Biomethane Production: Mass and Energy Balances of Alternative

Article pubs.acs.org/IECR

Cite This: Ind. Eng. Chem. Res. 2019, 58, 10951−10962

Biomethane Production: Mass and Energy Balances of Alternative Supply Chains Riccardo Bacci di Capaci, Andrea Luca Tasca,* Gabriele Pannocchia, Claudio Scali, Leonardo Tognotti, Elisabetta Brunazzi, Cristiano Nicolella, and Monica Puccini*

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Department of Civil and Industrial Engineering, University of Pisa, Largo Lucio Lazzarino 2, 561226 Pisa, Italy

ABSTRACT: Two supply chains for biomethane are here analyzed and modeled: gasification of short rotation forestry (SRF) poplar wood chips and anaerobic digestion of two species of microalgae, Chlorella vulgaris, and Nannochloropsis gaditana. Mass and energy balances are carried out along the whole systems, from the cultivation step to the stages of upgrading and injection into gas grid. A simulation model is employed to obtain gasification and upgrading parameters, energy requirements, and CO2 emission. Harvesting of microalgae requires high inputs of energy and fertilizers, while syngas upgrading is more demanding than biogas treatment, as high volumes of water and CO2 must be separated. Yields of the supply chains are very different: 7:1 biomethane:syngas ratio and 2:1 biomethane:biogas ratio have been estimated. Both of the supply chains could be optimized by using heat recovery methods. The highest removal of CO2 and steam can be attained through absorption by triethylene and monoethanolamine solution, respectively. scale.4,6 Recently, within the GoBiGas project, a 20 MW plant for biomethane production from syngas has been operative at a demonstration scale.7 The injection of bio-SNG in the gas grid allows the supplying of all of the utilities which operate in production of electricity, heat, or fuels and already use fossil natural gas. Conversely, raw syngas, not subjected to upgrading stages, is typically employed within the production site for applications of medium-low technology level, i.e., internal combustion engines for cogeneration.8−10 The proposed analysis of the whole production chain of biomethane on full scale has to be considered a novelty of the present paper. Here we assess the feasibility of poplar wood chips from Short Rotation Forestry (SRF) and two common species of microalgae as production chains of biomethane, from the agricultural operations to the final product. The main features of the analyzed biomasses are introduced in section 2; then, the studied supply chains are detailed in section 3. Mass and energy balances, including four alternative upgrading processes, have been estimated in section 4 with the aim of assessing and comparing the environmental sustainability and with the aim of identifying the most promising configuration.

1. INTRODUCTION Biomass is a renewable resource for the production of materials, chemicals, fuels, power, and heat. Energy from both forestry and agricultural crops, residual herbaceous fractions and wood processing waste, as well as the biodegradable part of urban solid waste and animal husbandry waste can be generated by combustion, gasification, anaerobic digestion, oil, or ethanol extraction. Biomethane is a gas obtained by anaerobic digestion or through thermochemical conversion of biomass and then refined by some upgrading stages. When obtained from thermochemical conversion, it is also known as bio-SNG (bio-synthetic natural gas).1 The interest toward biomethane is due to the possibility to use the physical and logistical infrastructures of the existent network for the distribution of conventional natural gas. Moreover, the production of this biofuel increases energy independence and well adheres to the European Union policies related to greenhouse gas reduction emissions.2,3 The transformation into bio-SNG allows combustion processes with lower emissions of dust and other pollutants with respect to the direct combustion of the biomass.4 The methods for obtaining biomethane are various and characterized by different levels of commercial development.5 The Energy Research Centre of The Netherlands carried out numerous campaigns of analysis and monitoring of supply chains for the production of green natural gas, as well gasification and methanation of woody biomass on experimental © 2019 American Chemical Society

Received: Revised: Accepted: Published: 10951

February 28, 2019 May 23, 2019 June 4, 2019 June 5, 2019 DOI: 10.1021/acs.iecr.9b01149 Ind. Eng. Chem. Res. 2019, 58, 10951−10962

