Valorization of OFMSW Digestate-Derived Syngas toward Methanol, Hydrogen, or Electricity: Process Simulation and Carbon Footprint Calculation

: This paper explores a possible waste-based economy transition strategy. Digestate from the organic fraction of municipal solid waste (OFMSW) is considered, as well as a low-added value product to be properly valorized. In this regard, air gasiﬁcation may be used to produce syngas. In this work, the production of methanol, hydrogen, or electricity from digestate-derived syngas was assessed by ChemCAD process simulation software. The process scheme of methanol production comprises the following parts: water gas shift (WGS) with carbon capture and storage units (CCS), methanol synthesis, and methanol puriﬁcation. In the case of hydrogen production, after WGS-CCS, hydrogen was puriﬁed from residual nitrogen by pressure swing absorption (PSA). Finally, for electricity production, the digestate-derived syngas was used as fuel in an internal combustion engine. of 2 ﬀ ect of CO 2 storage. In particular, CCS units were used for methanol or hydrogen production with the aim of obtaining high equilibrium yield toward these products. On the basis of 100 kt / year of digestate, results show that the global CO 2 savings were 80, 71, and 69 kt CO2eq / year for electricity, methanol, and hydrogen production, respectively. If carbon storage was considered, savings of about 105 and 99 kt CO2eq / year were achieved with methanol and hydrogen production, respectively. The proposed scenarios may provide an attractive option for transitioning into methanol or hydrogen economy of the future.


Introduction
Municipal solid waste (MSW) is a heterogeneous waste consisting of everyday items that are discarded by the citizens in a municipal context. It is composed of organic waste (e.g., food waste), waste paper, plastic waste, tin cans, aluminum cans, textile, glass, etc. Furthermore, the relative composition of municipal solid waste varies greatly from municipality to municipality, and it changes significantly with the season and along the years [1,2]. The global solid waste generation by urban residents is expected to increase from 3.5 Mtons/day in 2010 to 6.1 Mtons/day in 2025, with a total management cost of $375 billion in 2025 [3]. Therefore, the management of MSW is a very challenging issue to be addressed.
Municipal solid waste may be considered as a valuable source of carbon usable for both energy and chemical production. Incineration/thermo-valorization of MSW is the major solution adopted for unsorted MSW. On the contrary, an efficient MSW collection and sorting of MSW is the first step to recovering or recycling high-added value products pushing toward a circular economy system. In fact, converted into fuel, severely limiting the viability of the pyrolysis process for digestate valorization [36]. Furthermore, pyrolysis is an endothermic process and external energy is required. Pyrolytic liquid from digestate pyrolysis can also be fed into the anaerobic digester in order to increase the bio-methane yield. However, the presence of aromatic compounds which may have a microbial toxicity hinders direct utilization in anaerobic digestion units, and pre-conditioning is requested [37].
Compared to pyrolysis, air gasification shows more advantages. Gasification is a thermal conversion process, which converts biomass-or fossil-derived carbonaceous materials into a gaseous product which mainly consists of hydrogen, carbon monoxide, and carbon dioxide and fractions of methane and light hydrocarbons. Gasification is usually carried out at high temperature (700-900 • C) under a controlled amount of oxygen and/or steam. In contrast to pyrolysis, the partial combustion of organic matter provides the energy for the process [38,39]. Few research works are available in the literature concerning the gasification of digestate [40,41]. Chen et al. studied the air gasification of biogas-derived digestate in an electrically heated downdraft gasifier at the laboratory scale [42]. The authors found that the optimum temperature in terms of gas production and quality (H 2 /CO = 1.08) was 800 • C, while the equivalence ratio was 0.28. An interesting preliminary work was carried out by Gnanendra et al., who investigated on the use of digester briquettes for gasification in an open-top gasifier coupled with an internal combustion engine. The digester waste mass rate was 40 kg/h. Engine operation indicated that gas quality was consistent with specific fuel consumption of 1.6 kg·kWh −1 , amounting to an overall electrical efficiency of 15%. The authors were very careful to establish the correct ash extraction rate. The high inorganic content together with relatively high moisture of the fuel, in the first tests, caused large amounts of ash to settle inside the reactor. The clinkers fused to the walls of the reactor near the air nozzle, occasionally posing a problem to material movement [43]. Freda et al. found that a rotary kiln reactor is suitable for air gasification of digestate [44]. About 20 kg/h of digestate pellets were successfully gasified at a pilot scale under different equivalence ratios (0. 22-0.39). The collection of experimental data was mainly targeted at acquiring information about cold gas efficiency, permanent gas compositions, and gas yields. The syngas yields rose reasonably with the equivalence ratio. Specifically, values rising from 0.99 to 1.40 Nm 3 /kg feedstock were observed, on an average basis, when increasing the equivalence ratio from 0.22 to 0.39. Syngas composition lay in the following range: H 2 (9-12 vol.%), CO (10-14 vol.%), CO 2 (12-14 vol.%), CH 4 (3-4 vol.%), C 2 H 6 (ca 1 vol.%), and N 2 (56-62 vol.%), with a lower heating value of 4-5 MJ/Nm 3 dry gas . The highest cold gas efficiency of 47% was detected with an equivalence ratio 0.39.
The tests were carried out for several hours in steady state conditions without particular operative issues. In fact, rotary kilns are reliable in thermochemical processes of waste and biomass because they allow a good mixing of the feedstock, radial uniformity of bed temperature, and a continuous discharge of the process ashes [45][46][47][48][49] The good mixing of the bed prevents hot spots and the formation of slagging and clinkering; the continuous discharge of the produced ashes fits with the high ash content of the digestate. Moreover, the rotary kilns are easier to operate compared to a fluidized bed; compared to a fixed bed, they do not show the issues related to the ashes and are scalable up to several tens of thermal MW.
The interesting novelties of Freda's work compared to the available literature are that the trials were planned to scout self-sustaining gasification that is a paramount condition for industrial development, and that the reactor was a rotary kiln at a pilot scale. The kind of reactor and the investigated scale of the plant are particularly meaningful for the further scale up of the process toward a technological readiness level of industrial relevance. Aiming to improve the sustainability of the existing AD plants, several biorefinery schemes were investigated during the last few decades. Monlau et al. [50] reported a review on opportunities for agricultural digestate valorization. Thermo-valorization via pyrolysis was indicated as a suitable strategy to produce pyrochar as a soil amender, bio-adsorbent, or starting material to produce activated carbons. Antoniou et al. [51] proposed a gasification-AD integrated scheme, where the produced syngas from digestate air gasification was used for electricity production, while the residual char was considered as soil amendment, with energetic and environmental benefits. A similar concept was also adopted by Milani et al., who co-gasified digestate with biomass and the combustible fraction of MSW to produce electricity [52]. Recently, Pecchi et al. [40] reported a review on the valorization of digestate via gasification, pyrolysis, or hydrothermal carbonization. Concerning gasification, on the basis of few studies reported on digestate, the attention was focused on the CHP application of syngas and on the utilization of residual char as an amendment or to improve the biogas yield in AD. Therefore, to the best of our knowledge, few studies were devoted on the conceptualization of valorization of digestate-derived syngas, and no study reported the production of methanol or hydrogen. The novelty of our study is to propose a strategy for digestate valorization going beyond the production of electricity. Currently, green electricity is produced at low cost from other renewable energy sources, such as solar and wind energy. On the contrary, biomass or waste-derived organic residues can be considered to be the sole true alternatives to fossil sources for the production of chemicals and fuels. Therefore, the novelties of our work may be summarized as follows: (i) proposing new scenarios for the valorization of MSW-digestate toward methanol and hydrogen, (ii) performing process simulations for syngas upgrading and conversion, and (ii) assessing the environmental impact in terms of equivalent CO 2 emission for the three investigated cases (i.e., digestate to methanol, hydrogen, or electricity), pushing toward alternative strategies for valorizing the digestate-derived syngas.

