Techno-Economic Study and Environmental Analysis for the Production of Bio-methanol Using a Solar-Aided Dual-bed Gasifier

The study aimed to evaluate the effect of utilizing solar energy as a heat source for gasification reactions during the production of methanol from corn stover. For this purpose, two biorefinery scenarios were modelled: a standalone scenario where gasification was performed in a conventional dual-bed gasifier, and a solar-aided scenario where solar energy was used to drive the gasification reactions. In the solar-aided scenario, biochar was exported as a co-product rather than combusted. Results obtained revealed that the incorporation of solar energy could enhance the net gasification efficiency by 10 to 24%, depending on the biomass moisture content. Also, the biorefinery energy conversion efficiency was found to be 48% for the standalone scenario and 61% for the solar-aided scenario. Moreover, the export of biochar as a co-product resulted in a 35% decrease in potential environmental impact. Furthermore, the methanol production costs could be 0.31 $/litre for the standalone scenario and 0.50 $/litre for the solar-aided scenario. While the minimum biochar selling price was estimated to be 13.04\ $/GJ (0.37 $/kg). These results suggest that the adoption of solar-aided gasification could be one way to advance the circular bioeconomy concept, where lignocellulose is used to produce not only fuels but also bio-products capable of gradually substituting fossil-based alternatives.


