Economic Analysis For Sustainable Utilization of Ethanol Production Residues From Grain and Inedible Plant Wastes


 The global initiative to find alternative fuel sources to fossil fuels is an ongoing process. As such, bioethanol is used as a fuel blend with petrol. However, large number of solid wastes is produced from ethanol plants sourcing from grain and inedible plant wastes, for example, WDGS (wet distiller’s grain with soluble) and DDGS (dry distiller’s grain with soluble) produced from ethanol plants using corn. This study investigates alternative methods for using these co-products through combustion and anaerobic digestion. Process simulation and economic analysis were conducted using current market prices to evaluate the viability of the processes. Products in the form of energy are produced. Optimization of the corn ethanol plant was also explored for re-using the heat and electricity produced in those processes. The profits of combustion and anaerobic digestion were compared. It was found that these processes will supply more viable options to simply selling the grain as feed for livestock. The anaerobic digestion of WDGS to produce electricity scenario was found to have the biggest profit among the four scenarios which can bring the annual income of 14.1 million Australian dollar to the ethanol plant. An environmental analysis of the CO2 emissions was also conducted. Using the Australian state emission factor, the amount of CO2 offset through both combustion and anaerobic digestion can be seen. The anaerobic digestion of WDGS to supply heat to the plant was proved having the largest CO2 abatement with the value of 0.58 kg-CO2e/L-EtOH.

used as a fertilizer or soil amendment. Thus, research into expanding the solid co-product uses is ongoing.
Cellulose recovery by extraction Xu et al. [13] has investigated the extraction of the cellulose (approximately 9-16 wt% in DDGS) from DDGS as well as directly from corn kernels. The cellulose obtained has potential to be used as an absorbent due to its capability to absorb water up to nine times of its weight. Additionally, it could be used for paper, textile materials, lms, bres, and chemical lters.

Production of biochar and carbon materials
Another alternative method currently being researched by Wang et al. [10] is the conversion of DDGS to form 3-dimensional porous activated carbons using microwave-assisted chemical activation. Properties of activated carbon is eligible for removing dyes and heavy metals from liquids.
In a study carried out by our group [11], the pyrolysis of grape marc to produce biochar, bio-oil, and biogas was proposed and has shown big advantage in energy compensation for the wine industry in comparison with combustion.

Combustion
Combustion of DDGS or WDGS from corn is a path currently being explored. In the research done by Wang et al. [19], the corn derived ethanol production plant was theoretically modelled so that all steam required can be provided by the combustion of WDGS. The bene ts of this process are to eliminate the requirement in drying WDGS and to replace the use of fossil fuels, hence reducing greenhouse gas emissions and optimising the overall production process. In our previous research [11], the technoeconomic analysis for the combustion of grape marc was also studied.
However, due to the lower content of carbon and higher content of oxygen in comparison to fossil fuels, solid co-products produced from ethanol industry has lower heating values. The low heating value can also cause ame stability issues. Moreover, the high moisture content can lead to ignition delay and consume more heat in moisture removal [20].

