Comparison of molasses conversion to biomethanol by biohydrogen pathway with biogas route in engineering and cost assessment: Thailand case

ABSTRACT Biomethanol is a significant chemical in biochemicals and biofuels. Molasses is interested in producing biogas and biohydrogen for biomethanol. Biohydrogen, Enterobacter aerogenes digested molasses obtaining value organic chemicals and biohydrogen in appropriate ratios of H2/CO2 then transforming to H2/CO by RWGS. Biogas was converted to syngas then methanol synthesis. The biogas pathway was 4 steps and it was appropriate for sailing single product as biomethanol. The biohydrogen pathway was 3 steps and obtained income both valuable substances and biomethanol. Operating expenditure for 1 kg methanol by biohydrogen experiment and theory were 4.4148 and 4.0912 USD comparing with biogas 0.3446 USD based on commercial methanol price 0.449 USD/kg. The sale prices per kg of biomethanol by biohydrogen were 6.7243 USD (Exp.) and 5.7500 USD (Theory) comparing with biomethanol sailing from biogas pathway at 0.4486 USD. Margin caps were 30.19%, 39.66%, and 40.55% for biogas pathway, biohydrogen experiment and theory route respectively.


Introduction
Considering global warming, petroleum had been substituted by biofuels, such as ethanol and biodiesel, over the past three decades.Biofuels strongly supported the agricultural sector and bio-based industries.Biofuels are renewable and can act as replacement for petroleum-based fuel but they also emit carbon dioxide after getting into combustion engines.Considering the advantage of battery storage, electric vehicles are currently dominating internal combustion vehicles, which causes disruption effects on the transformation of the biofuel era to the carbon neutral era.The domino effect is on ethanol and biodiesel production systems and has a chain effect on the agricultural raw materials that are sources of these biofuel chains.Molasses is obtained from the sugar industry and is commonly used as a raw material for fertiliser production, animal feed ingredients, and biogas production for use in electricity generation and bioethanol generation.Ethanol was produced for blending in different ratios in gasoline depending on the policy of each country.Electric vehicles have currently gradually dominated gasoline vehicles, and using molasses for ethanol production has gradually decreased.Moreover, global molasses price sourced from Tridge global market has been continuously decreasing since 2011 (over 1 USD/kg) till now (0.10 USD/kg) in each year (Tridge Exhibition 2021) Molasses is a significant bio-renewable source for producing biomethane and biohydrogen (Detman et al. 2017).Both cases generate CO 2 in the gas phase: biogas contains methane (CH 4 ), carbon dioxide (CO 2 ), and a small amount of hydrogen sulfide (H 2 S), while biohydrogen comprises hydrogen (H 2 ) and CO 2 (Detman et al. 2017).Directly using biogas and biohydrogen leads to CO 2 emission to the atmosphere.Therefore, integrating biotechnology with thermochemical research aims to innovate the current research on using molasses for biogas and biohydrogen production and transforming both to biomethanol.
Methanol is crucial for chemical and fuel industries and acts as a solvent, thinner, and a reactant in biodiesel and petrochemicals.Thus, methanol is forecasted in the methanol market to grow from 110 million metric tons in 2018-220 in 2030(Infoholic Research LLP 2019;Imarc Impactful Insights 2021;Grand View Research 2022;Prismane Consulting 2018;Mordor Intelligence 2022;Polaris Market Research 2019;Triton Market Research 2022;Inkwood Research 2019;Statista Research Department 2022).Commercial methanol (black/gray methanol) had been traditionally produced from steam reforming of coal or natural gas.However, the last decade focused on bio/ green methanol, which has been widely studied to substitute conventional methanol.Consequently, the methanol in the market is divided into two groups: non-renewable and bio/green methanol.The methanol projection by the Council on Energy, Environment and Water and International Energy Agency advised that the cost of green methanol will be gradually decreased due to its competition with gray/black methanol in 2030 (Joshi 2019).The advantages of transforming molasses passing biogas and biohydrogen pathway to produce biomethanol would not only substitute bio/green methanol but also support increasing molasses usage and reduce CO 2 from bioresource applications emitting to the atmosphere.