Materials and Methods
In Table 1, the results of the proximate analyses and heating value of digestate pellets are reported. The main feature that can be argued is that, despite the high ash content, the volatile matter content and the heating value are interesting to investigate the feedstock for thermal treatments. Briefly, a feeding system, with a capacity of 20 kg/h, conveys feedstock to the rotary kiln reactor where gasification occurs. The produced raw gas crosses the cleaning system where particulate and tar are removed. The stainless-steel rotary kiln reactor is 3 m long and its diameter is 0.3 m. Freda et al. [44] studied the effect of feeding rate and equivalence ratio on syngas quality. In this study, we adopted the syngas with the highest CO concentration and the lowest N 2 concentration, as reported in Table 2.

Process Simulation
Process design tools [53] were used in order to identify all process streams of plants to valorize syngas obtained by digestate gasification.
As previously reported, gasification-deriving syngas can be used to produce either energy or high-added value products, such as chemicals or fuels. In this section, three different scenarios for gas stream valorization are assessed according to the scheme reported in Figure 1.

Process Simulation
Process design tools [53] were used in order to identify all process streams of plants to valorize syngas obtained by digestate gasification.
As previously reported, gasification-deriving syngas can be used to produce either energy or high-added value products, such as chemicals or fuels. In this section, three different scenarios for gas stream valorization are assessed according to the scheme reported in Figure 1. In this paper, the production of electricity (Case-EL), methanol (Case-MeOH), or pure hydrogen (Case-H2) was considered. In this regard, the commercial process simulation software ChemCAD was used. The Redlich-Kwong-Soave thermodynamic equation of state was adopted to simulate high-pressure systems, while NRTL-RK (NRTL: non-random two liquids model, RK: Redlich and Kwong model) was used for distillation columns and flash separation units. As the main aim of the paper was to assess the environmental impact of electricity, methanol, or hydrogen production from digestate-derived syngas, particular attention was focused on carbon dioxide capture. In this regard, in the cases of MeOH and H2 production, the removal of carbon dioxide is necessary. The syngas from gasification of solid material is often subjected to a water gas shift (WGS) process in order to enrich the mixture in hydrogen, to adjust the H/C ratio, or to convert CO into CO2 to proceed to the carbon capture subsequently [54]. In the case of methanol production, the syngas should have a ratio (H2 − CO2)/(CO + CO2) equal to the optimal one for the production of MeOH, i.e., 2. Since this is not verified for the adopted digestate-derived syngas, a WGS step followed by CO2 capture is implemented in order to increase the hydrogen content [55,56]. Similarly, in the case of production of pure hydrogen, the syngas must be enriched as much as possible in hydrogen in order to both maximize the yield in hydrogen and facilitate subsequent purification by PSA.
To facilitate the conversion of CO into CO2 + H2, two different methodologies can be carried out: (1) Increase the steam-to-CO ratio in the inlet stream to the reactor, although the maximum energy-sustainable value of the ratio is 3.5; for higher values, the production of steam at high temperature (about 400 °C) is too expensive [57].
(2) Remove one or both of the reaction products (namely, CO2 or H2) from the mixture in intermediate steps of the reaction (for example, between the high and low temperature shift (HTS and LTS) stages) or continuously along the reactor (for example, through the use of membranes or material adsorbents) [58,59].
In the case of hydrogen production, both methods are suitable in order to favor the production of hydrogen as much as possible. In particular, an H2O/CO ratio of 3.5 may be set at In this paper, the production of electricity (Case-EL), methanol (Case-MeOH), or pure hydrogen (Case-H 2 ) was considered. In this regard, the commercial process simulation software ChemCAD was used. The Redlich-Kwong-Soave thermodynamic equation of state was adopted to simulate high-pressure systems, while NRTL-RK (NRTL: non-random two liquids model, RK: Redlich and Kwong model) was used for distillation columns and flash separation units. As the main aim of the paper was to assess the environmental impact of electricity, methanol, or hydrogen production from digestate-derived syngas, particular attention was focused on carbon dioxide capture. In this regard, in the cases of MeOH and H 2 production, the removal of carbon dioxide is necessary. The syngas from gasification of solid material is often subjected to a water gas shift (WGS) process in order to enrich the mixture in hydrogen, to adjust the H/C ratio, or to convert CO into CO 2 to proceed to the carbon capture subsequently [54]. In the case of methanol production, the syngas should have a ratio (H 2 − CO 2 )/(CO + CO 2 ) equal to the optimal one for the production of MeOH, i.e., 2. Since this is not verified for the adopted digestate-derived syngas, a WGS step followed by CO 2 capture is implemented in order to increase the hydrogen content [55,56]. Similarly, in the case of production of pure hydrogen, the syngas must be enriched as much as possible in hydrogen in order to both maximize the yield in hydrogen and facilitate subsequent purification by PSA.
To facilitate the conversion of CO into CO 2 + H 2 , two different methodologies can be carried out: (1) Increase the steam-to-CO ratio in the inlet stream to the reactor, although the maximum energy-sustainable value of the ratio is 3.5; for higher values, the production of steam at high temperature (about 400 • C) is too expensive [57].
(2) Remove one or both of the reaction products (namely, CO 2 or H 2 ) from the mixture in intermediate steps of the reaction (for example, between the high and low temperature shift (HTS and LTS) stages) or continuously along the reactor (for example, through the use of membranes or material adsorbents) [58,59].
In the case of hydrogen production, both methods are suitable in order to favor the production of hydrogen as much as possible. In particular, an H 2 O/CO ratio of 3.5 may be set at the reactor, and the CO 2 obtained in the HTS reactor may be partially removed before the LTS reactor. For Case-EL the gas stream produced from digestate gasification was used as fuel to an internal combustion engine for the production of electricity, according to the scheme reported in Figure 2.
Processes 2020, 8,526 6 of 22 the reactor, and the CO2 obtained in the HTS reactor may be partially removed before the LTS reactor.

