, O. Le Correc Abstract I. Andrić Thanks to the high energy density and to the extended range it can ensure, LNG is an attractive energy vector especially in a IN+ Center Innovation, Technology and Research - Instituto Superior Técnico, Av. Rovisco Pais 1,energy 1049-001 Portugal heavy-duty This fuel isand even moreextended interesting if it produced starting biogas because of vector theLisbon, reduced carbon Thanks totransportation. theforhigh energy density toPolicy the range can ensure, LNGfrom is an attractive especially in b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520biogas Limay, France footprint. Biogas production occurs in anaerobic digestion plants and various production strategies can be pursued to transform heavy-duty transportation. This fuel is even more interesting if produced starting from because of the reduced carbon c Département Systèmes et Environnement - IMT Atlantique, 4 rue Alfredtreatment Kastler, 44300 France has been biogas intoBiogas bio-LNG. In this studyÉnergétiques theanaerobic anaerobic digester plants of the municipal plant Viareggio footprint. production occurs in digestion and variouswastewater production strategies can Nantes, beofpursued to transform analyzed. The plant is equipped with a Capstone C600s co-generative micro gas turbine and with a boiler for the sludgehas heating. biogas into bio-LNG. In this study the anaerobic digester of the municipal wastewater treatment plant of Viareggio been Three different bio-LNG production been considered. micro In the gas firstturbine strategy baseline thesludge biogasheating. fueled analyzed. The plant is equipped with strategies a Capstonehave C600s co-generative and(the with a boilercase), for the boiler heat necessary for strategies the sludgehave heating. electric energy for wastewater treatment upgrading Three provides different the bio-LNG production beenThe considered. In therequired first strategy (the baseline case), and the for biogas fueled Abstract and liquefaction processes is bought the electric grid. the second gas turbine is operated thermal boiler provides the heat necessary for from the sludge heating. TheInelectric energystrategy, requiredthe formicro wastewater treatment and for in upgrading following mode and the electric energyfrom required by the plant is partly self-produced acquired from the grid. In and liquefaction processes is bought the electric grid. processes In the second strategy, the microand gaspartly turbine is operated in thermal District heating networks are energy commonly addressed in following theprocesses literature one self-produced of thetomost solutions decreasing the the third strategy, the micro-turbine is operated inbyelectric mode, attempting covereffective all the electric loadfor necessary to the following mode and the electric required the plant isaspartly and partly acquired from the grid. In greenhouse gasof emissions from theis building sector. These systems require high investments which are returned through the heat process part themicro-turbine heat necessary to the sludge heating. The plant wasattempting analyzed in by necessary considering the thirdand strategy, the operated in electric following mode, tosteady-state cover all theconditions electric load to the sales. Duepart tobythe changed climate conditions and building policies, demand inand thethefuture could decrease, heat requested thethe anaerobic digesters, the sludge efficiency curves ofrenovation the micro turbine and the boiler,conditions heat offprocess and of heat necessary to the heating. The plant was gas analyzed inheat steady-state by exchangers considering the prolonging the return period. design behavior. Results showed that, according to the economic theand solution with and the high profitability canoffbe heat requested byinvestment the anaerobic digesters, the efficiency curves of reference the micro scenario, gas turbine the boiler, the heat exchangers Thewith main ofofthis paper tomicro assess theturbine feasibility of using the heatfollowing demand outdoor temperature function for heat demand that thescope boiler thatshowed with is the gas in thermal design behavior. Results that, according to theoperated economic reference scenario,–mode. the solution with the high profitability can be that with theThe boiler of that the micro gas turbine operated in thermal forecast. district of with Alvalade, located in Lisbon (Portugal), wasfollowing used as amode. case study. The district is consisted of 665 ©buildings 2018 Thethat Authors. by Elsevierperiod Ltd. and typology. Three weather scenarios (low, medium, high) and three district vary Published in both construction © 2018 2018 The Authors. Published by Elsevier Elsevier Ltd. © The Authors. by Ltd. This is an open accessPublished article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) This is an and open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection peer-review under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines compared with results from a dynamic heat demand model, previously validated of by theItalian authors. Selection and peer-review under responsibility of the scientific committeedeveloped of the 73rdand Conference the Thermal Machines Selection andAssociation peer-review under responsibility of the