The first pathway (A) involves the application of molasses for biogas.Generally, biogas had been produced from fermented organic residues but is mostly in small and medium scales.Molasses is an abundant source for large scales of biogas production.Biogas contained approximately 55% (v/v) CH 4 , 43% (v/v) CO 2 , 2% (v/v) water, and a small amount of H 2 S (Janke et al. 2015;Suwanasri et al. 2015;Chaiprasert 2011).The transformation of molasses to biomethanol requires a fourstep process as shown in Figure 1.Pathway A: (1) Biogas Production (BG), wherein molasses was diluted by water and fermented under anaerobic condition to obtain crude biogas.(2) Biogas Refinery (BGR), wherein crude biogas was refined by removing H 2 S by bioscrubber (Nishimura and Yoda 1997) or Fe-EDTA solution (Thongkhao et al. 2019).(3) Biogas Reforming (BGF), wherein CH 4 and CO 2 were reacted by dry reforming (1) and steam reforming (2) to obtain syngas under Metal/Al 2 O 3 catalyst under 600°C to 900°C in the atmosphere (Zhao et al. 2020).However, the optimised condition of biogas reforming was used followed by Jitrwung et al. (2022a).(4) Methanol synthesis (MS), wherein H 2 in syngas was hydrogenated on CO/CO 2 by (4)/(5) and both reactions occurred under Cu/ZnO/Al 2 O 3 catalyst (Sun, Metcalfe, and Sahibzada 1999); the optimised condition of CO hydrogenation was 170°C and 40 barg.After molasses passed through four steps, it was then transformed into crude biomethanol containing over 96% (v/v) methanol and contaminated 3% (v/v) of water, 0.02% (v/v) ethanol, and a small amount of impurities (Jitrwung et al. 2022b).
The second pathway (B) involves an innovation route, namely passing biohydrogen route, which had been researched by using biosugar sources, such as glucose, fructose, molasses, and glycerol.Molasses was extensively investigated due to its wide use and abundance.Moreover, molasses is the cheapest source among other sugar sources.Producing biohydrogen is close to the biogas fermentation because it is produced from biosugar sources by bacterial digestion (Özgür et al. 2010;Kumar et al. 2015).Biohydrogen obtained from molasses comprised only hydrogen (H 2 ) and carbon dioxide (CO 2 ).The ratio of H 2 /CO 2 was produced on the basis of bacterial strains and conditions; thus, in the use of Enterobacter aerogenes, the ratio of H 2 /CO 2 was 1 following the metabolic pathway of glucose (Ren et al. 2006).Enterobacter aerogenes was utilised in biohydrogen fermentation because its temperature of 30°C is similar to equatorial room temperature and facultative condition.H 2 /CO 2 ratio of biohydrogen was in the range of 0.5-1.0(Jitrwung and Yargeau 2015).Only three steps were required to convert molasses to biomethanol as shown in Figure 1.Pathway B: (1) Biohydrogen Fermentation (BHF), wherein molasses was diluted by nutrient solution and then fermented under facultative condition obtaining crude biohydrogen.(2) Biohydrogen Conversion, wherein CO 2 and H 2 were reacted by reverse water gas shift (3) to obtain syngas (Daza and Kuhn 2016) under Cu/ZnO/Al 2 O 3 catalyst under 500°C and atmospheric pressure; a side reaction of CO and H 2 O and reversed CO and H 2 was observed and referred to as water gas shift (WGS) (6) (Wang et al., 2017).(3) MS, wherein H 2 was added into CO by (4) or added into CO 2 by (5), demonstrating the occurrence of both reactions under Cu/ZnO/Al 2 O 3 catalyst at 170°C and 40 barg.Biomethanol was only three steps when passing biohydrogen pathway.
This article aims to provide a comparison using molasses for biomethanol.The comparison involved the four and three steps of molasses to biomethanol by biogas pathway and biomethanol by biohydrogen route to obtain the engineering view and cost.