Introduction
Global carbon emissions have steadily increased over the last few decades, and unless proactive steps are taken, this trend is expected to continue.The adoption of a bioeconomy strategy, via the substitution of fossil-based fuels with biobased alternatives, may help to mitigate this trend [1].One way of embracing the bioeconomy approach is by producing syngas from biomass instead of fossil fuels.Indeed, syngas can be used not only for electricity generation but also for the synthesis of various fuels such as Fischer Tropsch liquid fuel, dimethyl ether (DME), and methanol.
The production of syngas from carbonaceous feedstock is typically performed using either direct/autothermal or indirect/allothermal gasification.During autothermal gasification, the thermal energy required for endothermic chemical reactions is supplied through partial oxidation within the gasifier.In this case, the gasification process is performed with air or oxygen-enriched air.The utilisation of air leads to the dilution of syngas with atmospheric nitrogen, which results in inferior quality syngas with a relatively low calorific value (~ 4 to 7 MJ/ Nm 3 ).Pure oxygen or oxygenenriched air can be substituted for air in syngas to increase its calorific value.This approach, however, requires expensive air separation units [2].Although autothermal gasification is effective in supplying the heat required for the gasification process, it results in the consumption of at least 30% of the biomass thermal energy for process heat [3].
In contrast to autothermal gasification, allothermal gasification is conducted in an oxygen-free environment.Hence, it does not require air separation.Here, steam, which is used as a gasification agent, reacts with biomass to generate syngas with a relatively high hydrogen concentration.The syngas produced is nitrogen-free, due to the air-free environment.Also, the syngas product obtained has a calorific value relatively higher than autothermal gasification (15-20 MJ/N m 3 ) [2].The heat needed to maintain the gasification temperature is supplied to the reactor either using heat exchangers or by circulating hotbed materials between a combustor and the gasifier [4].Fuel burned in the combustor includes carbonaceous by-products such as char.In some cases, a portion of biomass feedstock or syngas product is burned in the combustor for heat generation.As with auto-thermal gasification, one of the drawbacks of this gasification approach is the partial combustion of feedstock or product gas in the combustor, which minimizes the gas yield per unit of feedstock [5].
An alternative option to the conventional autothermal and allothermal gasification schemes is the utilisation of concentrated solar energy as the energy source of high-temperature process heat.This approach, which is often referred to as "solar-aided" or "solar-driven" gasification, is analogous to allothermal gasification in the sense that the heat needed to drive the gasification reactions is supplied by an external source of energy.The solar-aided gasification process, however, has several unique features that distinguish it from conventional allothermal gasification.For instance, it eliminates the need to partially combust the biomass feedstock or syngas products for heat generation.This results in more syngas produced per unit of feedstock, with less discharge of pollutants into the environment [6,7].Moreover, solardriven gasification enables the upgrade of biomass energy content by an amount equivalent to the enthalpy change of endothermic reactions.Furthermore, because of solar energy input, solar-driven gasification can be assimilated as a way of storing intermittent solar energy in a readily dispatchable chemical form, which can then be used on-demand as it is with fossil fuels [7].In addition, given that solar-driven gasification does not require oxygen-enriched air, the need for expensive oxygen separation technology is avoided.Also, solar-driven gasification has the potential to achieve temperatures as high as 1200 °C, which results in high-quality syngas [8][9][10].
In contrast to conventional gasification systems that have already been extensively studied and reviewed [11][12][13][14], solar-aided configurations have only gained substantial attention in recent years.Indeed, their multiple benefits have led to several studies being conducted on the subject.These include Ansari, et al., [15] who used computer simulation software to model the gasification of wheat straw, coconut shells, groundnut shells, and corncobs, utilizing concentrated solar thermal as an intermittent heat source.The process was designed to co-produce electricity and bio-fertilizer.The heat was supplied to the gasifier using concentrated solar power during sun availability and syngas combustion during sun unavailability.Biochar and ash generated in the gasifier were exported as bio-fertilizer, while syngas was combusted for electricity generation.Syngas combustion for electricity generation was found to be more energy-efficient than combusting biomass for power generation.Furthermore, in the solar-assisted configuration, the net electricity output saw a 34% increase per unit biomass feed.
Nickerson, et al. [16], developed five biomass gasification designs for economic assessment.Four of these were solaraided and one was based on conventional state-of-the-art gasification technology.The solar-aided configuration made use of a concentrated solar power system which comprised heliostats reflecting solar radiation to a focal point.Solar energy reflected on the focal point was absorbed by the cavity wall and subsequently transferred to a eutectic blend of carbonate molten salts.Biomass was then injected into the molten salt where the gasification process took place.It was found that solar-aided gasification could compete with conventional gasification in certain situations.The breakeven prices for solar-aided gasification ranged from 10.90 $/GJ to 4.04 $/GJ.
Vidal & Martín, [17] disclosed an integrated polygeneration system utilizing biomass gasification and concentrated solar power to produce electricity.The study involved the development of a superstructure considering both autothermal and allothermal gasification with two reforming modes (partial oxidation and steam reforming).The system was coupled with a concentrated solar plant consisting of a tower collector, molten salt storage tanks, and a regenerative Rankine cycle.Three options were considered for syngas utilisation: water gas shift for hydrogen production, an open Brayton cycle, and syngas combustion in a furnace to heat the molten salt.The optimal integration consisted of allothermal gasification, steam reforming and a Brayton cycle.This integration resulted in electricity generation of 340 MW, along with 97-kilo tons of hydrogen per year, which could translate into 0.073 €/kWh if the hydrogen is viewed as a credit.
Further studies conducted on biomass gasification with solar radiation as a source of energy include Ansari & Liu, [18].The study was performed with computer simulation software and employed concentrated solar power (CSP) as a heat source for a dual bed gasifier.Sand was used as a heat carrier and was fed into the gasification bed along with steam.The syngas intermediate product was used to generate power and Fischer Tropsch liquid fuel.During sun unavailability, syngas or recycled tail gas was combusted to supply heat to the gasifier.The hybrid configuration was found to have a peak net efficiency that was 45% higher than hybrid biorefineries previously disclosed in the literature.Moreover, it was reported that 17-18 kg of liquid fuel could be produced for every GJ of solar energy supplied to the biorefinery.
Although numerous works have been conducted on solar-aided biomass gasification, a comprehensive study assessing not only the production performance but also the environmental impact and manufacturing costs of potential co-products such as biochar is yet to be conducted.The present study addresses this knowledge gap by performing a techno-economic study and environmental analysis of a corn stover-to-methanol biorefinery integrated with a concentrated solar thermal system.The configuration employed for solar energy integration is a novel concept based on a modified TNEE (Tunzini Nessi Equipment Companies)-type dual bed gasifier, and utilising silica sand as a heat carrier.Two scenarios are considered: a standalone scenario where bio-char is combusted to generate the heat required in the gasifier bed and a solar-aided scenario where a solar thermal system is used to provide gasification heat, and bio-char is commercialised as a co-product.
The main objectives of the study are to: • Develop a model that can predict the syngas yield of a TNEE-type gasifier • Evaluate the impact of solar-hybridization on gasification efficiency, and biomass-to-methanol energy conversion efficiency • Assess the effect of solar hybridization on the potential environmental impact • Estimate the methanol manufacturing cost for standalone and solar-aided configurations, as well as the minimum biochar selling price

Method
The methodology used in the study involved the screening of existing literature on the thermochemical conversion of lignocellulosic biomass into liquid fuel, with an emphasis on solar-aided biomass gasification and biomass-to-liquid technology.The study conducted by Batidzirai, et al., [19] on lignocellulosic biomass supply for energy application in South Africa was used as a guideline to select an adequate biomass feedstock (corn stover) and the geographical location (Free State, South Africa).A process flow diagram was subsequently drafted.This was followed by a process simulation exercise involving various conversion scenarios.Mass and energy balance data obtained from the process simulation were then used to evaluate energy conversion efficiency as well as economic performance and environmental impact.
A graphical illustration of the methodology used during the study is shown in Fig. 1.