Anaerobic Digestion
One of the greatest issues faced when disposing of ethanol co-products is the high biochemical oxygen demand (BOD) with strict environmental laws in place. Anaerobic digestion of those solid co-products using microorganisms to produce biogas and liquid biofuels in the absence of oxygen is a sustainable and environmentally e cient method. The biogas produced typically consists of methane, carbon dioxide, ammonia, as well as hydrogen sulphide and water [21]. The methane produced from the digestion can be consumed locally inside the ethanol plant to reduce the consumption of fuel, with minimal clean up after its production.
In a study conducted by Belhadj et al. [22], the production of biogas was measured by assessing the stability of the process and the biodegradability of the bacteria applied. The stability of the reaction was optimal at a neutral pH range between 6.5 -7.5. The production rate of biogas was dependant on the rate of reactions and degradability of the microorganisms during anaerobic digestion. The anaerobic digestion was generally carried out at the temperature between 35 -55 ºC [23,24].
The anaerobic digestion of stillage can be tracked back to 1950s, mainly focusing the treatment of wastewater. Until 1980s the energy recovery by biogas from stillage was integrated in a biofuel plant in Germany [25]. Ziganshin et al. [26] illustrated the digestion of DDGS and WDGS. Aceticlastic methanogens and hydrogenotrophic methanogens were proved the proper catalysts for the digestion. Alkan-Ozkaynak and Karthikeyan [27] directly used the thin stillage generated from corn-ethanol plants in anaerobic digestion to produce larger volumes of biogas. Baez-Smith [28] investigates the potential to anaerobically digest vinasse from sugarcane through a mathematical model. It described anaerobic digestion as a highly e cient treatment for reducing BOD in wastewater, as well as producing biogas. It was noted that substrates highly rich in lipids and easily degradable carbohydrates exhibit higher methane potential than lignocellulosic materials [29].
The current process of selling DDGS and WDGS as food for livestock might not be economically viable due to the uctuating market value of the abundant product. The process of producing bioethanol from corn as well as from inedible plant wastes has potential to be optimised, through re-purposing its coproducts. Re-purposing solid co-products such as WDGS and DDGS will encourage the growth of the bioethanol industry in countries such as Australia and deter reliance on non-renewable energy sources, fossil fuels. This paper investigated alternative methods for utilising the co-products of the bioethanol industries such as DDGS and WDGS through combustion and anaerobic digestion. An economic analysis was conducted to assess the quality of products. To establish a basis of comparison, the value of selling the WDGS and DDGS as feed for livestock was also calculated. Optimisation of the corn ethanol process was explored through the recovery energy produced via combustion and anaerobic digestion of the biomass. This would ultimately improve the e ciency of the overall system and provide economic bene ts.

Methodology
The comparison of anaerobic digestion and combustion processes was performed using the mass and energy balance capabilities of Aspen Plus (v10). The non-random two-liquid (NRTL) uid package was selected in Aspen Plus when modelling. A techno-economic analysis was performed on the results from these simulations to assess the economic viability. An environmental analysis was also performed based on the modelling results.

Composition of the feedstocks