scientific committee of the 73rd Conference the Italianfor Thermal Machines Engineering 2018). The results showed that(ATI when only weather change is considered, the margin of error could beof acceptable some applications Engineering Association (ATI 2018). Engineering Association (ATI 2018). (the error in annual demand was lower than 20% for all weather scenarios considered). However, after introducing renovation Keywords: Biogas, bio-LNG, Micro Gas Turbine, Operation scenarios, the error valueLiquefaction, increased up to 59.5% (depending on Mode the weather and renovation scenarios combination considered). Keywords: Biogas, bio-LNG, Liquefaction, Microon Gasaverage Turbine,within Operation The value of slope coefficient increased theMode range of 3.8% up to 8% per decade, that corresponds to the decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations. © 2017 The Authors. Published by Elsevier Ltd. Corresponding author. Tel.: +39 050 221 7132; fax: +39 050 221 7333. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and * E-mail address: author. [email protected] Corresponding Tel.: +39 050 221 7132; fax: +39 050 221 7333. Cooling. E-mail address: [email protected] *
1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Keywords: Heat demand; Forecast; Climate change This is an open access under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 1876-6102 © 2018 Thearticle Authors. Published by Elsevier Ltd. Selection under responsibility of the scientific of the 73rd Conference of the Italian Thermal Machines Engineering This is an and openpeer-review access article under the CC BY-NC-ND licensecommittee (https://creativecommons.org/licenses/by-nc-nd/4.0/) Association 2018). under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines Engineering Selection and(ATI peer-review Association (ATI 2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. 1876-6102 © 2018 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) Selection and peer-review under responsibility of the scientific committee of the 73rd Conference of the Italian Thermal Machines Engineering Association (ATI 2018). 10.1016/j.egypro.2018.08.154
A. Baccioli et al. / Energy Procedia 148 (2018) 655–662 Author name / Energy Procedia 00 (2018) 000–000
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1. Introduction Biogas is an attractive fuel since it has a zero-carbon footprint and can be produced from waste organic materials such as manure, organic waste from agricultural activities, sewage from water treatment plant and municipal biowaste. Sewage treatment may represent a significant contribution to the increase of biogas production [1] especially if co-digested with municipal bio-waste to increase methane concentration of the resulting biogas [2-5]. Food waste is an optimum substrate to improve the digester yields of methane, due to its high biodegradability. The produced biogas is often used as fuel for a co-generator to produce electricity and heat for the digester. Internal combustion engines (ICE) or micro gas turbines (mGT) are usually employed [6]. Un interesting development could be the conversion of biogas in liquefied natural gas (LNG). Recently, the interest towards LNG as a fuel for heavy-duty terrestrial and marine transport has increased thanks to its lower carbon content and pollutants emission in comparison to diesel oil. The high energy density of LNG in comparison to compressed natural gas guarantees ranges similar to those achieved with diesel oil. As a result, several vehicle manufacturers began to produce LNG-fueled trucks and an increasing number projects has been financed by public funding to promote LNG use. To produce bio-LNG, the biogas from anaerobic digestion should be firstly depurated and upgraded to remove corrosive compounds (H2S, Siloxanes) and to increase the content of methane. Various upgrading techniques are currently available on the market which can provide a high-quality bio-methane, with a content of CH4 which varies from 95 to 97% [7]. The liquefaction process for small-scale system is a technology which is recently available on the market but is characterized by an elevated specific energy consumption [8-10]. The production of bio-LNG highly increases the electric demand of the plant and opens the way to new opportunities in system management. In this study, the anaerobic digester of the town of Viareggio has been considered as a case study. The digester serves the wastewater treatment plant and has been recently updated to perform a co-digestion of organic municipal food. In the near future, the plant will be equipped with a Capstone C600s micro Gas-Turbine (mGT). The opportunity of installing a micro-scale liquefaction unit for bio-LNG production has been analyzed. Three different scenarios were considered to produce the electricity and the heat necessary for plant operation and sludge heating: 1) Biogas boiler provides the heat necessary to the process and electric energy is entirely bought from the national grid; 2) mGT operated according to an electric follow mode and waste heat used to cover part of the sludge heating needs 3) mGT operated according to a thermal follow mode and only part of the electricity need is covered by the cogeneration unit. The system has been analyzed from both a thermodynamic and an economic point of view. The plant performance was evaluated in four different climatic conditions of the year by considering components off-deign behavior. The configurations were compared in terms of profitability and potential reduction of CO2 emission for one year Nomenclature 𝑄𝑄̇ 𝑚𝑚̇𝑠𝑠 𝐶𝐶𝑝𝑝 𝑇𝑇𝑑𝑑 𝑇𝑇𝑖𝑖𝑖𝑖