Article

Industrial & Engineering Chemistry Research

2. POPLAR SHORT ROTATION FORESTRY AND MICROALGAE CULTIVATION 2.1. Poplar Short Rotation Forestry. Short rotation forestry (SRF) is an intensive cultivation of trees, with associated cutting, fertilization, and control of pests. Poplar wood has a limited content of ashes and microelements with respect to most herbaceous biomass. However, it has to be noted that, when biennial rotation is adopted, relevant ash content is obtained (0.5−2.5% by weight on a dry basis), due to the significant amount of cortex. Cultivation techniques adhere to two main models: European and American.11,12 The former implies high density (over 10 000 trees/ha) and biennial or triennial rotations; soil exploitation is high and productivity decreases over years. Conversely, the American model employs low density (≃1500 trees/ha) and rotation lasts 5−6 years.13,14 This approach allows larger flexibility, with the possibility of anticipating or postponing the rotations of 1−2 years according to market demands, light farming practices and easy control of weeds, production of high quality chips, and reduced exploitation of soil with the chance to extend cultivation up to 15 years.13,14 Biomass gasification is a thermochemical process, autothermal, if a fraction of the biomass is burned to reach the temperature necessary for gasification, or allothermal, when all the heat necessary for the gasification comes from the outside. Autothermal biomass gasification steps are shown in Figure 1.

scrubbing columns. Hydrogen sulfide is usually removed through absorption with hydroxide or oxide of iron, adsorption with active carbon, liquid phase oxidation, and also with chemical or physical scrubbing technologies. HCl removal is carried out by scrubber towers with NaOH which also removes H2S, or by dry techniques, where as adsorbent materials calcium oxide and sodium carbonate are used.6 Upgrading steps provide the removal of steam, CO2, and N2.5 Note that biogas upgrading techniques are similar to those used for syngas refining. Water vapor separation is possible by condensation through the gas compression at low temperatures, adsorption on alumina and silica gel, absorption on glycol solutions or on hygroscopic salts.24 CO2 separation reduces the density and enhances the HHV of the final product. The following CO2 removal and upgrading techniques are available:9,25 (1) physical absorption on scrubbing columns, by using water or polyethylene glycol;26 (2) chemical adsorption with amine solutions; (3) adsorption on activated carbon, as pressure swing adsorption (PSA) and vacuum swing adsorption (VSA);27 (4) separation with semipermeable membranes; (5) cryogenic methods. These techniques require a syngas without sulfur content, as H2S would be absorbed by the same means used for CO2 removal, generating acidic compounds which may cause corrosion issues.28 2.2. Microalgae cultivation. Microalgae are still less used than wood from SRF as primary source of biomass for biomethane chains. Microalgae are unicellular photosynthetic eukaryote organisms which can live individually or in colonies, suspended (phytoplankton) or fixed to a substrate (microfitobenthos).29 Using microalgae to produce energy leads to the following main advantages: (1) intensive growth in waters of poor quality (salted, freshwater, urban and zootechnical waste); (2) high productivity per unit of cultivated area or volume; (3) good capacity of CO2 absorption. Biodiesel can be obtained from the microalgae lipidic fraction, whereas biogas is generated by anaerobic digestion of the nonlipidic part or the entire microorganism and by codigestion with common substrates.30,31 Actually, there is a lack of standardization of cultivation and harvesting techniques to get biogas. Cultivation can be carried out in transparent photobioreactors, which is very expensive when used for large-scale productions, or in open-air raceway ponds, typically built in unused or fallow land. Low concentration of microalgae, water evaporation, and eventual poisoning of the culture by external organisms are the main issues of raceway ponds.29 Discontinuous, semicontinuous, and continuous approaches can be used for raceway ponds. Discontinuous methods bring the culture to the maximum possible concentration, and then collection of the whole biomass is carried out. Whereas, with semicontinuous methods, a suitable fraction of the biomass is periodically removed and initial conditions are restored by fresh water addition. The discontinuous approach guarantees higher purity of the phytoplankton population, but it requires very strict programming of the crops. On the other hand, a continuous approach is simple, but it tends to put crops at greater risk of pollution. The digestate is generally treated by centrifugation to separate the solid fraction from the liquid: the first is disposed of as waste or used in agriculture, while the liquid part

Figure 1. Autothermal gasification phases. Adapted from ref 18.