In this study, corn dry-grind ethanol by-products in the form of WDGS and DDGS as the model feedstocks (Table 1) were used to compare the energetic and economic balances within combustion and anaerobic digestion processes. It is noted that the dry base compositions of WDGS and DDGS are the same, but they have different moisture contents. Sulphur, nitrogen, and chlorine concentrations are negligible and hence omitted. All carbon and hydrogen will be fully oxidised to produce carbon dioxide and water, respectively, during the combustion. However, the processing via anaerobic digestion is more complicated and more detailed composition analysis is needed for the process simulation. The lower heating value of the dry matter was estimated by the contents of hydrogen and carbon with where C i is the mass content (wt%) and LHV i is the lower heating value of the combustible component i in the biomass, i refers the element hydrogen or carbon. The lower heating values of hydrogen and carbon are 119.96 and 32.8 MJ/kg, respectively [32].
It is well known that the structural composition of biomass can be characterized with the ratio of cellulose, hemicellulose, and lignin. 90% of lignocellulosic and 80% of herbaceous biomass are composed by cellulose, hemicellulose, and lignin inside the lignocellulosic substrates [33]. Both hemicellulose and amorphous cellulose can be easily hydrolysed, while crystalline cellulose and lignin are resistant to bioconversion [34]. Therefore, the composition of cellulose, hemicellulose, and lignin still cannot give us enough information about how much biomass is anaerobically digestible.
For the application of pyrolysis or gasi cation, the biomass is normally divided into volatiles, xed carbon, and ash, which is called proximate composition [33]. For anaerobic digestion, we are more interested in the digestible part, which overlaps but does not equal the volatiles content. Solvent extraction in addition of high-performance liquid chromatography (HPLC) analysis are applied in proximate analysis which supplies more meaningful information for anaerobic digestion. A Standard extraction procedure developed by the National Renewable Energy Laboratory (NREL) [35]. Kim et al. [35] further modi ed the procedure by doing the acid hydrolysis for the raw material. The composition of DDGS and WDGS from a variety of sources, as well as the values adopted in this study are listed in Table 2.
The proximate analysis groups the dry matter of DDGS and WDGS into crude protein, crude fat, carbohydrates, and ash. Carbohydrates contains crude bre (9.73 wt% of DDGS in this study) and the remaining nitrogen-free extracts (39.00 wt% of DDGS in this study). Thus, the item of carbohydrates refers to all other nitrogen-free extracts except crude bre.
Crude protein is the measurement of protein content, which can be digested into amino acids during the hydrolysis. The distribution of various amino acids was reported by Kim et al. [35], as shown in Table S1. The content of crude protein has a link with a so-called nitrogen factor as [35]: where C CrudeProtein and C Nitrogen are the mass contents of crude protein and the element nitrogen within the biomass, respectively, in wt%; NF represents the nitrogen factor with the value of 5.9 for DDGS and 5.4 for WDGS, depending on the resources of the biomass [35].
Crude fat, represented by the ether extract or the free lipid content, refers to the crude mixture of fat-soluble material present in a sample [36]. As the content of crude fat in DDGS and WDGS is small, we simply use oleic acid (C 18 H 34 O 2 ) to represent the average the formula of the soluble part of crude fat.  [39]. In this study, we assume that the carbohydrates include all nitrogen free extracts except for crude bre, such as acid detergent bre (ADF) and the extractives without the fat, with the average structure of dextrose (C 6 H 12 O 6 ) after the water treatment.
Fixed carbon is the solid carbon in biomass which remains in the char during pyrolysis [43].
Ash is composed of inorganic solid residua left after combustion, with the primary ingredients of silicon, aluminium, iron, and calcium oxides, and small amounts of magnesium, titanium, sodium, and potassium oxides, as well as the anthropogenic ash collected during harvest and handling [33,43]. During the anaerobic digestion, ash refers the inorganic contents in biomass left after all proximate analysis processes.