Apparatus and analytical methods for biomethanol
1.The gas composition was measured by a gas analyzer from the MRU model Vario Luxx. 2. Bioreactor: 10 L bioreactor with a 5 L working volume from Marubishi in model MDFT-10 L (Figure 2). 3. A pilot scale fixed-bed reactor comprised two reactors: 5 kg Ni/Al 2 O 3 catalyst filled in the reactor (Reforming case)/5 kg Cu/ZnO/Al 2 O 3 catalyst filled in the reactor (RWGS case) and 5 kg catalyst filled in the MS reactor.The maximum capacity for producing 5 L/day of biomethanol is given in Figure 3. 4. Biogas/Biohydrogen pathway to biomethanol Biogas composition was obtained from biogas plant companies.Simulation biogas CH 4 55% (v/v) and CO 2 45% (v/v) were then mixed followed by the biomethanol process comprising two step reactions: Reforming (RF) and MS reactions.By contrast, biohydrogen composition was obtained from molasses fermentation.Simulation varying ratios of biohydrogen (H 2 /CO 2 : 40/60, 50/50, 60/40, and 70/30% (v/v)) were mixed followed by the biomethanol process comprising two step reactions: RWGS and MS reactions.
The process flow diagram (PFD) is shown in Figure 4.The PFD presents two types of feed: the flow rate of biogas (mixture of ratio CH 4 /CO 2 ) or biohydrogen (mixed gas of ratio H 2 /CO 2 ) was controlled by mass flow controller (MF-1), preheated in a heat exchanger (HX-1), and then fed to the first reactor  (RX-1) for RF or RWGS reaction.The reaction was controlled temperature 600°C (RF case) or 500°C (RWGS case).After the reaction was continued in the reactor, the fluid product was generated and cooled down in a heat exchanger (HX-2).The product was condensed and trapped, and then the fluid was separated into two phases.The liquid phase was water, which was separated by opening the valve (CV-1).The gas phase was mixed gas (syngas) comprising H 2 , CO 2 , and CO, and its composition was measured by a gas analyzer before storing in a low-pressure vessel (V-1).The syngas was compressed by a compressor (CP-1) to reach the required pressure (40-50 barg) and then sent to absorption units (AB-1/2) for trapping contaminates, such as water and oil, before passing the syngas to the second reactor (RX-2).The syngas was then fed and controlled by mass flow controller (MF-2), and the methanol reaction was induced in the reactor (RX-2) under an operating temperature of 170°C and pressure of 40 barg.The fluid product was cooled down and trapped in a heat exchanger (HX-3), and the liquid or methanol was then condensed and pulled out by the valve (CV-2).The gas phase pressure was reduced to atmospheric level by a relief valve (CV-3) for recycling of gas to RX-1 or adjusting to atmospheric pressure for sending to the environment by control valve (CV-7).This vent gas composition was measured with a gas analyzer through valve (CV-5).Considering performing only the MS reaction, the mixed gas of H 2 /CO 2 /CO was directly fed into the MF-2 and the RX-2 procedure was continued until the methanol product was obtained.

Results and discussion
First, data of molasses to biogas pathway and BGR were obtained from two biogas companies in Thailand, namely RE Power Service Co., Ltd. and Tanachewasap Co., Ltd., Thailand.Considering molasses to crude and refinery biogas, simulation biogas was then experimented to generate syngas and produce biomethanol in a 5 L/day continuous process.Finally, data of biohydrogen pathway were obtained from 10 L bioreactor, and simulation biohydrogen gases were then experimented to obtain syngas and produce biomethanol in a 5 L/day continuous process.