Logistic Configuration and Biomass Characteristics
2000 metric tons of corn stover (dry basis) were processed by the biorefinery per day, which is equivalent to an average of 83,333 kg/hr.Corn stover feedstock was assumed to be collected from regional biomass processing depots (RBPD) as proposed by Carolan, et al., [20].These facilities owned by farmers would be located near selected corn farms.They would serve as points of storage, transfer, pretreatment and collection after harvest using a configuration similar to that proposed by Yakan & Patel [21].An illustration of the biomass transportation operation is shown in Figure S-1 of the supplementary material (online resource).The pretreatment process would mainly include size reduction (to about 5 mm diameter), after which the biomass will be transported to the main biorefinery site for thermochemical processing.The characteristics of the corn stover feedstock (Ultimate and proximate analyses) are summarised in Table 1.
Corn stover's higher heating value (HHV) was calculated using the correlation developed by Noushabadi, et al. [22].The correlation is presented in Eq. 1, and was derived from experiments performed with 535 biomass samples.The lower heating value (LHV) on the other hand was estimated from the HHV, by subtracting the heat of evaporation of water present in the biomass feedstock.
Where N, H, C, O and S represent respectively the mass percentage of nitrogen, hydrogen, carbon, oxygen and sulphur in the biomass.

Design Scenarios
The biorefinery consisted of four conversion sections: pretreatment, gasification, methanol synthesis, and heat and power generation.Two scenarios were considered in the gasification section: a standalone scenario and a solar-aided ( 1) scenario.In the standalone scenario, biomass gasification was performed using an indirect gasifier in which biochar was combusted to generate gasification heat.While in the solar-aided scenario, gasification reactions were driven using concentrated solar power, and biochar was commercialised as a co-product.The raw syngas generated in each scenario was fed to the tar reformer where tar compounds are converted to various other compounds including CH 4 , CO, C 6 H 6 .The product from the tar reformer is fed to two consecutive water-gas-shift reactors where additional hydrogen is produced along with CO 2 .To meet the required stochiometric number and carbon oxide ratio, excess CO 2 is removed in a Monoethanolamine (MEA) absorption unit.The product gas exiting the MEA absorption unit is then sent to a methanol reactor where it is converted into methanol via CO 2 hydrogenation.A portion of the unconverted syngas is recycled back into the reactor to maximize conversion yield.While the remaining portion is combusted in the heat and power generation area to generate heat and electricity required in the processes.Methanol is purified using a distillation column.Overviews of standalone and solar-aided scenarios are provided in Figs. 2 and 3.The conversion processes were modelled and simulated using CHEMCAD 7.2.Further details on the gasifier operation for each scenario are provided in the subsequent sections.

Gasifier's Description and Operation
An indirect fluidized bed gasifier was selected to model the gasification process.This type of gasifier was chosen for its ability to uniformly transfer heat between biomass and oxidisers.Also, indirect fluidized bed gasifiers are known to achieve high carbon conversion, and low tar production compared to fixed bed gasifiers [23].Moreover, operation does not require pure oxygen.Hence, the need for an air separation unit (ASU) is avoided.In addition, it generally has a relatively long reactant residence time which leads to higher quality syngas [24].The setup used in the indirect gasifier was similar to the one developed by Tunzini Nessi Equipment Companies (TNEE).It consisted of two separate beds: a dense lowvelocity fluidized bed (LVFB) for gasification reactions, and a combustion bed comprising a high-velocity pneumatic riser (HVPR) for char combustion [25].It should be noted that one particularity of the TNEE indirect gasifier is its ability to process carbonaceous fuels with a relatively high moisture content (38% wt).Moreover, its operation does not require steam [25].
During the operation of TNEE indirect gasifier, biomass is fed to the bottom of the gasification bed, while hot sand along with unreacted char is conveyed to the bottom of the

Cold sand
Unconverted Syngas pneumatic riser.Low-velocity fluidization is achieved in the gasification bed by recycling a portion of the product gas into the gasifier.While in the combustion bed, air is injected bottom-up to create fluidization.Moreover, the pneumatic riser allows the bed to expand and subsequently overflow to the adjacent gasification bed.This results in hot sand circulation, and heat is exchanged from the combustion bed to the gasification bed.Exhaust gas and product gas remain unmixed throughout the process, which leads to high-quality syngas being produced.A detailed description of the type of dual fluidized bed gasifier design considered in this work can be found in the literature [24,25].