Process description
Based on a report in 2013, the mean production capacity across all 214 existing ethanol plants in the United States is 107.5 million gallon (321 thousand tonnes) per year, equalling to a median plant capacity [44]. In our simulation, a plant with an annual ethanol production of 100 million gallons (equating to 378.5 million litres or 298.7 thousand tonnes) was applied. Using the information supplied by Urbanchuk [45], the amount of DDGS and WDGS produced annually was calculated and the following assumptions were made: (1) the plant with capability of 2.99 x 10 5 tonnes of ethanol production, equaling to 3.79 x (assuming the same with WDG) is produced annually, (4) 2.8 x 10 5 tonnes of DDGS with moisture content of 10.12% is produced annually, and (5) it is assumed that the corn is already peeled and only the corn grain is to be used and there is no loss in grain mass during the drying process, other than the water removed.
The simulation was carried out with the following 4 scenarios: (1) combustion of DDGS to produce electricity, (2) combustion of WDGS to produce electricity, (3) anaerobic digestion of WDGS to produce heat, and (4) anaerobic digestion of WDGS to produce electricity.

Combustion of WDGS and DDGS
Simpli ed block diagrams of the combustion processes of DDGS and WDGS can be seen in Figures 2a and b. More details can be found in Figures S1a and b in the Supplement. The combustion of those solid co-products can be simply expressed as [11]: The combustion of WDGS involves drying the biomass to 10.12% of moisture before entering the combustor (Figures 2b and S1b) at 70 ºC [46,47]. The recycled hot ue gas from the combustion is fed into a separator to separate the dry biomass and the exhaust. The air for combustion enters the reactor as a separate stream. Heat from the combustor is used to heat water in a boiler. Ash is removed from the stream and the hot ue gas is recycled back to the drier.
Water entering the boiler is superheated to generate high pressured steam before entering the turbine. The resulting steam from the turbine then passes through a condenser cooling by ambient air. The turbine is assumed to operate at 70% isentropic e ciency.
The combustion processes of DDGS and WDGS are modelled very similarly. However, in the combustion of DDGS (Figures 2a and S1a), the drying section (the yellow section in Figure S1b) are eliminated, and the feed with 10% moisture enters the combustor directly. However, it should be noted that extra energy is already applied to evaporate the moisture of WDGS down to approximately 10% in DDGS.
Generally, a no-biological step so called disintegration which converts biomass particulate to carbohydrate, protein and lipids is included before anaerobic digestion, which is not discussed in this work. Thus, anaerobic digestion involves four main processes: hydrolysis, acidogenesis, acetogenesis and methanogenesis [23,[48][49][50]. Each process involves different bacteria and microorganisms and different optimal conditions. The overall reaction can be expressed as [21]: For anaerobic digestion of biomass, a biodegradable factor is applied to calculate the maximum conversion [29,51] with where f D is the substrate biodegradable factor, COD D is the degradable chemical oxygen demand (mg/L) representing the speci c methane yields, and COD T is the total chemical oxygen demand (mg/L).
Kim et al. [35] calculated the total digestible nutrients (TDN) in wt% of the DDGS by estimating the digestive factor of each part with where C i is the mass content (wt%) and The rst stage of anaerobic digestion is hydrolysis. The purpose of hydrolysis is to covert the degradable portion of biomass (carbohydrates, proteins, and lipids) into free monomers, water soluble fragments including sugars, amino acids, and fatty acids. Pre-treatments such as oxidation, and alkali and acid addition for structural modi cation of lignocellulosic substrates are necessary to increase the enzymatic digestibility of cellulose [52]. During the hydrolysis, cellulose is converted into glucose and hemicellulose into both pentoses and hexoses. Hydrolysis is not discussed in detail in this paper. We just simply assume that all soluble COD within stillage and syrup is hydrolysable. The soluble COD contents in the stillage of different inedible resources were reported by Cesaro and Belgiorno [23]. The amino acid contents generated from the hydrolysation of WDGS was reported by Kim et al. [35].
The hydrolyses of cellulose and carbohydrates result in the production of sugar molecule, glucose, formulated as C 6 H 12 O 6 [53]. The hydrolysed products of crude protein are a series of amino acids with the average molecular formula of C 3.7 H 7.2 O 2 N in this study, a little bit heavier than C 3 H 7 O 2 N of alanine. The mass balance needs to be calculated by considering that the production of one mole of glucose or amino acid consumes one mole of water during the hydrolysis step. The consumed water is in the WDGS already. All possible involved components are listed in Table S1, and the stream composition of WDGS after hydrolysis in this simulation is listed in Table S2.
Acidogenesis is a biological reaction where simple monomers are further hydrolysed into volatile fatty acids and gas components by facultative anaerobic bacteria, while acetogenesis is a biological reaction where volatile fatty acids are degraded into acetic acid, carbon dioxide, and hydrogen [54] with presence of acetogenic bacteria which is an obligatory H 2 producer. During the acetogenesis, amino acids are mainly degraded through Stickland reactions in which one amino-acid acts as an electron donor and the other as an acceptor [55], for example, Amino-acid can also be fermented with the presence of hydrogen-utilizing bacteria, a homo-acetogenic microorganism, for example,

R7
All hydrolysed organic compounds are assumed to be converted into methane by methanogens in this simulation.
There has been a lot of research published on the kinetics of anaerobic digestion by different groups [56-60]. However, the kinetic reaction rates and parameters vary a lot with the types of biomass and reactor, and reaction conditions. To simplify the simulation, yield-based reactors were applied by assuming all dissolved monomers and small molecules can be fully converted to produce biogas. The solubilities of the proximate components equalise the digestive factors showed in equation 4. The only exception is the conversion of hydrogen in Reaction 7 which was set as 93%. These processes are modelled with Aspen Plus (v10) using speci c amino acid reactions sourcing from the model applied by Serrano [56], including 45 compounds (as shown in Table S1).
The simpli ed reactions applied in this simulation were listed in Table S3. It is notable that the reactions applied in this study are only for the simulation purpose to meet the overall mass balance, the actual reactions may occur in different ways as discussed above.
Figures 2c and d depict simpli ed block diagrams of the anaerobic digestion process in two approaches: the biogas is simply burnt to produce heat ( Figure  2c) or is used to drive a steam turbine to produce electricity (Figure 2d). More detailed diagrams are shown in Figures S1c and d in the Supplement. The anaerobic digester was modelled as a two-stage plant with a ash vaporiser. The rst digester conditions are set for acidogenesis to undergo optimally at 55 ºC and atmosphere pressure, and the second digester for acetogenesis and methanation also at the same temperature and pressure [56]. Biogas and waste are separated from the ash unit. The biogas produced is cooled down to further reduce the moisture content and then is combusted producing heat (Scenario 3) or electricity (Scenario 4), and the waste is centrifuged to separate the liquid with the solid wastes.