Gasification is commonly performed in bubbling fluidized beds (BFBs) or in circulating fluidized beds (CFBs).15−17 As the gasification agent, air, pure oxygen, carbon dioxide, or steam can be employed.18 A mixture of air (or oxygen) and steam could be recommended as gasification agent for two main reasons: (i) the easy control of the reactor, to balance exothermic oxidation reactions and endothermic reduction reactions, and (ii) enhanced content of methane in the final product (i.e., high lower heating value). However, some drawbacks of the use of air and steam, as the consequent high tar content, have been highlighted in refs 19 and 20. If air is used, N2 has to be removed during the upgrading stages. Downstream of the gasifier, tar, H2S, NH3, and HCl need to be removed from syngas, prior entering the methanation stage,10,21 which is employed to enhance the low initial content of CH4 in the syngas (1.5% in volume). Methanation is generally operated in specific reactors with nickel-based catalysts, arranged in different configurations and working at high pressures (up to 70 bar).22 Cyclones or filters (bag or ceramic type) usually provides dust removal and separation of tar is carried out by catalytic or thermal cracking reactors or with oil or water in scrubbing columns. A comparative analysis of economic and practical feasibility of the various solutions has been presented in ref 23. Ammonia is collected when the gas stream is dehumidified or removed by countercurrent 10952

DOI: 10.1021/acs.iecr.9b01149 Ind. Eng. Chem. Res. 2019, 58, 10951−10962

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Figure 2. Stages of the anaerobic digestion process.

and Nannochloropsis gaditana, have been analyzed in this paper as alternative biomasses, since they represent common examples of biomass extensively studied in the literature, for which reliable sets of data are available. Stages of the selected supply chains are detailed in section 3.

can be sent to the culture of microalgae to recover the contents of fertilizers. Cultivation stage also includes the injection of carbon dioxide into the water of cultivation to promote microalgae growth;32−35 the most interesting solutions are the recycling of the CO2 emitted along the microalgae chain and the use of CO2 provided by carbon capture and storage (CCS). The production of fertilizers implies high energy consumption, as showed by various life cycle assessment studies.32−34 Hence, it is advised to reuse and recirculate the aqueous stream coming from the processes of separation, containing nonseparated microalgae and nonabsorbed nutrients. In addition, when the cultures of algae are intended for the production of substrate to feed anaerobic digesters, nutrients can be recycled by exploiting the residue of the anaerobic digestion process, that is, the digestate. The digestate is generally treated by centrifugation to separate the solid fraction from the liquid: the first is disposed of as waste or used in agriculture, while the liquid part can be sent to the culture of microalgae to recover the contents of fertilizers. Cultivation stage also includes the injection of carbon dioxide into the water of cultivation to promote microalgae growth;32−35 the most interesting solutions are the recycle of the CO2 emitted along the microalgae chain and the use of CO2 provided by CCS. Harvesting processes include sedimentation or flotation which allow the separation of the microalgae from the cultivation medium. The obtained product has a content of dry material from 0.5 to 6%, suitable for processes involving anaerobic digestion. Biodiesel production requires secondary techniques, like mechanical squeezing systems (presses, centrifuges), to increase dry matter content of the material up to 20%. Cationic flocculation can also be implemented.29 After microalgae harvesting, biogas, with approximately 60% CH4 content is produced from anaerobic digestion of the concentrated microalgae flow.29,33 Biogas is generated by the combined action of different bacteria, through four phenomena: hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Figure 2). Temperature and pH should be kept constant. Digestion of the algal substrate with the inoculum can be carried out in small reactors in mesophilic (35−37 °C)36−38 or thermophilic conditions (around 50 °C),39 with a hydraulic residence time (HRT) of 20−40 days. Finally, NH3 and H2S are separated from the biogas by means of techniques analogous to those used in the syngas production (section 2.1). The addition of iron salts and biological desulphurization in packed columns are also used.1 Note also that biogas upgrading techniques are analogous to those used for syngas refining, as discussed in the previous section. Considering all this, Poplar wood chips from Short Rotation Forestry (SRF) and two species of microalgae, Chlorella vulgaris