Gasifier Modelling Approach
To simplify the modelling process, the gasification bed was divided into two main zones: a primary reaction zone where the biomass was pyrolyzed into a mixture of CH 4 , H 2 , CO 2 , CO, char and tar compounds, and a secondary reaction zone where the pyrolysis product underwent further chemical reactions.To remain consistent with TNEE's gasifier design, it was assumed that 5% of the char would be conveyed to the secondary reaction zone.The remaining 95% would exit the gasifier bed and be sent to the combustion bed along with sand. Figure 4 illustrates the configuration of the dual fluidized bed employed, and also illustrates material flow through the system.

Modelling of the Pyrolysis Zone
Biomass pyrolysis is a complex process resulting in the formation of three major products: bio-char, bio-oil and non-condensable gases.The product yields are a function of several parameters, such as the type of biomass feedstock, processing parameters, and reaction pathways [26].Because of the complex nature of biomass pyrolysis, the pyrolysis product yield was predicted based on experimental data disclosed by You et al., [27], on corn stover pyrolysis.It is worth mentioning that You et al., [27]'s study is one of the few experimental works reporting on the effect of temperature on the product yield of corn stover pyrolysis/carbonisation.Most of the other studies are generally conducted with wood as feedstock.Table 2 shows the product yield of the pyrolysis correlation derived from [27].It is worth noting that the experimental results disclosed by You et al., [27] (refer to Table 2) reveal a marginal drop in char yield as the temperature gradually increases from 477 to 877 °C.On the other hand, the yield of condensable vapour shows a more pronounced decline, while gas yield sharply increases with increasing temperatures.Tar was modelled as a mixture of four compounds, mainly phenol, benzene, naphthalene and toluene.These compounds are among the prominent constituents of tar [28].

Kinetic Modelling of Gasification Zone
Products generated during pyrolysis were conveyed to the gasification zone where they were further heated up and underwent secondary reactions.The conversion taking place in the gasification zone was modelled using kinetic data.The sand temperature in the combustion zone was varied by manipulating the fraction of biochar combusted, which subsequently impacted the temperature in the gasification and pyrolysis zones.Table 3 shows the chemical reactions considered in the secondary reaction zone along with the kinetic data used during modelling.modelled using two separate forward and reverse kinetic rate laws obtained from Bustamante, [29] and Bustamante, [30].This is because as shown by [25], the combination of these forward and reverse WGS rate laws gives a more accurate prediction of syngas composition for a TNEE-type dual-bed gasifier.

Modelling of Combustion Sub-process
Char combustion was modelled using a stoichiometric reactor.The thermal energy released during char combustion was calculated from the heat of reaction.This energy was assumed to entirely passed on to the sand, which resulted in the sand being heated.For the sake of simplicity, sand was modelled as 100% silica and remained inert during the entire conversion process.It is worth noting that 95% of the char generated in the pyrolysis bed is sent to the combustion bed.The remaining 5% is conveyed to the secondary reaction zone.Complete oxidation of char is assumed in the combustor.