Economic Analysis
An economic analysis is conducted to gauge the feasibility of their real-world applications. Capital costs are calculated using the equipment costs from reference values. The exponential method is used to estimate the equipment costs, which is based on existing costs and data from credible sources like a company or published data to establish a capacity-ratio exponent: When calculating the total cost of the plant, the following assumptions were made: (1) the non-variable operating costs equates to 5% of the total capital costs, (2) the investment term is 10 years, (3) the operation term is 10 years, (4) construction is undergone in 1 year, (5) interest rate is 5% of capital costs, and (6) depreciation is 5% of capital costs.
Selling the WDGS and DDGS at market value at the end of 2019 was used in comparation with the pro ts made from combustion and anaerobic digestion. , which is 0.14 A$/Nm 3 (N refers to the value at the standard conditions at 25 ºC and atmospheric pressure). The natural gas is assumed to be consumed for heating requirements in the corn ethanol process, which is 220 btu/bushel-corn (equivalent to 9.14 MJ/t-corn) [67] where btu refers British thermal unit.
Drying of WDGS to DDGS is also required eternal heat generated by combustion of natural gas. where LP is the loan payment (A$), LA is the loan amount (A$), and DF is the discount factor, r is the interest rate, and n is the loan life in years.

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Page 8/15 The environmental analysis was conducted accounting for the CO 2 emissions from the digestion processes as well as in comparison of combustion. The CO 2 offsets by combustion and anaerobic digestion are calculated.

Results And Discussions
The products from the combustion and anaerobic models are energy in the form of electricity or heat. The processes were modelled with the scale for the annual DDGS production of 2.8 x 10 5 tonnes and WDGS of 7.2 x 10 5 tonnes by assuming the plant is continuously operated for 42 weeks per year, in responding to the ow rates of DDGS and WDGS at 40.3 and 102.0 tonnes/hour, respectively, in which the dry mass equals 36.3 tonnes/hour.

Combustion of WDGS and DDGS
When modelling the combustion processes, the air was input with an excess of 25% of the stoichiometric requirement to ensure full combustion.
The combustion of DDGS yields a greater electricity output than the combustion of WGDS: 33.8 and 27.2 MW for combustion of DDGS and WDGS after offsetting the electricity consumption of the pump, as seen in Table 3. However, considering the large heat requirement for drying (153.7 MW) in the production of DDGS, using WDGS is bene cial as the waste heat of the combustion can be integrated into the drying of WDGS. In this simulation, the temperature of the drier was controlled at 75 ºC. Conversely, as the combustion of DDGS does not require a drier, the ue gas is discharged to the atmosphere at 110 ºC.