3. CONSIDERED SUPPLY CHAINS The study is comprehensive as including all of the possible stages, from cultivation and harvesting of the biomass, up to the final product: biomethane. The main features of the supply chains are represented in Figure 3. Mass and energy balances include: (1) electrical and thermal energy; (2) diesel consumption for the operation of agricultural machines and transportation of biomass; (3) fertilizers and pesticides; (4) fluids and auxiliary materials for upgrading operations; (5) air and steam for gasification and methanation stages; (6) emission of carbon dioxide. 3.1. Cultivation. The European model of SRF is used for the considered poplar cultivation. Phytosanitary treatments, fertilization, and harvesting occur every two years. Discontinuous consumption of water, fertilizers and fuel is considered over the entire cultivation cycle (i.e., 10 years). The following values were obtained: 42.85 kg diesel ha−1 year −1; 80.3 kg N ha−1 year −1; 24.8 kg P ha−1 year −1; 29.9 kg K ha−1 year −1; irrigation water 140 m3 ha−1 year −1. The poplar areal productivity has been set to 17.67 tons dry matter ha−1 year −1.40−42 Two different species of microalgae have been considered: Chlorella vulgaris and Nannochloropsis gaditana. The algal material is grown in open raceway ponds where mixing is guaranteed by blade systems. Fertilizers and CO2 are provided into the ponds. Data on biomass composition (Table 1) have been used to determine the amount of fertilizers, water, and CO2 necessary for the growth of biomass, according to the photosynthetic mechanism. The areal productivity for the two microalgae is assumed to be 25 and 18 g m−2 d−1 for C. vulgaris and N. gaditana, respectively.32−34,43 Water tends to evaporate from the open nurseries: 0.6 cm s−1 is assumed as mean loss. A demand of 0.05 kWh m−3 is considered for the makeup of fresh water; the whole water wake-up includes also water recovered from the subsequent operations: sedimentation, flotation, and centrifugation. Carbon dioxide injection and water mixing require further electricity: 0.0222 kWh kg−1 CO2 and 50 kWh ha−1 d−1, respectively, to ensure an agitation velocity of 25 cm s−1. A complete absorption of fertilizers has been considered, while a percentage of CO2 is assumed to leave the ponds. The ratio between 10953


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Figure 3. Analyzed supply chains: main stages and features.

Table 1. Mass Percent Composition of Different Biomass (Dry-Basis) biomass

moisture [%]

C [%]

O [%]

H [%]

N [%]

poplar SRF4 C. vulgaris32−34 N. gaditana43

55

48.78 36.7 51.4

43.11 55.11 38.3

5.99

0.18 6.1 7.54

P [%] 0.81 1.24

K [%]

S [%]

Cl [%]

ash [%]

0.01

1.88

0.66 0.89

0.05 0.62 0.62

VS/TS [% w/w] 80.0 90.0

concentration in the cultivation media is only 0.5 kg m−3. The associate consumption of electricity is 0.05, 0.3, and 1 kWh m−3, respectively. For all three operations, a yield (η) and a concentration factor (CF) of the microalgae is considered, defined as the ratio between output and input biomass, and the ratio between output and input algae concentration, respectively. In detail, η = 0.65, CF = 20 for sedimentation, and η = 0.9, CF = 5 for centrifugation and DAF have been assumed.32−34 3.3. Biomass Conversion. Poplar wood chips undertake gasification, while microalgae are anaerobically digested. In the modeled poplar supply chain, biomass is processed in a dual-bed gasifier, which represents an allothermal process, analogous to the ones discussed in refs 4 and 6. Steam and preheated air enter the gasification chamber and the combustion chamber, respectively. The unreacted char moves between the two chambers, while flue gases and ashes are removed from the combustion chamber. Even though air is used, the peculiar configuration of dualbed gasifier provides a syngas with a very low nitrogen concentration, as it exits only with flue gases, avoiding subsequent expensive separation stages. Downstream of the gasifier,