Biogas refinery (BGR)
Crude biogas required two stages for H 2 S removal.Stage (1) 50,000 ppm of hydrogen sulfide (H 2 S) in biogas was removed by bioscrubber, reducing H 2 S concentration from 50,000 to 500 ppm.The OPEX (H 2 S-Bio) cost was around 0.40 THB/m 3 biogas as reported by the RPS biogas company.Stage (2) 500 ppm of H 2 S was separated by chemical absorption techniques using the Fe-EDTA solution to adsorb H 2 S in the form of sulfur (S) solid using the method of absorption (Thongkhao et al. 2019).The optimum condition for the adsorption was feeding of 4 L/min blending of 0.1 mol/L Fe-EDTA with 2 mol/L NaOH in counter current contacted with 1 L/min biogas.The final H 2 S in biogas product remained lesser than 10 ppm.The OPEX (H 2 S-FeS) cost was around 0.10 THB/m 3 biogas.The concentration of refined biogas remaining at 60% CH 4 and 40% CO 2 based on the total OPEX-BGR was 4.00 THB/m 3 biogas (0.114 USD/m 3 biogas).

Refined biogas to syngas (BS) and syngas to biomethanol (SM)
The 10.0 L/min refined biogas combined with 6.0 ml/min water was sent to the reforming reactor, which was filled with 5 kg of Ni/Al 2 O 3 commercial catalyst operated under 600°C and pressure atmosphere.The product was water and syngas contained H 2 , CO 2 , and CO with the ratio of H 2 /CO around 2.10.The syngas was then separated from water and compressed to 50 barg for sending to the MS reactor, which was packed with 5 kg of Cu/ZnO/Al 2 O 3 commercial catalyst by operating conditions (temperature 170°C and pressure 40 barg pressure).The raw biomethanol generated by 5.0 kg/day contained 96% methanol and was contaminated with small amounts of water, ethanol, and isopropanol (Jitrwung et al. 2022b).Figure 5 shows that material balance based on 1 kg of crude biomethanol required in 1.35 kg of refined biogas containing 60% CH 4 and 40% CO 2 reacted with 0.61 kg of water.The byproduct is off gas containing 0.76 kg CO 2 , 0.13 kg CO, and 0.07 kg H 2 , with a heating value of 9.80 MJ/kg MeOH.This off gas can be returned and mixed with biogas for electricity generator.Considering 1 kg of crude biomethanol refined and obtained 0.72 kg refined biomethanol and 0.28 kg off grade biomethanol, the OPEX-BMR (Biomethanol Refinery) was 0.60 THB/kg biomethanol (0.0171 USD/kg biomethanol) (Jitrwung et al. 2022a).

Molasses to biohydrogen (MH)
The optimal condition of biohydrogen production was experimented in a 10 L batch reactor with a working volume of 5 L containing 40 g/L molasses concentration in media solution and applying 4 g/L CaO (ranging from 0 to 5 g/L CaO).The optimal result of biohydrogen was 0.0089 g H 2 /g molasses with H 2 /CO 2 ratio of 0.67 compared with 0.0090 g H 2 /g molasses with H 2 /CO 2 ratio of 0.425 by Kumar et al. (2015) (Kumar et al. 2015).Therefore, the OPEX calculation of this research would be 0.009 g H 2 /g molasses and H 2 /CO 2 ratio of 0.67.The OPEX-MH was 20.93 THB/m 3 biohydrogen (0.598 USD/m 3 biohydrogen).The biohydrogen price was more expensive compared with biogas because 1 mol of molasses was digested to 1 mol of H 2 and 1 mol of CO 2 following glucose metabolic pathway, resulting in H 2 /CO 2 equal to 1 by theory and obtaining other organic substances, such as ethanol, acetic acid, propanoic acid, and pyruvic acid, as reported by Ren et al. (2006).