Case-EL
For Case-EL the gas stream produced from digestate gasification was used as fuel to an internal combustion engine for the production of electricity, according to the scheme reported in Figure 2. After cleaning, the syngas is considered free of particulate or tars. An engine [60] was considered for the combustion of syngas after mixing with air. Air flowrate was optimized in order to maximize the electricity production.

Case-MeOH
Methanol is one of the most important compounds of the chemical industry, with an annual production of about 40 million tons and an annual market increase rate of 4%. Currently, methanol is mainly produced from syngas obtained via steam reforming of fossil carbon sources, mainly natural gas and coal. In particular, a higher CO2 amount is emitted from coal-based plants. Aiming to reduce both CO2 emission and fossil source supply, alternative carbon sources may be considered in light of a sustainable energy economy. In this concern, the syngas obtained from gasification of biomass, residuals, and waste may be considered as a raw stream to obtain methanol.
In this regard, for Case-MeOH considered in this work, the clean syngas was sent to a (i) WGS section to convert CO into CO2 and hydrogen, followed by (ii) a carbon capture section using the Selexol process to obtain a (H2 − CO2)/(CO + CO2) ratio equal to 2. After that, (iii) a methanol production and purification section was considered for producing high-purity methanol. A flowsheet scheme is reported in Figure 3. Below, such sections are briefly detailed.

Water Gas Shift (WGS) Section
The high-pressure capture of carbon dioxide and the production of methanol or pure hydrogen require the conversion of carbon monoxide and steam into CO2 and H2, coming with the clean syngas stream, by means of the water gas shift (WGS) reaction: The WGS reaction is carried out in two steps, namely high temperature shift (HTS) and low temperature shift (LTS). The temperature was set to 400 °C and 200 °C for HTS and LTS, respectively. Similar conditions are adopted for industrial processes, where Fe2O4/Cr2O3 and Cu-ZnO-Al2O3 are used as catalysts for HTS and LTS, respectively. This sequence allows taking advantage of the faster conversion rate in the HTS path and finally approaching a higher equilibrium conversion value (larger than 99%) for the most favorable equilibrium conditions in the LTS [61]. The main operating parameter of this process is the H2O/CO molar ratio (steam to carbon, SC). The clean gas is mixed with a superheated steam (1 bar, 400 °C). The adopted feed ratio was chosen to obtain a H2O/CO molar ratio (steam to carbon, SC) equal to 2.5. The two fixed-bed reactors were modeled as After cleaning, the syngas is considered free of particulate or tars. An engine [60] was considered for the combustion of syngas after mixing with air. Air flowrate was optimized in order to maximize the electricity production.

Case-MeOH
Methanol is one of the most important compounds of the chemical industry, with an annual production of about 40 million tons and an annual market increase rate of 4%. Currently, methanol is mainly produced from syngas obtained via steam reforming of fossil carbon sources, mainly natural gas and coal. In particular, a higher CO 2 amount is emitted from coal-based plants. Aiming to reduce both CO 2 emission and fossil source supply, alternative carbon sources may be considered in light of a sustainable energy economy. In this concern, the syngas obtained from gasification of biomass, residuals, and waste may be considered as a raw stream to obtain methanol.
In this regard, for Case-MeOH considered in this work, the clean syngas was sent to a (i) WGS section to convert CO into CO 2 and hydrogen, followed by (ii) a carbon capture section using the Selexol process to obtain a (H 2 − CO 2 )/(CO + CO 2 ) ratio equal to 2. After that, (iii) a methanol production and purification section was considered for producing high-purity methanol. A flowsheet scheme is reported in Figure 3. Below, such sections are briefly detailed.

Water Gas Shift (WGS) Section
The high-pressure capture of carbon dioxide and the production of methanol or pure hydrogen require the conversion of carbon monoxide and steam into CO 2 and H 2 , coming with the clean syngas stream, by means of the water gas shift (WGS) reaction: The WGS reaction is carried out in two steps, namely high temperature shift (HTS) and low temperature shift (LTS). The temperature was set to 400 • C and 200 • C for HTS and LTS, respectively. Similar conditions are adopted for industrial processes, where Fe 2 O 4 /Cr 2 O 3 and Cu-ZnO-Al 2 O 3 are used as catalysts for HTS and LTS, respectively. This sequence allows taking advantage of the faster conversion rate in the HTS path and finally approaching a higher equilibrium conversion value (larger than 99%) for the most favorable equilibrium conditions in the LTS [61]. The main operating parameter of this process is the H 2 O/CO molar ratio (steam to carbon, SC). The clean gas is mixed with a superheated steam (1 bar, 400 • C). The adopted feed ratio was chosen to obtain a H 2 O/CO molar ratio (steam to carbon, SC) equal to 2.5. The two fixed-bed reactors were modeled as equilibrium reactors. The pressure drop in the sequence of the two catalytic fixed bed was assumed of about 2 bar [57].
For the case of the traditional process sketched in Figure 4, the section for CO2 separation with the Selexol ® technology was located after the WGS reactor and, thus, the inlet stream to the section was a clean syngas enriched in H2 and CO2 [45]. On the other hand, for the case of the innovative process sketched in Figure 3, the Selexol ® section was located after the partial H2 separation by the membrane and, thus, the inlet stream was richer in CO2.
The gas stream leaving the two WGS stages was cooled down to 30 °C to condense the steam. CO2 Capture Section Figure 3 shows the flowsheet of the section with the Selexol ® technology. The separation of CO2 from the enriched syngas stream was carried out by absorption in the proprietary solvent Selexol ® , a mixture of dimethyl ethers of polyethylene glycol (CH3O(C2H4O)nCH3), where "n" is between 3 and 9, in a packed tower with 75 mm IMTP ® packing of Koch-Glitsch ® .
The Selexol ® solvent was compressed and sent to the top of the tower 30 °C. Solvent regeneration was carried out downstream the absorption tower to separate carbon dioxide and to recycle the solvent. However, a small make-up stream was necessary to replace the solvent entrained in the gaseous stream leaving the packed tower. In particular, the regeneration was performed by lowering the pressure in two stages by means of two expansion valves up to 1 bar.
A flash was used to recover the CO2 from the solvent. The regenerated solvent was mixed with the make-up stream and recycled back to the absorption tower after proper recompression and cooling. Low-pressure captured CO2 was considered emitted for the comparison with case of CO2 captured and stored at 110 bar. In this last case, the recovered CO2, which had the required purity (at least 98.5% by weight) was compressed up to 110 bar by a compressor train, formed by three compressors with intercooling by two heat exchangers, for storage conditions.