carbon dioxide absorbed by microalgae and the total amount injected is supposed equal to 0.90. 3.2. Harvesting and Pretreatments. Tractors with specific equipment are used in the harvesting stage of poplar trees, in which biomass is also chipped and disposed on field for natural drying. Pretreatments are used to get a more homogeneous and dense product, simplifying and improving the subsequent phases: transport, storage, and combustion. Typical processes are chipping, drying, and densification, as pelletizing, roasting, and briquetting.40−42 Here, operations of harvesting and chipping occur simultaneously; hence, a diesel consumption for tractors and chippers is established as 61 + 50 kg diesel ha−1 year−1.11−14,42 The moisture of wood chips is reduced to 15% by natural drying, and a dry matter loss of 20% is registered. Wood chips are transported to the gasifier for a distance of 150 km; a load capacity of 40 tons is assumed, both full and empty travels are considered.44 The diesel consumption of trucks is assumed tobe 0.012 L diesel ton−1 km−1.45,46 Composition of the biomass from poplar SRF is detailed in Table 1. For the algae digestion chain, sedimentation, flotation (DAF), and centrifugation are necessary pretreatments, as algal cells 10954


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A large set of data useful for model implementation of a gasifier are available in ref 48. With regards to the alga supply chain, a continuous anaerobic digester was considered and modeled as a continuous-flow stirred-tank reactor (CSTR), similarly to that in ref 32. The process produces biogas and also digestate, that is, the residual fraction of the algal substrate. Data related to electricity and thermal energy have been included in the model; the demand due to centrifugation of the digestate has been estimated assuming a 6% w/w of solids content. The organic load rate and the Substrate to Inoculum Ratio are the min process control parameters, defined as kg of volatile solids (VS) fed on cubic meter of liquid in the reactor and as the ratio between VS of substrate and VS of inoculum. The reference batch digester operates under mesophilic conditions, and a constant temperature of 35 °C is maintained for a HRT of 30 days, until the saturation of CH4 productivity. The amount of methane extracted daily from the continuous reactor is considered equal to the amount removable from a batch reactor after the same HRT. A value of 60% v/v has been considered as average methane content in the biogas;29,33 the remaining part is comprehensive of CO2, H2O, NH3, and H2S, whose compositions are evaluated by considering the gas mixture as ideal and by assuming liquid−vapor equilibrium by standard thermodynamic data. Note that CH4 g−1 VS is used to express methane productivity. Time trend of the amount of methane produced into the digester is given by fitting data from biochemical

thermal energy is recovered by syngas cooling and used to preheat the air required from the gasification process. The simulation software Aspen Plus was used to model the dualbed gasifier by simulating two reactors, one for gasification and one for combustion, and introducing suitable recycles. The flow rate, temperature, and composition of syngas were computed by varying the equivalence ratio (ER) and steam to biomass ratio (STBR) parameters, defined as follow: ER =

mair m + mmoist ; STBR = steam mairst mdrybio

(1)

where mair, mair st, msteam, mmoist, and mdry bio are the mass flow rate of air, stoichiometric air, steam, moisture, and dry biomass, respectively. The values of ER and STBR were finally assumed as 0.28 and 0.75, respectively. Moreover, steady-state conditions have been assumed. Reactors have been considered as isothermal (650 and 700 °C) and no pressure drops (1 bar), no tar production, a 100% carbon-made char, and a complete conversion of N, S, and Cl to NH3, H2S, and HCl have been assumed. Note that other interesting results of biomass gasification have been recently presented in refs 16 and 47. Table 2. Average Energy Consumptions of the Upgrading Techniques Analyzed energy [kWh/m3]

water scrubbing

electricity heat

0.3

selexol amine 0.3 0.12

0.2 1

PSA

membranes

cryogenic separation

0.5

0.3

0.76

Figure 4. Flow diagram of the poplar gasification chain (part 1). 10955


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Figure 5. Flow diagram of the poplar gasification chain (part 2).