Anaerobic digestion of WDGS
As anaerobic digestion requires the biomass to be wet, WDGS is chosen as the feed, skipping the energetic costly drying process. The reactors are set to be operated at 55 °C. The yield of biogas is 1127.6 Nm 3 per tonne of dry mass by converting the ow rate of biogas into volume under standard conditions. As the biogas is a mixture of methane, CO 2 and hydrogen, and small amount of NH 3 , H 2 S etc. (Table S4), the puri cation system will be complex which has been well studied [70][71][72].
The biogas can be combusted to generate heat or electricity. The produced heat can be recycled back to the ethanol plant to offset the natural gas requirement. In addition of this, the biogas could also be used for (1) the production of synthetic methane via CO 2 methanation reaction [73,74]; (2) the production of renewable hydrogen via bi-reforming [75] followed by water-gas shift reaction [49,76]; and (3) biofuel production via reverse water-gas shift reaction [77] followed by fuel synthesis [48,78], depending on the need of market and the scale of plant.
As seen from the energy balance in Table 3, biogas combustion can generate 28.95 MW of electricity, even slightly larger than the electricity output of the direct combustion of WDGS. It is because the anaerobic digestion does not need the drying step. As we know, the electricity generation requires a higher capital cost as the turbine and boiler are the two most expensive items. Moreover, the energy loss during the energy conversion makes the process less e cient. The biogas can be directly burnt to generate 159 MW of heat. By considering the lower heating value of methane is 50 MJ/kg [32], the plant can save about 8.08×10 4 tonne of natural gas every year, which is 1.21×10 8 m 3 /year. For a small size of ethanol plant, this scenario might be more affordable. The waste sludge from the digester has little value and is centrifuged prior to disposal. The non-digestible solid waste separated by the centrifugation still maintains enough nutrition which can be applied as the compost and fertiliser after proper neutralization [79].

Economic Analysis
The capital expenditure (CAPEX) and main operating expenditure (OPEX) are also listed in Table 3 and plotted in Figure 3, in which the positive values represent the cash owing in and the negative ones refer the cash owing out. The OPEX includes the production and non-manufacturing costs. The production cost can be decomposed into variable and xed costs, in which the variable cost includes raw material and utility consumptions, and the xed cost can be further divided into the labour cost, plant overheads, the maintenance, the insurance, and the property tax. The non-manufacturing cost covers the corporate administration and selling expenses, and the investment in the research and the development, etc. [77]. Here we just simply separate the variable cost out of other operating costs. The main cash ow segments of all simulated scenarios are shown in Figure 3.
As seen in Table 3 The electricity value produced from the combustion of DDGS (Scenario 1) is higher than those produced from the combustion (Scenario 2) and anaerobic digestion (Scenario 4) of WDGS. However, the negative natural gas offset for the combustion of DDGS describes the additional energy costs consumed in the dryer to produce DDGS. Thus, a third of the natural gas requirement in the corn ethanol plant can be eliminated by direct processing WGDS in which the heat transferred from the recycled ue gas can offset the heat requirement for drying.
Like combustion process of WDGS, the natural gas value shown in Table 3 and Figure 3 depicts the value of natural gas offset by using the heat produced from the combustion of the biogas. As the current natural gas price is at a low level, The Scenario 3 is not pro To optimise the corn ethanol process, it is assumed that electricity produced from combustion is used in the corn ethanol plant, which offsets portion of the electricity requirement. It is also possible to sell the electricity back to the grid. In addition, the solid wastes could be used as the soil mediation purpose after neutralisation. Moreover, gas combustor will be more e cient than solid combustor.

Feasibility and sensibility analyses
It is clearly demonstrated from Table 3 and Figure 3 that the pro t of the plant is mainly determined by the prices of electricity, natural gas, DDGS, and WDGS.
The OPEX is affected by the prices of various parameters such as the feed cost, utility cost, product price, labour cost, as well as the scale of the plant [77]. To perform the sensitivity analysis, we only focus on the feed and utility costs within the range of k i (1 ± 20%) where k i refers to individual parameters. Then the OPEX changes relative to the base value are calculated as: which are plotted in Figure 4.
The prices of electricity and natural gas play a critical role in the sustainable utilization of ethanol production residues from grain and inedible plant wastes.
The electricity values generated from all processes except Scenario 3 are quite similar. When the price of electricity increases, it is expected that all electricity production scenarios will have more pro t. The price of natural gas has opposite effects to Scenarios 1 and 3. If the price of natural gas increases, the negative gain of Scenario 3 may drop down to an acceptable range and even will be overturned into positive when the price reaches 0.37 A$/m 3 as shown in Figure 5. Moreover, when the price of natural gas reaches 0.48 A$/m 3 , Scenario 3 will take turns Scenario 4 becoming the most pro table choice. In this case, drying WDGS to make DDGS will be more costly. The prices of feedstocks (DDGS and WDGS) have similar effects to all scenarios. In the extreme case, where the DDGS and WDGS have good market, they would be sold directly.