Methanol Production and Purification Section
The following reactions may be adopted to simulate methanol synthesis via both CO and CO2 hydrogenation: For the case of the traditional process sketched in Figure 4, the section for CO 2 separation with the Selexol ® technology was located after the WGS reactor and, thus, the inlet stream to the section was a clean syngas enriched in H 2 and CO 2 [45]. On the other hand, for the case of the innovative process sketched in Figure 3, the Selexol ® section was located after the partial H 2 separation by the membrane and, thus, the inlet stream was richer in CO 2 .  The PSA is a batch process consisting of a sequence of four steps: adsorption, depressurization, purge, and pressurization [57]. The adsorption pressure and the purge ratio (P/F) are the operating variables of the PSA process. In our case, the operating pressure was fixed equal to the output pressure from the absorption column. In the adsorption step, the adsorption time is a fraction of the breakthrough time of N2. In the purge step, the pure hydrogen, obtained during the adsorption step, is sent, in countercurrent, through the bed in order to allow further desorption of previously adsorbed species, primarily N2 and CO. Purge gas is sent to the engine to produce electricity. A lower content of H2 in the purge corresponds to higher pure hydrogen recovery and lower power output. Purge gas and air are mixed before the compression and engine section. Engine pressure was optimized to maximize the electricity production. The gas stream leaving the two WGS stages was cooled down to 30 • C to condense the steam. The Selexol ® solvent was compressed and sent to the top of the tower 30 • C. Solvent regeneration was carried out downstream the absorption tower to separate carbon dioxide and to recycle the solvent. However, a small make-up stream was necessary to replace the solvent entrained in the gaseous stream leaving the packed tower. In particular, the regeneration was performed by lowering the pressure in two stages by means of two expansion valves up to 1 bar.

Environmental Impact Analysis
A flash was used to recover the CO 2 from the solvent. The regenerated solvent was mixed with the make-up stream and recycled back to the absorption tower after proper recompression and cooling. Low-pressure captured CO 2 was considered emitted for the comparison with case of CO 2 captured and stored at 110 bar. In this last case, the recovered CO 2 , which had the required purity (at least 98.5% by weight) was compressed up to 110 bar by a compressor train, formed by three compressors with intercooling by two heat exchangers, for storage conditions.

Methanol Production and Purification Section
The following reactions may be adopted to simulate methanol synthesis via both CO and CO 2 hydrogenation: In addition, unwanted side reactions, such as formation of methane, dimethyl ether, higher alcohols, and other oxygenates, e.g., aldehydes, can also take place. In this paper, WGS was considered as a side reaction only. Because methanol formation is an exothermic reaction, methanol synthesis is favored at low temperature, from a thermodynamic point of view. Nevertheless, kinetic aspects have to be taken into account. For these reasons, methanol synthesis is usually carried out in the temperature range 250-280 • C by using Cu-ZnO/alumina as catalyst. The inlet stream to the methanol reactor is mixed with the recycle stream (unreacted syngas), heated up to 200 • C, and fed to the methanol synthesis adiabatic reactor. An equilibrium reactor was adopted for the methanol synthesis unit by considering the reactions (ii), (iii), and (iv). A pressure drop equal to 2 bar was assumed along the reactor [61]. Concerning operating pressure, a pressure of 80 bar was considered to favor thermodynamics.
As mentioned before, crude methanol needs to be purified from water and impurities. As only WGS is considered, dissolved gases, such as CO 2 , CO, and H 2 , are the main impurities that need to be removed from the methanol stream. For this purpose, the output stream of the methanol synthesis reactor was cooled down at 40 • C, and the light gases were separated in a flash phase separator. A part of the separated gases was re-compressed at 80 bar and recycled to the reactor, while the rest was used as fuel for electricity production. The liquid streams, mainly consisting of methanol and water, were expanded in a throttling valve at 2 bar and distilled to produce a methanol-rich stream which was further purified in a flash phase separator to obtain 99% methanol.

Case-H 2
Hydrogen is considered one promising alternative to fossil fuel, especially because of its combustion release energy with water as a "by-product". The great interest toward hydrogen as an energy carrier is because it can be generated from a wide variety of renewable or non-renewable primary energy sources. Actually, most of the hydrogen is produced by fossil source-based processes, such as (i) steam reforming of methane, (ii) partial oxidation, (iii) auto-thermal reforming, and (iv) dry reforming [62]. Although most of the hydrogen is actually produced from fossil sources (about 95%), the utilization of renewables, such as biomass, received considerable attention in recent years. In particular, biomass gasification may be used as a suitable technology to produce syngas and then hydrogen. In fact, the syngas produced from gasification may be converted into a H 2 -rich stream via the water gas shift reaction [58,63].
In this regard, for Case-H 2 considered in this work, the clean syngas streams obtained from the digestate gasification unit were sent to the WGS section. In this case, pure hydrogen production was maximized; thus, a higher H 2 O/CO ratio was set (equal to 3.5); after HTS-WGS, CO 2 was removed by Selexol ® . Intermediate CO 2 removal coupled to a high H 2 O/CO ratio resulted in a higher conversion of CO to CO 2 and a higher yield of H 2 . After HTS-WGS, CO 2 capture, and LTS-WGS, the H 2 -rich syngas was sent to the pressure swing adsorption (PSA) section for purifying and then producing 99.99% hydrogen. A scheme of the simulated process is reported in Figure 4.
The PSA is a batch process consisting of a sequence of four steps: adsorption, depressurization, purge, and pressurization [57]. The adsorption pressure and the purge ratio (P/F) are the operating variables of the PSA process. In our case, the operating pressure was fixed equal to the output pressure from the absorption column. In the adsorption step, the adsorption time is a fraction of the breakthrough time of N 2 . In the purge step, the pure hydrogen, obtained during the adsorption step, is sent, in countercurrent, through the bed in order to allow further desorption of previously adsorbed species, primarily N 2 and CO. Purge gas is sent to the engine to produce electricity. A lower content of H 2 in the purge corresponds to higher pure hydrogen recovery and lower power output. Purge gas and air are mixed before the compression and engine section. Engine pressure was optimized to maximize the electricity production.