methane potential (BMP) tests, and it is expressed by a first order equation: CH4 = K (1 − e−t / τ )

series of three methanators, after a third stage of cooling.22 In this work, a two-block Aspen Plus model of water−gas shift reaction (first block) and methanation reactions (second block) has been used to obtain the refined syngas composition, as well as the steam flow required for the conversion of CO2 and CO. The model gives also the amount of heat recovered from gas cooling and the level of electricity consumed for the gas compression. 3.5. Upgrading Techniques. Separation of steam, carbon dioxide, and nitrogen from syngas and biogas represent the main upgrading techniques. These operations allow one to get high quality biomethane, ready to be injected in the gas grid. To ensure a high HHV, N2 removal may be necessary. As a matter of fact, the use of a dual-bed gasifier significantly limits the nitrogen content. Steam is used as unique gasification agent, which avoids the mixing of nitrogen with the syngas stream, as typical for other gasifying schemes. Otherwise, nitrogen has to separated by pressure swing adsorption (PSA), cryogenic separations, and membranes. However, these processes are very demanding, by resulting in high investment costs (special membranes) or high operating costs (cryogenic separation). Advantages arise only when nitrogen removal is integrated with the separation of CO2. Best methodologies for biomethane productions have been proven to be PSA and scrubbing with amine solutions, due to the high efficiency of carbon dioxide separation.51 Nevertheless, PSA implies high electricity consumption and amine generation has high heat demand, as shown in Table 2. In this work, removal of N2 is not considered and the following four upgrading configurations are studied: (1) H2O condensation, and CO2 adsorption with PSA;27,52 (2) H2O absorption with triethylene glycol26 and CO2 adsorption with PSA;

(2)

−1

where K [ml CH4 g VS] is the productivity of methane at saturation and τ [days] is the time constant of the considered microalgal biomass. Three different correlations were derived from all available experimental data, one based on the averaged value of the data, and two based on maximum and minimum production values. Mean values for C. vulgaris and N. gaditana are K = 251 and 265 mL CH4 g−1 VS and τ = 6.9 and 4.8 d, respectively. 3.4. Gas Treatments. Gas treatments have been modeled as an unique and ideal operation. The complete removal of H2S, NH3, and HCl is supposed. Emissions of methane are considered and equivalent value of CO2 has been associated. Within the poplar supply chain, CO and CO2 are converted into CH4 and H2O by methanation exothermic reactions:49,50 CO + 3H 2 → CH4 + H 2O

−206 kJ/mol

CO2 + 4H 2 → CH4 + 2H 2O

−165 kJ/mol

(3) (4)

The water−gas shift reaction (5), promoted by the addition of steam to methanators, generates hydrogen by adjusting the H2/CO molar ratio of the gas around 3−4 and allows the control of reaction temperature, preserving the catalysts employed.9 CO + H 2O → CO2 + H 2

−41.2 kJ/mol

(5)

Heat recovered from the syngas can be used to generate the steam required from gasifier and methanators. Hence, compressors are not needed for gas recirculation, but they can operate only on the outlet gases from the last one of a 10956


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Figure 6. Flow diagram of the poplar gasification chain (part 3).

Figure 7. Energy consumptions of the poplar gasification chain.

(3) H2O condensation, and CO2 absorption with monoethanolamine (MEA) solution;53

(4) H2O absorption with triethylene glycol, and CO2 absorption with MEA solution. 10957


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Industrial & Engineering Chemistry Research Table 3. CO2 Balance of the Poplar Gasification Chain

CO2 [103 t/year]

biomass

cultivation + harvesting + transport

−54.97

2.10

bio-SNG gasification methanation upgrading 1 upgrading 2 upgrading 3 upgrading 4 combustion 13.22

3.30

25.94

29.54

24.28

28.03

16.55

overall [4.49 ÷ 9.75]

Figure 8. Flow diagram of the anaerobic microalgae (C. vulgaris) digestion chain (part 1).

higher separation of steam and carbon dioxide is possible through Upgrading 4. Further heat recovery would increase the performance of the supply chain. The energy demand of steam generation and air preheating could be partially covered by cooling hot streams, that is, the syngas as well as the enriched gas exiting the methanation stage. Thermal recovery would cover about 70% and 60% of the thermal energy demand of the chain, considering Upgrading 3 and 4 configurations, respectively. This would result in the reduction of the self-consumption of bio-SNG for thermal energy production. The CO2 balance is shown in Table 3. Note that a high amount of carbon dioxide can be separated by all four upgrading techniques. All solutions lead to similar greenhouse gas emissions, due to electricity and heat consumption, and to removal of CO2 from the syngas and CH4 losses, assumed to be 2% of the input. Carbon dioxide emitted from the combustion of the final bio-SNG has also been computed. The cultivation phase does not affect heavily the balance, since some operations occur every two years (harvesting, fertilization, transport), while sowing and soil reclamation happen only once, at the beginning and at the end of the crop, respectively. 4.2. Microalgae Cultivation and Digestion. Results of the examined chain are shown by the flow diagrams of

Note that these technologies are well-established, since they are applied in tradition coal-fired plants.