Solar-Aided Gasification Scenario
The gasifier employed in the solar-aided scenario had a gasification bed similar to the standalone scenario.The difference, however, was in the adjacent bed where char combustion was substituted for a concentrated solar power system (CSP) analogous to the one employed in power tower systems.Thus, sand was heated using solar energy rather than char combustion heat.Solar tower systems typically comprise a central receiver mounted on a tower.The receiver is then surrounded by a sizable heliostat field.The system is configured in such a way that sunlight from the heliostat's mirrors is directed to the receiver.To enable thermal energy absorption, the receiver is filled with a working substance such as water, molten salts, liquid sodium or air.Such systems can achieve temperatures as high as 2000 °C [32].Water is generally used as working substance when the goal is to produce steam for heat and power generation via the Rankine cycle.While air is used as working substance when the intent is to produce hot gases for turbines operating on the Brayton cycle [33].Molten salts and liquid sodium on the other hand are employed when the objective is to store thermal energy, which can be used later for steam generation or other heat demanding processes [34,35].In the present design however, desert dune sand is utilized as a working substance.The system is configured in such a way that a pneumatic riser drops the cold sand into the receiver, where it is heated to 980 °C before being fed to the secondary reaction zone of the gasifier's bed.Thus, sand plays the role of a working substance in the CSP system, and a heat carrier in the gasifier.To minimize the formation of hot spots, a static mixer is placed at the receiver's exit.The mixer is also expected to prevent sand agglomeration which, as demonstrated by Diago, et al., [36] could occur at temperatures in the vicinity of 1000 °C and above.Heat lost during mixing was assumed to be marginal, hence the hot sand exited the receiver at a temperature of 980 °C.An illustration of material flow in the solar-aided gasifier is shown in Fig. 6.
Furthermore, it was assumed that both the gasification and pyrolysis reaction mechanisms were similar to the ones taking place in the standalone dual bed gasifier.The mixture of char and cold sand exiting the gasification bed is fed to a settling tank separator where the sand is recovered at the bottom, and the less dense char is captured at the surface and then exported as a co-product.A natural convection dryer is then used to remove excess moisture from the sand before being recycled back to the solar receiver.Considering that the heat capacity of sand (710 J kg −1 K −1 ), is almost half that of molten salts (1542 J•kg −1 K −1 ) generally utilized for thermal energy storage [37], the amount of thermal energy required to raise the sand temperature to 980 °C was assumed to be half as large as the one required with molten salt.This resulted in a solar field twice as small as the one that would have been used if molten salt was the working substance.
Given the intermittent nature of solar power and the relatively high temperature required in the gasifier, the field used to harness solar energy was designed to supply a heat load 3 times greater than the gasifier thermal energy requirements.The relatively large solar field employed was a way of ensuring that surplus solar energy would be continuously stored for nighttime operations and days of low solar irradiation.This would minimize any unplanned production interruptions.

Modelling of the Solar Field for the Solar-Aided Scenario
The design parameters of the solar receiver are shown in Table 5.The System Advisor Model (SAM) was used to estimate the solar field size as well as the capital and variable costs of the solar power system.This simulation software, which was developed by the National Renewable Energy Laboratory (NREL), can provide information on the technical performance and approximate costs of a solar power system.The biorefinery was assumed to be located in Free State province, South Africa.Weather files

R13
of this location were downloaded from NREL National Solar Radiation Database (NSRDB) and used during the simulation to obtain location-specific results.Detailed information on the methodology employed for syngas cleaning and conditioning, syngas upgrade into methanol can be found in the supplementary material (online resource).

Evaluation of the Potential Environmental Impact
Chemical plants are prone to having a direct impact on their surrounding environment.This impact can be evaluated using the waste reduction algorithm (WAR).The WAR operates by quantifying the environmental impact a chemical would have if released into the environment.Unlike

CombusƟon zone
Fig. 5 Flowsheet of the biomass gasification process 1 3 conventional life cycle analysis which assesses the environmental impact of a process at every single stage (from raw materials extraction to product disposal), the WAR only focuses on a specific conversion process [38].Hence, this methodology is very useful at the design stage of chemical processes, as it can provide an early indication of the potential environmental impact across the boundaries of the system [39,40].
In the present work, the WAR algorithm embedded in CHEMCAD 7.1 was used to determine the potential environmental impact of both biorefinery scenarios.Considering that the feed streams for both scenarios were identical, the focus was placed on the output rate of potential environmental impact, also referred to as ̇I (t) out or PEI output index.Eight environmental impact categories were evaluated: Ozone Depletion Potential (ODP), Global Warming Potential (GWP), Smog Formation Potential (PCOP), Acid Rain Potential (ARP), Human Toxicity Potential by Ingestion or (HTPI), Human Toxicity Potential by Inhalation or Dermal Exposure (HTPE), Aquatic Toxicity Potential (ATP), Terrestrial Toxicity Potential (TTP).Essentially, a process with a relatively low ̇I (t) out is more likely to effectively dissipate the waste emitted than a process with a relatively high ̇I(t) out .Thus, the earlier process represents a more environmentally friendly design.