thermal power [kW] sludge mass flow rate [kg/s] specific heat [kJ/kg/K] Digesters temperature [°C] Sludge inlet temperature [°C]
2. Case Study description
𝑈𝑈 𝐴𝐴𝑑𝑑 𝑇𝑇𝑎𝑎 𝐶𝐶 𝐴𝐴
Overall heat transfer coefficient [kW/m2/K] Digesters surface [m2] Ambient Temperature [°C] Cost [$] Capacity factor
A plant scheme with the mGTis reported in Figure 1. The plant is made up of two cylindrical anaerobic digesters with an operating volume of about 3300 m3 and 1300 m3 respectively. Before entering in the plant, the sewage is mixed with municipal bio-waste in the ratio of 4:1 and a final methane content in the biogas up to 65% is achieved. The mixture of bio-waste and sludge is diluted in water with a solid concentration of 7%. The total mass flow rate of the mixture entering in the plant is 10.8 t/h. The biogas capacity is about 276.6 kg/h [11, 12]. The digestate is currently
A. Baccioli et al. / Energy Procedia 148 (2018) 655–662 Author name / Energy Procedia 00 (2018) 000–000
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discharged and stored in a tank, but the opportunity of sludge regeneration was considered in this study. This solution can reduce the heat requested to increase the sludge temperature and keep the digester at 37°C [11-13].
Fig. 1. Scheme of the system with mGT.
3. Methodology The system has been modeled through equations in steady-state conditions, by considering, for each component the mass and energy conservation. The design ambient temperature of the system was 0°C and the design sludge temperature was 9.8°C, which are the lowest value recorded during winter condition. The system was simulated in off-design in four different days, representative of the average conditions of the various seasons in Viareggio. The average ambient and sludge temperatures, the system electric energy request (without considering upgrading and liquefaction) and the thermal power request (without considering the regenerator) are reported for each season in Table 1. Annual data are evaluated by multiplying the results of the average seasonal conditions for the number of the days of the season and summing the obtained values of each season. Table 1. Average ambient and sludge temperature in Viareggio and seasonal energy requests. Season
Air temperature [°C]
Sludge temperature [°C]
Electric Energy request [kWh/day]
Heat request [kWh/day]
Winter
7.3
10.3
4094
9069
Spring
13.2
14.6
4094
7578
Summer
22.9
24.8
4094
4393
Autumn
16.3
19.8
4094
5936
For each configuration, the avoided CO2 emissions and the profitability index (PI) were evaluated: the first index was calculated by considering the difference between the avoided emission due to the replacement of diesel oil with bio-LNG in heavy-duty vehicle and the CO2 emissions due to the production of the electric energy acquired from the grid. The emission of carbon dioxide in kg produced by the electric system was calculated by multiplying the required energy in kWh by the factor 0.325 [kg/kWh], as from [14], typical of Italian generation plants. The avoided emissions were evaluated by considering the equivalent emissions of the amount of diesel oil necessary to obtain the same heating value of bio-LNG. In other terms, bio-LNG heat content in kWh was multiplied for 0.27 [kg/kWh], according to the values reported in [15]. For the economic analysis, the profitability index was evaluated as the ratio between the actualized cash flows and the capital investment of the system.
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3.1. Digesters The heat load necessary to keep the digesters at the temperature of 37°C was evaluated considering the convective losses through the wall and the energy necessary to warm up the sludge entering in the plant (or after the regenerator if this component is present):
Q = ms C p (Td − Tin ) + U Ad (Td − Ta )
(1)
The first term represents the heat necessary to warm up the sludge and the second term is the convective heat losses to the ambient. The first term of this equation has the highest impact on the thermal energy need, especially when the sludge regeneration is not adopted, due to the high sludge mass flowrate and to the low temperature of the sludge (especially during winter). 3.2. Boiler The boiler was designed to provide the heat necessary to the two digesters. The off-design behavior was simulated by considering a typical efficiency curve from the literature. The costs of the device were evaluated from manufacturers catalogues [16]. 3.3. mGT The mGT is a Capstone C600s micro-turbine, composed by three 200 kW modules. The turbine efficiency curve as a function of the load and the ambient temperature is reported in Figure 2, as well as the variation of the exhaust gas mass flow rate and temperature. The cost of the whole turbine was provided by the plant manager and was equal to 1.1 million of Euro. The total cost was divided by three to obtain the order of magnitude of the cost of the single 200 kW module.