4. RESULTS OF BALANCE ANALYSIS Analysis of supply chains has been developed with reference to a typical size of thermal power output, in terms of biomethane. In this paper, detailed results related to 10 MW are presented, by considering poplar wood chips and C. vulgaris as biomasses; the reader can refer to ref 54 for other case studies of smaller size. Results were obtained by taking 1 day as time reference and by assuming 8000 annual working hours. As a final step of the upgrading stage, a gas compression up to 70 bar before injection into the network is considered, and the relative electrical energy demand is computed. 4.1. Poplar Wood Chips Gasification and Upgrading. Results for the gasification chain are shown in Figures 4−6; the composition and the energy content of each flow are therein specified. Referring to Figure 7 it can be observed that the stages of cultivation, harvesting, and transport do not involve high energy consumption since they occur only every 2 years, while all of the examined upgrading configurations are highly demanding (Figure 6), as syngas leaving the methanation stage contains large volumes of CO2 (≃50% on dry basis). To be noted that the 10958


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Figure 9. Flow diagram of the anaerobic microalgae (C. vulgaris) digestion chain (part 2).

those required for microalgae cultivation. A high provision of water is necessary for microalgae growth in open raceway ponds, due to the high evaporation rates and to the digester demand. Moreover, a high continuous supplying of fertilizers are fundamental. The different methane content of syngas and biogas leads to different biomethane yields indeed, a bio-SNG:syngas 7:1 ratio and a biomethane:biogas ratio of 2:1 have been estimated. Upgrading syngas to bio-SNG needs high quantities of energy (electricity and heat), as discussed in the previous section; the upgrading stage is 2−3 times more demanding for syngas than biogas treatment. Results are summarized in Table 5: values related to poplar SRF, Chlorella vulgaris, and Nannochloropsis gaditana are reported. The NER (net energy ratio) has been chosen to describe the energy efficiency of the supply chains; this index is defined as the ratio between the output thermal energy obtained by biomethane combustion, and all of the energetic inputs considered along the chain (electricity, heat, and diesel). NER values of the examined supply chains, considering the four upgrading configurations, are presented in Table 6. Highest values have been always obtained for the upgrading 1, that is, H2O condensation and CO2 adsorption with PSA. Solar energy is a free input absorbed by biomass during its growth; hence, NER values are bigger than 1. Similar values of NER have been obtained for all three supply chains. NER values related to poplar wood gasification are heavily affected by the energy demanded for the generation of steam required by methanation and gasification processes, as well as by the air preheating and regeneration of scrubbing liquids used in the upgrading operations, as thermal recovery is not considered. The microalgae supply chain has

Figures 8 and 9. The energy demand of this supply chain is heavily affected by cultivation and harvesting. Harvesting is an expensive stage, due to the low cells concentration. The stream exiting raceway ponds contains only 0.5 kg of microalgae per m3; hence, separation and filtration of biomass require very high electricity consumption: around 11000 MWh/y (Figure 10). Sedimentation has the lowest energy demand, as electricity is necessary only for water pumping. DAF unit separates the algae biomass by means of the injection of air bubbles which move the cells toward the top of tank; the compressor used in this process requires high electrical power input, while the major consumption is due to the electric engine used for the centrifugation phase. Raw biogas produced by anaerobic digestion has a high content of methane (≃60%). Low energy input is required for mixing and maintaining mesophilic conditions (35 °C). As observed in poplar supply chain, between the four configurations considered, Upgrading 4 removes the highest levels of steam and carbon dioxide. Upgrading of biogas is far simpler than syngas treatment, as lower volumes of CO2 and H2O have to be separated from the gas. Equivalent carbon dioxide emissions concern fertilizers (emission factors have been defined), electricity required for cultivation and harvesting operations, anaerobic digestion (mixing) and from compressors and scrubbed pumps used for biogas treatment and purification stages (Table 4). 4.3. Comparison of Production Chains. In this section a comparison between the analyzed supply chains for the production of biomethane is presented. The areal productivity of poplar cultivation is lower than that of microalgae; large surfaces can be cultivated according to the SRF approach, by using lower amounts of water and fertilizers with respect to 10959


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Figure 10. Energy consumptions of the anaerobic microalgae digestion chain (C. vulgaris).