Biohydrogen to syngas (HS) and syngas to biomethanol (SM)
Considering biomethanol production on the biohydrogen pathway, the biohydrogen contained only CO 2 and H 2 and then the RWGS reaction was used to transform CO 2 to CO through H 2 following Equation (3).RWGS was a reverse reaction, which was balanced with WGS, as shown in Equation ( 4).The H 2 /CO 2 ratio obtained from the biohydrogen product was 0.67.However, H 2 / CO 2 ratios were varied depending on methods, type of bacteria, and raw materials.The results of varying H 2 /CO 2 ratios for observing syngas products and MS are reported in Table 2. Varying H 2 /CO 2 in the range of 0.67, 1.00, 1.50, and 2.30 by feeding % ratio H 2 /CO 2 of 40/60, 50/50, 60/ 40, and 70/30, respectively, were experimented in 5 kg of Cu/ZnO/Al 2 O 3 commercial catalyst operated under 500°C and pressure atmosphere.The product was water and syngas contained H 2 , CO 2 , and CO with the ratio of H 2 /CO around 1.75, 2.16, 2.48, and 2.95.The syngas was then separated from water and compressed to 50 barg for sending to the MS reactor, which was packed with 5 kg of Cu/ZnO/Al 2 O 3 commercial catalyst by operating conditions (temperature 170°C and pressure 40 barg pressure).The raw biomethanol was generated by 1.92, 2.23, 2.95, and 3.06 kg/day, containing 96% methanol.CO 2 conversions were 16.56, 19.70, 35.95, and 44.69% and H 2 conversions were 98.49, 78.88, 67.59 and 65.74% respectively.In addition, the RWGS of CO 2 by using H 2 at H 2 / CO 2 minimum ratio yielded the highest in H 2 conversion but not CO 2 conversion.However, the   1 and 2. Thus, 1 kg of biomethanol produced by the experiment was different from the 1.16 kg by theory (approximately 16%).

OPEX comparison
The price of raw materials and energy are based on prices in Thailand and exchange conversion (35 THB equal to 1 USD).The material and energy balance of molasses to biomethanol pathway between biogas and biohydrogen pathways are presented for economic evaluation (Table 3).
Price assumption is shown in Table 4.The OPEX of both pathways were calculated and presented in Table 5.
The OPEX for 1 kg of crude methanol by passing biohydrogen pathway experiment and theory were 4.4148 and 4.0912 USD compared with that of passing biogas pathway 0.3446 USD based on commercial methanol price 0.449 USD/kg.The OPEX of biohydrogen pathway were more expensive than that of the biogas route around 12.82 and 11.87 times, respectively.However, when adding by products, such as valuable organic substances, which were produced by the biohydrogen pathway, the total sale prices per kg biomethanol produced were 6.7243 USD (Exp.) and 5.7500 USD (Theory) compared with only single biomethanol obtained from biogas pathway at 0.4486 USD/kg of produced biomethanol.The margin caps were around 30.19%, 39.66%, and 40.55% for biogas pathway, biohydrogen experiment, and theory route, respectively.

Conclusion
Commercial methanol can be replaced by biomethanol using molasses as an abundant raw material.
Molasses can be digested under bacteria by biogas and biohydrogen pathway, syngas conversion, and MS.The choices were dependent on the following presented conditions.(1) The biogas pathway was appropriate in the case of sailing single products as biomethanol.
(2) The biohydrogen pathway was suitable for adding organic refinery to obtain income from valuable organic substances along with sailing biomethanol.

Figure 4 .
Figure 4. Process flow diagram of two options for Pathway A: Biogas (CH 4 /CO 2 ) experiment containing RF and MS in a Fixed-Bed Reactor, Pathway B: Biohydrogen (H 2 /CO 2 ) experiment containing RWGS and MS in a fixed-bed reactor.

Figure 5 .
Figure 5. Block flow diagram of molasses to biomethanol by passing biogas direction.

Figure 6 .
Figure 6.Block flow diagram of molasses to biomethanol by passing biohydrogen direction.

Table 1 .
Gas compositions by varying H 2 /CO 2 feed ratios.

Table 2 .
Gas compositions and biomethanol produced by varying H 2 /CO 2 feed ratios.

Table 3 .
Materials and energy balance of molasses to biomethanol by biogas and biohydrogen pathway.

Table 4 .
Price assumption (based on Thailand 35THB/USD) and catalyst duration.

Table 5 .
OPEX calculation for molasses to biomethanol by passing the biogas and biohydrogen routes.