Environmental Impact Analysis
The environmental impact analysis was conducted through the use of equivalence relationships to calculate the CO 2 -equivalent emissions (or savings if negative values) for each inlet or outlet stream from the system [64]. Figure 5 shows a simplified scheme of the environmental analysis approach. Dashed lines indicate streams present in only some cases. In particular, the following inlet/outlet streams are associated with CO 2 equivalent emissions: -Residual biochar disposal after the gasification process in a landfill (BC); -Process water utilization for the WGS reaction, also considering detoxification and demineralization processes (PW).  The PSA is a batch process consisting of a sequence of four steps: adsorption, depressurization, purge, and pressurization [57]. The adsorption pressure and the purge ratio (P/F) are the operating variables of the PSA process. In our case, the operating pressure was fixed equal to the output pressure from the absorption column. In the adsorption step, the adsorption time is a fraction of the breakthrough time of N2. In the purge step, the pure hydrogen, obtained during the adsorption step, is sent, in countercurrent, through the bed in order to allow further desorption of previously adsorbed species, primarily N2 and CO. Purge gas is sent to the engine to produce electricity. A lower content of H2 in the purge corresponds to higher pure hydrogen recovery and lower power output. Purge gas and air are mixed before the compression and engine section. Engine pressure was optimized to maximize the electricity production.

Environmental Impact Analysis
The environmental impact analysis was conducted through the use of equivalence relationships to calculate the CO2-equivalent emissions (or savings if negative values) for each inlet or outlet stream from the system [64].   The direct CO 2 emission streams are as follows: -CO 2 in the flue gases of the combustion of purge gas or of syngas for the case EL (FG); -CO 2 derived by carbon capture section for the cases without CO 2 compression and storage (CC).
A negative CO 2 -equivalent emission value (CO 2eq saving) is associated with digestate (DIG). The digestate valorization by gasification avoids the disposal in a landfill of this solid material; thus, the equivalent CO 2 emitted from the landfill disposal [65] is avoided.
Savings are also associated with bio-based products for each case. Bio-based production of power, fuels, or chemicals substitutes the equivalent fossil-based product. Emissions associated with fossil-based production are avoided. In this work, the environmental savings estimated for the investigated bio-methanol (MeOH) production plants were considered for the production of methanol from coal (1643 kg CO2eq /t MeOH ) [66]. The environmental savings for pure hydrogen (H 2 ) production were considered for the case of utilization of the natural gas as feedstock for the steam reforming process (12000 kg CO2eq /t H2 ) [67]. Finally, the electricity (EL) production savings were considered equal to that of the fossil-based energy production system (600 kg CO2eq /MWhe) [50].
For each case, the values reported in Table 3 were adopted to calculate the CO 2 -equivalent emissions/savings [64].
SAV CO2,j = em CO2,j F j , j = DIG, EL, MeOH, H2, where Fi is the input/output flowrate of the emission stream i of Figure 5 (t/year), Fj is the input/output flowrate (energy stream) of the saving stream j of Figure 5 (t/year or MWhe/year), em CO2,i and em CO2,j are the equivalent CO 2 emission parameters of Table 3 related to the stream i or j, EM CO2,i is the CO 2eq emission for the emission stream i (kgCO 2eq /year), SAVCO 2,j is the CO 2eq saving for the saving stream j (kgCO 2eq /year), and TOT CO2 is the total annual CO 2eq emission (or saving, if negative) of the process (kgCO 2eq /y). Table 3. CO 2 -equivalent emission parameters.