Conversion Efficiency
The conversion efficiency is an essential key performance indicator used in energy conversion systems such as biorefineries.The present study employs three types of conversion efficiencies: carbon conversion efficiency (CCE), net gasification efficiency, and energy conversion efficiency.The former is illustrated in Eq. 2 and represents the percentage of carbon in the biomass that is converted into methanol.
The second, which is the net gasification efficiency, is applied to the gasification process (refer to Eq. 3).It's the ratio of the net energy in the product gas to the total energy supplied to the gasifier minus the co-product credit, which in this case, is the bio-char generated in the solaraided scenario.Note that, the LHV of syngas was calculated by adding up the LHV of H 2 , CO and CH 4 exiting the gasifier ( The third conversion efficiency is referred to as biomass energy conversion efficiency and it is applied to the entire biorefinery.Biomass energy conversion efficiency was calculated as illustrated in Eq. 4, and takes into account the solar energy input where applicable, as well as the thermal energy of all products fuels including biochar. The last efficiency used (refer to Eq. 5) was the liquid fuel efficiency, and was obtained from Hamelinck, et al., [41].It should be noted that, in order to account for the portion of feedstock energy contained in the co-product, the thermal energy of co-product credit was subtracted from biomass feedstock.Thus, Eq. 5 measures the capability of the process to produce liquid fuel from the portion of the feedstock energy that is effectively gasified, leaving out the portion converted to biochar.

Biomass Gasification: Standalone Scenario-Model Validation
Model validation was carried out by operating the gasifier using parameters typically used in TNEE gasifier technology (refer to Table 4), then comparing the syngas composition against experimental data disclosed by Gourtay, et al., [42].
Considering that Gourtay, et al., [42]'s results were based on woody biomass, the pyrolysis correlation applied during model validation exercise was for woodchips, and was similar to the one used by Abdelouahed, [25] for the modelling of wood gasification using a TNEE-type technology.
As can be seen in Fig. 7, there is good agreement between simulation results and experimental data.Hence, the gasification model used in the present work can accurately predict the syngas composition of a TNEE-type dual bed gasifier.Furthermore, the present model predicts H 2 and CO 2 concentrations comparable to modelling results reported by Abdelouahed, [25].Discrepancies were however observed in the CO and CH 4 concentrations, where the values predicted by the present model were respectively 34 and 16% greater than the ones reported by Abdelouahed, [25].These discrepancies could be attributed to the differences in kinetic parameters employed to model chemical reactions taking place in the secondary reaction zone.

Biomass Gasification: Impact of Solar Hybridization on Net Gasification Efficiency
The incorporation of solar energy as the unique thermal energy source for the gasification reactions implied that biochar did not have to be combusted.As a result, the gasifier could generate two energetic products: syngas and biochar.Thus, as promoted by the circular economy production model, carbon is kept in the system instead of ending up as CO 2 .By assuming syngas to be the main product and biochar the co-product, energy credit status can be assigned to the biochar as per Eq. 5, which

=
LHV methanol LHV biomass × Biomass input − LHV co -product credit results in enhanced net gasification efficiency as shown in Fig. 8. Therefore, it can be deduced that the incorporation of solar energy as the heat source of a TNEE-type gasifier could enhance the net gasification efficiency by 56 to 87%, depending on the biomass moisture content.The enhancement in net gasification efficiency is mainly due to exported biochar, given that sand is entirely heated using solar energy.
Furthermore, as shown in Fig. 8, regardless of the scenario used, the utilisation of high moisture content corn stover feedstock in a TNEE-type gasifier has a detrimental effect on the net gasification efficiency.This trend can be attributed to two factors: firstly, the syngas calorific value which increases with decreasing moisture content.Secondly, the drier the biomass, the lesser the fraction of biochar needed to be combusted to achieve 980 °C in the combustor, and the greater the fraction of biochar that can be exported as a credit.
It is worth emphasizing that despite the enhancement in net gasification efficiency, the syngas flow rate remains identical to the standalone scenario.Thus, the incorporation of solar energy does not mean that more syngas is produced; rather, it allows the storage of intermittent solar energy in a dispatchable bio-char form, which can then be used ondemand as bio-fertilizer, fuel, or starting materials for the production of other bio-materials.