Fig. 2. Capstone C600s m-GT Efficiency curve (left) and efficiency and power variation with the temperature (right).
3.4. Heat Exchanger and sludge regenerator All the heat exchangers of the systems, including the sludge regenerator were designed and modeled in off-design conditions through a ε-NTU approach. The average heat exchanger coefficients were taken from [17] and were considered as constants also in off-design conditions. In the case of mGT, an iterative procedure was used to solve the water loop, satisfying the ε-NTU equations on both the heat exchangers (flu-gas/water and water/sludge) of the loop. The cost of the heat exchangers was evaluated using the procedure reported in [18].
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3.5. Upgrading and Liquefaction Systems Before the liquefaction process, biogas needs a depuration to remove pollutants and acid substances (H 2S and siloxanes) and an upgrading process to reduce the CO2 content and obtaining high purity biogas (methane content higher than 95%). Various upgrading and depuration techniques are nowadays commercially viable. Among the upgrading techniques, pressure swing adsorption is one of the most used process. This process requires only electrical energy to drive the compressor that increases the biogas pressure for the adsorption of the CO2. Regarding the liquefaction plant, several types of refrigeration cycles can be implemented, but the reverse Joule-Brayton cycle seems to be the most promising technology for small-scale systems [19]. Commercial units are currently available for this plant typology [20]. The specific consumption of the upgrading and liquefaction system was set equal to 0.7 kWh/Stm3 [19] and was retained constant also during the off-design characterization since the mass flow rate of biogas does not have large variation during operation respect to the design point. The cost of the system was extrapolated from a previous published work [18], and then scaled down through a power law in the form:
A = C Cref A ref
0.6
(2)
Where Cref is the reference cost of the plant with the capacity Aref from [18], and A is the actual capacity of the liquefaction system. The cost of the upgrading system was supposed to be of the same order of magnitude of the liquefaction system. 3.6. Wastewater treatment plant and sludge pre/post processing plant The wastewater treatment plant and the pre/post processing plant of the sludge are characterized by an electric consumption which can be considered constant in all the investigated ambient conditions and equal to 4.09 MWh/day. 4. Results The heat requested for the sludge heating and for keeping the internal temperature of the digesters decreased with the regenerator surface (Figure. 3). The amount of heat necessary to heat the sludge had a deep impact on all the considered strategies both on the system design and on the management, since it represents a constraint of the system. The heat was supposed to be produced from biogas, without any external contribution from fossil fuels. The maximum amount of bio-LNG was produced in the baseline case, i.e. with the boiler (Figure 4, left). With the mGT, both in electric and in thermal follow mode, the amount of bio-LNG produced was lower. With the boiler, the bio-LNG amount increased with the regenerator area: the reduction of the heat necessary to the bio-digester, led to a lower consumption of biogas from the boiler, which was therefore available for bio-LNG production. The same trend was found with the mGT in thermal follow mode. Since the turbine was controlled to keep the operating temperature inside the digester, a reduction of heat consumption led to a larger availability of biogas for upgrading and liquefaction. In the case of mGT in electric follow mode the bio-LNG production was constant, since the consumption of the turbine did not depend on the regenerator size. It is also worth noting that in the case of electric follow mode, a minimum regenerator area of about 6 m2 was necessary to obtain enough heat for the sludge. The electric consumption (Figure 4 right) was obviously maximum in the baseline case. With the mGT operating in electric follow mode the electric consumption was zero, whereas in thermal follow mode the amount of electricity bought from the grid sharply increased with the regenerator area, due to the larger amount of bio-LNG produced and to the consequent increase of electric consumption.
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A. Baccioli et al. / Energy Procedia 148 (2018) 655–662 Author name / Energy Procedia 00 (2018) 000–000
Fig. 3. Annual (left) and seasonal (right) thermal power requested by the Anaerobic Digesters.
Fig. 4. Average daily produced bio-LNG (left) and acquired electric energy (right).