Results and Discussion
As previously reported, the valorization syngas was assessed by simulating the production of electricity, methanol, or pure hydrogen. Table 4 reports the main parameters adopted for the simulation. A plant size equal to 100,000 t/year of digestate was considered as a collective digestate treatment plant.  Table 5 shows the process results for the three cases considered. For the case MeOH, the obtained conversions of HTS-WGS and LTS-WGS were 31% and to 33%, respectively. These values are consistent with the literature [66], as the first WGS reaction step is influenced by the H 2 O/CO ratio and by the presence of products (CO 2 and H 2 ) within the currents entering the reactor. Lower H 2 /CO and CO 2 /CO ratios result in higher equilibrium conversion in the first HTS stage. In case-MeOH, the aim was to convert CO to CO 2 having the maximum energy saving. The H 2 O/CO ratio was set equal to 2.5, and products from the first stage HTS were not removed. Thus, inlet ratios H 2 /CO and CO 2 /CO were equal to 0.87 and 0.94, respectively.
LTS-WGS conversion is higher than HTS-WGS because LTS-WGS conditions are thermodynamically optimal due low inlet temperature (200 • C). In order to obtain a (H 2 − CO 2 )/(CO + CO 2 ) ratio equal to 2, a 90% carbon capture section was necessary. Consequently, a rich-CO 2 syngas at 40 bar was obtained.
For case-H 2 , HTS-WGS and LTS-WGS conversions were equal to 41% and 71%, respectively. In this case, CO and CO 2 were not reactants of the next process step; thus, CO conversion was to be maximized. To obtain that, a H 2 O/CO ratio equal to 3.5 was set, and the CO 2 removal was carried out between HTS and LTS stages. Therefore, in the first stage, a higher conversion than case-MeOH was obtained (41%) due to a higher H 2 O/CO ratio.
Moreover, in this case, a CO 2 capture equal to 90% was performed; consequently, a Selexol pressure equal to 56 bar was necessary.
After carbon capture, the inlet LTS-WGS CO 2 /CO ratio was equal to 0.23. This low ratio and low inlet temperature (200 • C) allowed reaching a LTS-WGS conversion higher than in case-MeOH (71%).
The global WGS CO conversion (HTS + LTS) was equal to 83%, enriching the current by about 7 kmol/h of hydrogen. Total carbon (CO + CO 2 ) conversion in the MeOH reactor was equal to 56%. This conversion was equal to 12% considering the single passage in the MeOH reactor. A purge split ratio equal to 10% was set in order to keep the (H 2 − CO 2 )/(CO + CO 2 ) ratio equal to 2 in the reactor inlet.
In the PSA process, an 85 % pure hydrogen recovery was obtained considering an inlet pressure of 35 bar.
Electricity production was equal to 7.78 MWe for case-EL. In case-MeOH, purge gas combustion produced a gross power equal to 7.39 MWe. The compression for the carbon capture (−4.19 MWe), for the methanol reactor (−0.51 MWe), and for the recycle (−0.09 MWe) decreased the gross power to a net power equal to 2.60 MWe. In Case-H 2 , the only compression for the carbon capture was considered. In this case, a gross power equal to 4.28 MWe was obtained from the combustion of purge gas.
In the cases of MeOH and H 2 , captured CO 2 compression was also considered. For this compression, an electrical power of 0.56 and 0.48 MWe for case-MeOH and case-H 2 was requested, respectively. The net power productions are summarized in Figure 6.
In the PSA process, an 85 % pure hydrogen recovery was obtained considering an inlet pressure of 35 bar. Electricity production was equal to 7.78 MWe for case-EL. In case-MeOH, purge gas combustion produced a gross power equal to 7.39 MWe. The compression for the carbon capture (−4.19 MWe), for the methanol reactor (−0.51 MWe), and for the recycle (−0.09 MWe) decreased the gross power to a net power equal to 2.60 MWe. In Case-H2, the only compression for the carbon capture was considered. In this case, a gross power equal to 4.28 MWe was obtained from the combustion of purge gas.
In the cases of MeOH and H2, captured CO2 compression was also considered. For this compression, an electrical power of 0.56 and 0.48 MWe for case-MeOH and case-H2 was requested, respectively. The net power productions are summarized in Figure 6. The low methanol yield obtained in this work, namely, about 8 kg MeOH /100 kg feedstock,daf (daf: dry and ash free) may be related to the syngas quality produced from the gasification of digestate. In particular, in order to have the optimal (H 2 − CO 2 )/(CO + CO 2 ) molar ratio, i.e., 2, in the methanol reactor inlet, WGS and carbon capture units are usually implemented, especially for hydrogen-lean produced syngas. These steps cause a carbon loss, mainly occurring in the carbon capture step, leading to a lower biomass-to-methanol conversion. Both CO 2 and N 2 content in the produced syngas should also be taken into account in order to rationalize the obtained results. In fact, from a thermodynamic point of view, the CO-to-MeOH hydrogenation step is more favored than CO 2 -to-MeOH in terms of theoretical carbon conversion and methanol yield. For this reason, a CO 2 -lean syngas should be preferred in order to increase the incorporation of biomass-derived carbon into methanol. N 2 content should be as low as possible in order to minimize its diluting effect, since a higher N 2 content results in a lower partial pressure of H 2 /CO/CO 2 reactants with a negative impact on the thermodynamic of methanol synthesis, which is favored at high reactant pressure. Furthermore, the presence of inerts, such as N 2 , increases both operational and capital costs, e.g., compression power, reactor volume, and so on. These conditions can be partially approached when the gasification agent is steam, as in the work of Zhang et al. [68] that simulated straw gasification in an interconnected gasifier-combustor fluidized bed. The authors calculated a methanol yield of about 40 kg MeOH /100 kg feedstock,daf when the straw was gasified at 750 • C and 0.1 MPa with a steam-to-straw weight ratio in the range 0.4-0.5. Under such conditions, a clean syngas with H 2 /CO x equal to about 1.5, CO 2 /CO equal to about 0.67, and N 2 concentration equal to 0.5% was obtained. Ptasinski et al. [69] reported a study on gasification of partially dried sewage sludge (80 wt.% of solids) for the production of methanol. The authors obtained a methanol yield high as 77 kg MeOH /100 kg feedstock,daf when the sewage sludge was gasified at 1000 • C to obtain a produced syngas with H 2 /CO x equal to about 1.3, CO 2 /CO lower than 0.02, and N 2 concentration equal to about 3%. High methanol yield was also obtained by Yang et al. [70]. The authors re-used the CO 2 captured by Rectisol in a dual-stage entrained flow gasification unit, by using pure oxygen and steam as gasification agents and operating at 900 • C to obtain a methanol yield equal to about 63 kg MeOH /100 kg biomass,daf under optimal conditions. In our study, we calculated a relatively low methanol yield, i.e., about 8 kg MeOH /100 kg feedstock,daf . Such a result may be related to the relative low quality of produced syngas from digestate air gasification which has a H 2 /CO x ratio <0.5 and a high CO 2 /CO ratio equal to about one, as well as high nitrogen content (i.e., about 56%). Nevertheless, a quantitative comparison between the proposed process schemes with the literature is very challenging because the boundary conditions are quite different, and the methods adopted in the simulation differ from one work to another. Furthermore, to the best of our knowledge, no studies on digestate-to-methanol conversion were previously published. As a general frame, steam gasification seems to perform better in terms of methanol yield, although external heat input is required. Heat may be provided by a gasification/combustion interconnected fluidized bed reactor that implies technical issues during the running. Similar results may also be obtained in nitrogen-lean oxygen/steam gasification, but addition costs of pure oxygen should be taken into account.
Similar considerations may be done for hydrogen production. In fact, a higher quality of produced syngas, such as higher H 2 content or lower CO 2 and N 2 content, results in a higher e hydrogen yield. Toonssen et al. compared several commercial biomass-based gasification plants for hydrogen production [71]. Although the hydrogen yield may be affected by performances of either gas cleaning (e.g., low-or high-temperature cleaning system) or and upgrading (e.g., steam reforming of methane, water gas shift) systems, there is an evident effect of the syngas quality on hydrogen yield. For processes producing nitrogen-lean and hydrogen-rich syngas, e.g., Battelle, IGT (Institute of Gas Technology), FICFB (Fast Internal Circulating Fluidized Bed), and Blau Turm processes, the hydrogen yield is in the range 6.8-10.7 kg H2 /kg feedstock,daf , higher than the value obtained in this work, i.e., about 3 kg H2 /kg feedstock,daf . Moreover, in that case, the production of a nitrogen-lean syngas requires steam, oxygen, or a mixture of them as gasification agents, or advanced technologies (e.g., indirect gasification) should be used [72,73]. Similarly, syngas with a higher CO 2 level leads to a lower hydrogen yield. In fact, the carbon contained in the feedstock should be mainly converted to CO which may be used in the WGS unit to increase the hydrogen content [74]. Despite the relatively low yield toward methanol and hydrogen, the strength of the process proposed in this work lies in a syngas obtained via a reliable and simple rotary kiln gasifier with air as the gasification agent. Therefore, the lower methanol or hydrogen yields might be balanced by the ease and the low cost in the rotary kiln gasifier management. Capital and operating expenditure analysis would be useful for a proper viability assessment of the proposed process.
The global electrical efficiency of the process calculated as the ratio between produced electrical power and the inlet digestate thermal power was 15.9%, which is lower compared to lignocellulosic biomass gasification for power production (about 20%). The electrical efficiency from syngas to power is 37%, and this value is close to data from the literature for internal combustion engine performance fueled by poor gas [75][76][77]. Therefore, the limiting step of the power production is digestate gasification. Digestate could be considered as a low-grade feedstock because of its high ash content (about 30 wt.%) that negatively affects gasification and other thermochemical valorization processes. In fact, the gas yield was relatively low (0.99 Nm 3 /kg feedstock ) compared to low ash content biomass (2-2.4 Nm 3 /kg feedstock ). Hence, the cold gas efficiency, i.e., the power that was converted from digestate to raw gas, was only 43%. Finally, despite the fact that digestate is not the best-performing biomass for gasification, its massive and localized production together with a not well-established fate highlights gasification as a solution to reduce its volume with useful power production.