Methanol Production
The single pass conversion of conversion of carbon monoxide and carbon dioxide to methanol was 19.6%.The relatively low conversion yield was due to chemical reactions being limited by thermodynamic equilibrium.After recycling the unconverted syngas back to the reactor, the overall conversion of carbon monoxide and carbon dioxide in the fresh syngas to methanol exiting the reactor was enhanced to 97% (mol/mol); a figure close to the 96% reported by Luyben, [43] using similar processing conditions, and the 99% reported by Yusup, et al., [44] during the simulation of methanol.
It is worth mentioning that despite enhancing the methanol conversion yield, the implementation of a recycling loop is energy intensive.This is because large volumes of unconverted syngas have to be compressed before being recycled back into the reactor.In turn, this impacts the energy consumption of the plant.Further information on the effect of recycle loop on methanol production can be found in Luyben, [43].
After purification via flashing and distillation, 1055 kmol/ hr of methanol was recovered from the distillation column.Hence, 31% (mol/mol) of carbon initially present in the fresh corn stover is converted into methanol.This is equivalent to 1 3 an energy conversion efficiency of 48% for the standalone scenario and 61% for the solar-aided scenario.

Potential Environmental Impact (PEI)
Non-product streams were released into the environment in four main conversion areas: biomass gasification (flue gases from the combustion bed were released into the atmosphere); syngas cleaning and conditioning (stripper's effluent); heat and power generation (flue gases from the combustor), and methanol purification (effluent from the distillation column).Indices representing the PEI of these non-product streams were assigned by CHEMCAD 7.1 for each category.The overall potential environmental impact per unit of methanol produced (PEI/kg methanol ) was found to be 0.46 for the standalone scenario and 0.30 for the solar-aided configuration.On the other hand, the overall PEI/hr for the standalone scenario was found to be 1.65E + 04.This value decreased to 1.06E + 04 in the solar-aided configuration.As shown in Table 6, the Aquatic Toxicity Potential contributes the most to the environmental impact, followed by the global warming and Smog Formation Potentials.In addition, the solaraided scenario had an overall PEI/hr almost 40% lower than the standalone scenario.This result can be attributed to the elimination of char combustion in the solar-aided scenario which leads to less CO 2 being emitted into the environment.The utilisation of solar energy for biomass gasification could therefore considerably mitigate the environmental impact of a biomass-to-methanol biorefinery.
ODP Ozone Depletion Potential.

Land Use
A summary of land requirements for the solar field is displayed in Table 7.As can be seen, the utilisation of solar energy to drive the gasification process could require a total land area of 3.83 km 2 .Note that this figure does not include the refinery site, which on its own, could be as little as 3 km 2 if the refinery was to match a medium size crude oil refinery such as the Fos-sur-Mer refinery located in southern France, [45].Or, as large as 8 km 2 if the refinery site was to have a surface area comparable to SASOL's coal and gas refinery located in South Africa, [46].It can therefore be said that, although the use of solar energy to drive the gasification reactions might reduce environmental pollution and human toxicity, it would also require a substantial physical footprint.

Investment Analyses and Sensitivity Study
The production cost from techno-economic studies is generally dependent on assumptions made during the study.For this reason, it is essential to conduct a sensitivity study and assess the impact of the main economic and technical parameters on the cost of production.Three main parameters were considered in the sensitivity study.These included: biomass cost, total capital investment, internal rate of return and fixed operating costs.The supplementary material (online resource) provides detailed information on the methodology used to conduct economic analyses.
The present TEA is based on factored estimates of major process equipment.An exercise that carries a cost accuracy of 30% [47].The capital investment was therefore varied within a 30% margin and the response in methanol production cost was evaluated.The biomass cost, interest rate and fixed operating costs were also manipulated to determine their impact on production costs.As can be seen in Fig. 9, variations in total capital investment (TCI) and biomass price appear to have a more pronounced impact on the methanol production cost compared to the other manipulated variables.Thus, the successful commercialisation of lignocellulosic bio-methanol in South Africa will therefore require some stability in the biomass price.Moreover, major technology breakthroughs allowing the reduction in capital costs could also play a major role in boosting the competitiveness of bio-methanol.
Also, according to the methanol institute, which is a member of Methanol Market Services Asia (MMSA), from December 2019 and December 2022, the global price of methanol in key regional markets (United States Gulf Coast, Netherlands-Rotterdam, Coastal China) fluctuated between 200 and 500 $/tonne.During this period, the global methanol price averaged 375 $/tonne (≈0.295 $/litre) [48].This value was used in Fig. 9 for comparison purposes.When compared to global prices, the solar-aided scenario is unlikely to become cost-competitive even with a 30% reduction in total capital investment.On the other hand, a 30% reduction in total capital investment might allow the standalone scenario to compete with the global methanol price.