Environmental Analysis
The processing of biomass is a carbon-neutral process. The production of electricity or fuel offset from the processing of biomass will result in a reduction of energy input of the ethanol plant, and therefore lead a negative carbon emission to the environment. From a report issued by Australian government in 2020 [81], the emission intensity of electricity production in Victoria was found to be 0.98 kg-CO 2 e/kWh (e stands for emission). Thus, from the energy production and fuel offset, the CO 2 equivalent emissions for the combustion and digestion processes can be easily calculated.
As expected, all processes have positive bene t to the reduction of carbon footprint ( Table 3). The combustion of DDGS has the least advantage from the view of CO 2 emission due to the signi cant energy penalty during drying WDGS. All other three scenarios have similar effect on carbon emissions with the anaerobic digestion to produce heat (Scenario 3) as the best due to the large offset in natural gas. The CO 2 abatement from the amount of CO 2 sequestered by the anaerobic digestion process is more than the direct combustion of DDGS and WDGS as the solid waste rich of carbon produced from the anaerobic digestion can be disposed without direct CO 2 emission.

Conclusion
This project aims to research alternative methods of sustainably utilising these co-products in an environmentally and economically viable manner. WDGS and DDGS were used as examples to perform the simulation in this study. The sustainability of bioethanol production from grain and inedible plant wastes can be increased and potentially the ethanol production process can also be improved by introducing the processes investigated in this study.
The two alternative methods, combustion of WDGS and DDGS and anaerobic digestion of WDGS, are modelled with Aspen Plus software. The products yielded from these processes were energies in the forms of electricity and heat. From the results, a huge difference between the combustions of WDGS and DDGS was shown. By incorporating the drying process into the combustion process of WDGS without additional energy input, it offsets the energy requirement for current drying process in corn ethanol plant. Furthermore, the electricity generation reduces the reliance on the electricity grid. By investigating the combustion and anaerobic digestion of both WDGS and DDGS, it caters to a wider range of existing corn ethanol plants. Although with the current prices of WDGS and DDGS, electricity, and natural gas, it is still economically bene cial to maintain current processes to sell WDGS and DDGS as feeds for livestock, the processes investigated in this study are still seen to be more valuable. These will supply more attractive options based on the market prices of those variable operating expenditures. The alternative method of anaerobic digestion to produce electricity has environmental bene ts, pro table and is more energy e cient. Moreover, this process also has potentials as a pathway to other products such as bio-hydrogen, biomethane, and biofuel. The solid wastes produced from the anaerobic digestion can also be used in soil mediation purpose.
An economic analysis was conducted which accounted for estimates of capital, operating, and waste handling costs, where necessary. The products were quanti ed using current market values for industrial electricity and natural gas in Australian dollars. The combustion of WDGS offset both heat and electricity from the corn ethanol process and hence was the more economically lucrative process from the combustion of DDGS with the pro t of 10.84 million Australian dollars per year. The large amount of heat requirement during the production of DDGS makes the combustion of DDGS non-pro table on the current market. The anaerobic digestion of WDGS to produce heat is even worse. The anaerobic digestion of WDGS to produce electricity scenario generates the biggest pro t among the four scenarios which can bring the annual income of 14.10 million Australian dollar to the ethanol plant, which equals to 3.72 ¢/L-EtOH.
An environmental assessment of the CO 2 emissions was also conducted. As expected, the combustion of DDGS has a signi cantly higher CO 2 equivalent emission compared to other scenarios, while the anaerobic digestion of WDGS to supply heat to the plant has the largest CO 2 abatement with the value of 0.58 kg-CO 2 e/L-EtOH.