CO 2 -Equivalent Emission Calculation
The annual CO 2 equivalent emissions are shown in Figure 7. The direct emissions concern the CO 2 captured in Selexol ® if it is not also compressed and stored, as well as the flue gas of the syngas/purge gas combustion.
which may be used in the WGS unit to increase the hydrogen content [74]. Despite the relatively low yield toward methanol and hydrogen, the strength of the process proposed in this work lies in a syngas obtained via a reliable and simple rotary kiln gasifier with air as the gasification agent. Therefore, the lower methanol or hydrogen yields might be balanced by the ease and the low cost in the rotary kiln gasifier management. Capital and operating expenditure analysis would be useful for a proper viability assessment of the proposed process.
The global electrical efficiency of the process calculated as the ratio between produced electrical power and the inlet digestate thermal power was 15.9%, which is lower compared to lignocellulosic biomass gasification for power production (about 20%). The electrical efficiency from syngas to power is 37%, and this value is close to data from the literature for internal combustion engine performance fueled by poor gas [75][76][77]. Therefore, the limiting step of the power production is digestate gasification. Digestate could be considered as a low-grade feedstock because of its high ash content (about 30 wt.%) that negatively affects gasification and other thermochemical valorization processes. In fact, the gas yield was relatively low (0.99 Nm 3 /kgfeedstock) compared to low ash content biomass (2-2.4 Nm 3 /kgfeedstock). Hence, the cold gas efficiency, i.e., the power that was converted from digestate to raw gas, was only 43%. Finally, despite the fact that digestate is not the best-performing biomass for gasification, its massive and localized production together with a not well-established fate highlights gasification as a solution to reduce its volume with useful power production.

CO2-Equivalent Emission Calculation
The annual CO2 equivalent emissions are shown in Figure 7. The direct emissions concern the CO2 captured in Selexol ® if it is not also compressed and stored, as well as the flue gas of the syngas/purge gas combustion. The indirect emissions concern the disposal of the solid residue of the char after the gasification process as water for the WGS reaction. The orange bars consider the savings derived from the avoided landfilling for the digestate disposal. This contribution is equal to −182 ktCO2eq/year; this value makes The indirect emissions concern the disposal of the solid residue of the char after the gasification process as water for the WGS reaction. The orange bars consider the savings derived from the avoided landfilling for the digestate disposal. This contribution is equal to −182 kt CO2eq /year; this value makes total emissions negative (i.e., saving), because, even without the syngas valorization, the carbon contained in the digestate is so low as to make the direct CO 2 emissions (burning directly the digestate) lower than the environmental impact of the digestate disposal, due to the extremely high ash content (32%).
In terms of emissions, the highest emissions derive from the disposal of the solid residue. In fact, 39% of the solid material fed to the gasifier remains as a solid residue to be disposed. From this effect, the direct emissions of syngas combustion are also derived; they are lower than biochar disposal emissions.
Direct emissions from the combustion of the purge gas (cases MeOH and H 2 ) are about 35% and 50% of the syngas combustion emissions for cases of MeOH and H 2 , respectively.
This occurs because the combustible material in the purge gases is lower. The CO 2 captured is approximately equal to the difference between the flue gas in the EL case and the flue gas in the case of the combustion of the purge gas for case-H 2 . For Case-MeOH, emissions from captured CO 2 are higher than Case-H 2 because of the position of the carbon capture process after both HTS-WGS and LHS-WGS. Total CO 2 direct emissions are lower than emissions for Case-EL and Case-H 2 because a fraction of the carbon in the feedstock was contained in the methanol product. Figure 8 shows the total emissions shown in Figure 7 for each case under "digestate disposal savings and char disposal emissions".
All cases show negative emissions (savings) because of high savings from avoided digestate disposal. Case-MeOH has higher savings due to lower direct CO 2 emissions. Furthermore, with CO 2 storage, the highest savings came from Case-H 2 , while the lowest ones came from Case-EL. This is due to the higher emissions derived from the syngas combustion. In Case-H 2 , the production of green power was reduced as the calorific value of the hydrogen was subtracted from the PSA, but the CO 2 was released into the atmosphere together with the flue of the purge gas. Figure 7 shows the savings of MeOH and pure H 2 products ("product savings") and the savings obtained from the production of green power. The total savings (product + electricity) were higher in the EL case and lower in the MeOH case. This is because the saving from green power is higher than the sum (product saving + green power) calculated for the other cases. This is due to the low overall yields of MeOH and pure H 2 products, equal to 4.7% and 1.7%, respectively. Producing chemicals reduces the amount of electricity produced but obtains reduced quantities of product. CO 2 storage strongly affects CO 2 emission as may be noted by comparing data reported in Figure 7. Considering the cases without CO 2 storage, higher savings are seen in the EL case and lower savings are seen in Case-H 2 . The reason is the low savings from products in the MeOH and H 2 cases and the high emissions in Case-H 2 , derived from the flue gas of the purge gas combustion. In Case-H 2 , the production of green power is reduced as the calorific value of the hydrogen is subtracted from the PSA, but the CO 2 is released into the atmosphere in the fumes of the purge gas. In the MeOH case, on the other hand, the post-capture residual CO 2 is partially converted into MeOH.
Considering the cases with CO 2 storage, the highest savings are obtained for MeOH and H 2 (very similar) and lower savings are obtained for EL. This is because 35.5 kt/year and 32.4 of CO 2 captured and stored for Case-MeOH and Case-H 2 , respectively, represent about 27% and 24% of the total emissions in the MeOH and H 2 cases, respectively. Figure 8 shows the comparison between the specific equivalent CO 2 emissions/savings of this work (without and with CCS) (tot CO2, i, i = EL, MeOH, H 2 ) and literature results using a fossil source or a bio-based source. In particular, this work's results show the convenience of utilizing digestate as feedstock.
For all three products, the lowest emissions (the highest savings) were obtained for the digestate-as-feedstock stream. In accordance with literature results, the highest specific savings consisted of the savings derived from the feedstock [78]. In an environmental impact analysis, the saving from the feedstock is the principal one. Typical savings using biomass as feedstock are 1434 kgCO 2 /t [79] for corn stover, 1456 kgCO 2 /t [80] for cereal straw, and 1597 kgCO 2 /t [57] for the organic fraction of municipal solid waste. For the present work, a value equal to 1643 kgCO 2 /t was used as feedstock savings from Li et al. [66].
In particular, Figure 9 shows the comparison between the specific equivalent CO 2 emissions/savings of this work (without and with CCS) (tot CO2,i , i = EL, MeOH, H 2 ) and literature results using a fossil source or a bio-based source. In particular, this work's results show the convenience of utilizing digestate as feedstock.
emissions/savings of this work (without and with CCS) (totCO2,i, i = EL, MeOH, H2) and literature results using a fossil source or a bio-based source. In particular, this work's results show the convenience of utilizing digestate as feedstock.
For case-EL, the value for fossil-based emission was taken from Rodrigues Gurgel da Silva et al. [65], equal to 0.60 kgCO2/kWhe, which is higher than the negative bio-based literature value of −0.20 kgCO2/kWhe from Caposciutti et al. [79]. For electricity production, the specific impact is equal to −0.82 kgCO2/kWhe, due to high savings from the avoided digestate disposal in the landfill. Only this saving leads to −3.25 kgCO2/kWhe.  . Equivalent CO2 emissions/savings comparison for each case and between fossil-based production (orange bars), biobased production in the literature from Reference [79] (yellow bars), and present work without (gray bars) and with (blue bars) CO2 storage.