Impact of Solar Hybridisation on Methanol Production Cost
The minimum methanol production cost in the solar-aided scenario were estimated to be 0.50 $/litre.Hence the methanol selling price needs to be inflated by 0.19 $/litre to cater for expenses related to the incorporation of solar energy.Furthermore, to lower the production cost back to 0.31 $/ litre, the bio-char by-product should be sold for 13.04 $/GJ or about 0.37 $/kg.This value, which could be referred to as the "minimum bio-char selling price" was determined by keeping the price of methanol fixed at 0.31 $/litre and adjusting the price of bio-char until an NPV of zero is obtained.

Conclusions
The production of methanol from corn stover was studied using two gasification scenarios.The first scenario employed a TNEE-type dual bed gasifier whereby biochar was combusted to generate the heat required in the gasification bed.While in the second scenario, biochar was commercialised as a co-product and concentrated solar power was used to drive the gasification process.The main findings of the study can be summarised as follows: • The incorporation of solar energy as the heat source of a TNEE-type gasifier combined with the production of biochar as a co-product could improve the net gasification efficiency by 56 to 87%, depending on the biomass moisture content.• The utilisation of wet corn stover feedstock in a TNEEtype gasifier has a detrimental effect on the net gasification efficiency.• The biorefinery energy conversion efficiency was found to be 48% for the standalone configuration, and 61% for the solar-aided scenario.• The utilisation of solar energy to drive the gasification process might require a total land area of 3.83 km 2 , which is substantial.• The commercialisation of biochar as a co-product could result in a 35% reduction in the potential environmental impact.• The minimum methanol production costs were respectively estimated to be 0.31 $/litre and 0.50 $/litre for the standalone scenario and solar-aided scenarios.While the minimum biochar selling price was estimated to be 13.04 $/GJ (0.37 $/kg) • A sensitivity study revealed that variations in TCI and biomass price have a more pronounced impact on the methanol production cost compared to the interest rate, fixed cost and income tax.Major technology breakthroughs enabling the reduction of capital costs could therefore play a major role in boosting the competitiveness of bio-methanol.
The study is expected to add value to the bioenergy field, mainly in the design of low-emission bio-refineries.Furthermore, although the study was conducted using South Africa as a case study, the data generated, and the methodology used could be extended to any other geographical location with adequate lignocellulosic biomass resources and solar irradiance.Finally, it is recommended that future studies on solar-aided lignocellulosic biorefineries consider developing experimental pilot plants based on the configuration discussed in this work.Such pilot plants could help to ascertain the results obtained and optimize the conversion performances.

Fig. 1
Fig. 1 Graphical illustration of the methodology used for the techno-economic study

Fig. 2 Fig. 3
Fig.2Overview of main conversion areas used for scenario 1

Fig. 4
Fig. 4 Material flow in the dual fluidized bed gasification system

( 2 ) 4 )Fig. 6 5
Fig. 6 Illustration of material flow considered during the modelling of the proposed solar-aided gasifier

Fig. 7
Fig. 7 Comparison between syngas composition obtained during experimental works and present model.The gasifier operating conditions can be found in Table 4

Fig. 8
Fig. 8 Net gasification efficiency for standalone and solar-aided scenarios

Table 1
Characteristics of corn stover feedstock(Li et al. 2017) *Value derived from HHV calculated using Eq. 1

Table 4
on the other hand shows the process parameters of TNEE dual bed gasifier technology.A flowsheet illustrating the gasification process can be found in Fig.5.Note that the water gas shift reaction was

Table 2
Pyrolysis product yield (kg/kg dry fuel)

Table 3
Chemical reactions and kinetic data considered in the secondary reaction zone

Table 4
Key process parameters of TNEE gasifier technology *These temperatures can only be manipulated by adjusting the combustor's temperature and/or the sand flow rate GWP Global Warming Potential.PCOP Smog Formation Potential.ARP Acid Rain Potential.HTPI Human Toxicity Potential by Ingestion.HTPE Human Toxicity Potential by Inhalation or Dermal Exposure.ATP Aquatic Toxicity Potential.TTP Terrestrial Toxicity Potential

Table 6
Potential environmental impact output indices for each category (PEI/kg methanol )

Table 7
Key modelling results of solar energy technologies