From the trend of bio-LNG production and electric energy purchasing, it is hard to identify the optimal strategy, since if, from one hand, the baseline case provided the highest bio-LNG production, from the other this is the strategy which requires the highest electric energy purchase. Similarly, the mGT in electric follow mode did not require any electric energy purchasing but provided less bio-LNG than that produced in all the other strategies. The evaluation of the avoided CO2 emissions (Figure 5) may provide an indication about the effectiveness of the various strategies. As stated above, the avoided CO2 emissions consider the CO2 emission for energy production and the avoided emission due to the replacement of diesel oil with bio-LNG for final users (heavy-duty vehicles). The trend of the CO2 avoided emission was the same both for the baseline case and for the mGT in thermal follow mode. The sludge regenerator has a benefic effect since avoided CO2 emissions increases with the size of this component. This trend highlights that the largest savings in CO2 emissions were obtained with large bio-LNG production, despite the highest amount of purchased electric energy. This is due to the greatest impact on CO2 emissions of the replacement of diesel oil with bio-LNG rather than of the electric energy production. For this reason, in the case of mGT operating in electric follow mode, although no electric energy purchasing was necessary, the CO2 avoided emission were lower than in the other cases. Since in this case the production of bio-LNG did not depend on the regenerator, the CO2 avoided emissions were constant with the regenerator area. The seasonal trend of bio-LNG production, acquired electric energy and CO2 avoided emission are reported in Figure 5 left in the case of a 40 m2 regenerator. The avoided emissions strongly depend on the heat requested by the system, and on the turbine efficiency. With a 40 m2 regenerator, the heat request is very low and therefore just one mGT module was operating. During winter the efficiency of the mGT was high, but the produced bio-LNG was lower with the mGT than with the boiler. In spring and autumn, the efficiency of the mGT decreases but the largest amount of bio-LNG lead to higher avoided emissions. In summer, the efficiency of the mGT was very low (the mGT module operated at very low load) and the amount of bio-LNG produced was not compensated by the produced electric energy in terms of CO2 emissions.
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These analyses did not univocally lead to the definition of an optimal bio-LNG production strategy and therefore an economic analysis was performed. The profitability indexes of the various cases as a function of the bio-LNG and electricity cost were estimated (Figure 6). The analysis was reported for the baseline case and for the mGT operating in thermal follow mode. The profitability indexes for mGT operating in electric follow mode were not reported since the values were largely lower than those obtained in the other cases. The highest values of the profitability indexes were obtained in the case of large regenerator area, which maximized bio-LNG production. From the analysis of the maps of Figure 6, it is apparent that with both the boiler and the mGT, the profitability index is positive in large ranges of electricity costs and bio-LNG sale price. The boiler was more convenient in the case of high bio-LNG prices and low electric energy costs (light blue area of Figure 7), whereas the mGT resulted more convenient at high electric energy costs and low bio-LNG prices (red area of Figure 7). The boiler became competitive in comparison to the mGT operated in thermal follow mode if the selling price of bio-LNG was higher than 0.63 $/kg and for electricity costs lower than 0.12 $/kWh.
Fig. 5. Average daily CO2 avoided emissions
. Fig. 6. Profitability index as a function of the electricity cost and of bio-LNG selling price for the baseline case (left) and for mGT operating in thermal follow mode (right)
Fig. 7. Convenience regions of mGT and of boiler as a function of the electricity cost and of bio-LNG selling price.
5. Conclusion In this study, a preliminary analysis on the possible operation modes of an anaerobic digestion plant to produce bio-LNG was investigated. The digestion plant of Viareggio was considered as a reference. Three different operation modes have been considered:
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1) a biogas fueled boiler provides the heat necessary to the digesters and the entire electric need is acquired from the grid; 2) a mGT operating in electric follow mode satisfies the electric energy need and part of the thermal one; 3) a mGT operating in thermal follow mode covers the entire thermal need and part of the electric one. The system was analyzed in off-design conditions over one year by considering as reference four characteristic days, one for each season. The bio-LNG production strongly depends on the operation mode and in the first and third case also on the size of the sludge regenerator: bio-LNG production largely improves with the size of this device. The potential avoided CO2 emissions are maximum (and practically the same) in the case of using the boiler or the mGT operating in thermal follow mode. The economic analysis shows that mGT is more suitable than the boiler for high electric energy costs, whereas the boiler could be convenient only if the bio-LNG selling price was higher than 0.63 $/kg and the cost of electric energy lower than 0.12 $/kWh. Acknowledgements The research is part of the project BIO2ENERGY funded by MIUR-Regione Toscana DGRT 1208/2012 and MIUR-MISE-Regione Toscana DGRT 758/2013 PAR FAS 2007-2013- Linea d'Azione 1.1 in sub-programme FARFAS 2014. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
[12] [13] [14] [15] [16] [17] [18] [19] [20]
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