Table 4. CO2 Balance of the Anaerobic Microalgae Supply Chain (C. vulgaris) microalgae cultivation harvesting digester upgrading 1 upgrading 2 upgrading 3 upgrading 4 CO2 [103 t/year]

−55.80

23.70

4.50

1.79

15.72

17.20

14.01

biomethane combustion

overall

14.92

[3.12 ÷ 6.31]

15.75

Table 5. Performance Indexes of Biomethane Production (10 MW) from Poplar Wood Chips, Chlorella vulgaris, and Nannochloropsis gaditana supply chain parameter

unit of measure

poplar SRF

C. vulgaris

N. gaditana

hectares of cultivations water consumption fertilizers consumption

[ha] [103 m3/year] N [ton/year] P [ton/year] K [ton/year] S [ton/year] [ton ds/year] electricity [kWh/Nm3 biomethane] heat [kWh/Nm3 biomethane]

1739.3 243.5 139.7 43.2 52.0 0.0 24 600 3.9 3.1

500 10 117.1 2529.3 335.9 273.2 29.4 44 630 0.9 1.8

575 11 651.5 260.3 428.3 307.4 29.4 34 520 0.9 1.8

feed of biomass energy consumption for upgrading 4

chips and anaerobic digestion of two species of microalgae, Chlorella vulgaris and Nannochloropsis gaditana. Mass and energy balances of the analyzed supply chains can be considered comparable; however, some peculiarities clearly arise. In the bio-SNG production, methanation and syngas upgrading stages are highly demanding, due to the low methane concentration in the raw syngas and to the high volumes of water and carbon dioxide to be removed. The highest removal efficiency is reached through absorption by triethylene and monoethanolamine solution. This supply chain is more consolidated than the biomethane production from microalgae; however, efficient heat recovery methods would further reduce the energy inputs, providing the energy required for steam generation and air preheating. Heat recovery would cover up to 70%

Table 6. Net Energy Ratios of the Considered Supply Chains supply chain

upgrading 1

upgrading 2

upgrading 3

upgrading 4

poplar SRF C. vulgaris N. gaditana

1.72 1.73 1.85

1.33 1.51 1.61

1.38 1.19 1.27

1.13 1.05 1.11

lower NER values than poplar wood, mainly due to the energy requirements of cultivation and harvesting stages.

5. CONCLUSIONS Different supply chains for the production of biomethane have been analyzed in this paper: gasification of SRF poplar wood 10960


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of the thermal energy demand of the chain, if upgrading 3 is considered; hence, self-consumption of bio-SNG would be reduced. Conversely, due to the low cells concentration in the cultivation media, microalgae harvesting is an expensive phase, while biogas upgrading requires lower energy input than syngas treatment, due to its high content of methane. A bio-SNG:syngas 7:1 ratio and a biomethane:biogas ratio of 2:1 have been estimated. Recirculation of the digestate is recommended, to reduce the fresh dose of fertilizers. Moreover, biomethane production could be coupled with the synthesis of biodiesel, by transesterification of microalgal lipids extracted from residues of the anaerobic digestion.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Riccardo Bacci di Capaci: 0000-0001-6339-6303 Andrea Luca Tasca: 0000-0002-6389-5724 Gabriele Pannocchia: 0000-0002-5578-344X Elisabetta Brunazzi: 0000-0001-6316-6946 Monica Puccini: 0000-0001-7039-0604 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the project PRA_2016_55 Biomethane production chain f rom renewable sources funded by the University of Pisa. The authors also thank Francesco Corsi for his contribution to simulation and experimental activity.

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REFERENCES

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Biomethane Production: Mass and Energy Balances of Alternative

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