Conclusions
Digestate may be considered a by-product of anaerobic digestion of the organic fraction of municipal solid waste. In this work, the valorization of digestate for the production of methanol, hydrogen, or green electric power was proposed. On the basis of previous experimental observation of syngas production via air gasification of digestate, process simulations were performed in order to assess the carbon footprint of the proposed valorization schemes. . Equivalent CO 2 emissions/savings comparison for each case and between fossil-based production (orange bars), biobased production in the literature from Reference [79] (yellow bars), and present work without (gray bars) and with (blue bars) CO 2 storage.
For case-EL, the value for fossil-based emission was taken from Rodrigues Gurgel da Silva et al. [65], equal to 0.60 kgCO 2 /kWhe, which is higher than the negative bio-based literature value of −0.20 kgCO 2 /kWhe from Caposciutti et al. [79]. For electricity production, the specific impact is equal to −0.82 kgCO 2 /kWhe, due to high savings from the avoided digestate disposal in the landfill. Only this saving leads to −3.25 kgCO 2 /kWhe.
For case-MeOH, the fossil-based emission was taken from Li et al. [66], equal to 1.64 kgCO 2 /kgMeOH, which is higher than the bio-based literature value 0.08 kgCO 2 /kgMeOH from Giuliano et al. [53]. Without CCS, for methanol production, the specific impact is equal to −13.59 kgCO2/kgMeOH. In this case, the saving from digestate leads to −38.75 kgCO 2 /kgMeOH. Considering also the storage of captured CO 2 , the saving for each kg of methanol is equal to 20.62 kgCO 2 /kgMeOH, whereas the captured CO 2 has an impact of −7.03 kgCO 2 /kgMeOH.
For case-H 2 , the fossil-based emission was taken from Bareiß et al. [68], equal to 12.00 kgCO 2 /kgH 2 , which is higher than negative bio-based literature value −19.60 kgCO 2 /kgH 2 from Dean et al. [80]. Without CCS, for methanol production, the specific impact is equal to −27.60 kgCO 2 /kgH 2 . In this case, the saving from digestate leads to −104.52 kgCO 2 /kgH 2 . Considering also the storage of captured CO 2 , the saving for each kg of methanol is equal to 45.01 kgCO 2 /kgH 2 , while the captured CO 2 has an impact of −17.41 kgCO 2 /kgH 2 .

Conclusions
Digestate may be considered a by-product of anaerobic digestion of the organic fraction of municipal solid waste. In this work, the valorization of digestate for the production of methanol, hydrogen, or green electric power was proposed. On the basis of previous experimental observation of syngas production via air gasification of digestate, process simulations were performed in order to assess the carbon footprint of the proposed valorization schemes.
In particular, two different process flowsheets were considered for the methanol and for pure hydrogen production. In the case of pure hydrogen production after the first step of HTS/water gas shift, the carbon capture unit was carried out in order to convert more CO to CO 2 and H 2 . Calculations indicate that from syngas obtained via air gasification of 100 kt/year of digestate, about 4700 t/year of methanol, or 1700 t/year of hydrogen may be produced.
Syngas may also be used for the production of 560 kWhe/t DIGESTATE . In particular, the production of green electricity maximizes the saving of CO 2 equivalent (about 790 t CO2eq /kt DIGESTATE ). If the storage of CO 2 is considered in addition to the capture, the optimal CO 2eq -saving is similar for the production of methanol and pure hydrogen, i.e., 1050 t CO2eq /kt DIGESTATE and 990 t CO2eq /kt DIGESTATE , respectively. Although a cost analysis should be performed to complete the assessment, this study pushes toward new scenarios for the valorization of digestate from anaerobic digestion of municipal waste.