Biofuel production by solar-powered microwave pyrolysis of biomass: a case study on process optimization, environmental, and techno-economic aspects

doi:10.21203/rs.3.rs-1530278/v1

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Biofuel production by solar-powered microwave pyrolysis of biomass: a case study on process optimization, environmental, and techno-economic aspects

https://doi.org/10.21203/rs.3.rs-1530278/v1

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Microwave pyrolysis of corn stover has been optimized by Response surface methodology under different microwave power (500, 700, and 900 W) and three ratios of activated carbon additive (10, 15, and 20%) for obtaining maximum bio-oil yield followed by biochar. The optimal result has been evaluated and the environmental and techno-economic impacts of using solar-powered microwave heating have been tested. The optimal pyrolysis condition found to be 700 W microwave power and 10% of activated carbon. The yields of both bio-oil and biochar were about 74 wt.% under optimal condition. The higher heat values of 26 MJ/kg and 16 MJ/kg were respectively achieved for biochar and bio-oil. The major components of bio-oil were hydrocarbons (36%) and phenols (28%) with low oxygen-containing compounds (2%) and acids (2%). Using the solar-powered system, 20549 tonnes of CO₂ can be mitigated over the lifetime of the set-up, resulting in USD 51373 in carbon credit earnings, compared to 16875 tonnes of CO₂ mitigation and USD 42167 in carbon credit earnings from a grid electricity system. The payback periods for solar-powered and grid-connected electrical systems are estimated to be 1.6 and 0.5 years, respectively, based on biochar and bio-oil income of USD 39700 and USD 45400.

Biomass

Biochar

Bio-oil

Response surface methodology

Solar PV system

Techno-economic environmental assessment

Process optimization, techno-economic, and environmental assessment have been evaluated.
The yields of both bio-oil and biochar represent about 74 wt.% under optimal condition.
The HHV of 26 MJ/kg and 16 MJ/kg for biochar and bio-oil respectively were achieved.
The payback period are1.6 and 0.5 years when using solar-powered and grid electricity.
Monthly income of USD39700 and USD45400 earned using solar-powered system.

Valorization of underutilized agricultural residues represents a significant challenge in the context of their safe disposal and the potential recovery of energy in an environmentally sustainable manner. About 40 million tonnes of crop residue are generated every year in Egypt [1, 2]. Crop residues are mostly burned in the field. However, around 60% of the gross residue generated remains surplus [1–3], which becomes a great concern from the environmental point of view. Such a sustainable resource of bioenergy in Egypt (crop residues) needs to be properly utilized to strengthen the poor socio-economic conditions of the crop growers [4]. One such attempt is to derive biofuel and chemicals from underutilized crop residues in a sustainable manner. This option would ultimately assist to earn extra income and reduce environmental pollution by disposing of neglected and surplus agro-residues safely and effectively [4]. One of the most important crop residues in Egypt and across the world is corn stover (CS). The annual production of CS was approximately 7.2 Tg (Tera-gram) in Egypt [3]. Therefore, CS has been selected as a feedstock in this study because it can be considered a sustainable source of bioenergy.

Among the two usual processes, i.e., biological and thermochemical, for conversion of biomass into energy, the latter is preferable to convert the biomass into more useful products [5–6]. Pyrolysis is the rapid thermal decomposition of biomass into pyrolytic products such as biochar, bio-oil, and gas in the absence of oxygen [4, 7], with the goal of recovering both its chemical and energy value with high efficiency and low emissions [5].

Because of their easy storability and transportability, bio-oil and biochar produced by the pyrolysis process have more applications, both at the user and commercial levels, than pyrolytic gas (syngas) [6–8]. Biochar, a solid biofuel having a higher carbon content, offers many applications in day-to-day life because of its environmental friendliness [9–11]. Its applications are mostly in domestic cooking, boilers, production of activated carbon, absorbing microwaves, preparing nanotubes, conditioning the soil, etc. It is, therefore, easy to trade among the farming community [12, 13]. Likewise, bio-oil, one of the pyrolytic products from the thermochemical decomposition of biomass, is a dark brown and viscous organic liquid that offers several utilizations due to the presence of various chemical compounds in it [14, 15]. The availability of a number of chemical compounds, in bio-oil, increases its use as an alternate fossil fuel along with its enhanced utilization in the biochemical, packaging, and pharmaceutical industries [6, 16].

Microwave-assisted pyrolysis of biomass (MAP) in recent years has been considered as an emerging pyrolysis technology for saving time, energy, achieving higher heating efficiency, precise control over the process, and a higher yield of the desired products compared to conventional pyrolysis (CP) [17, 18]. One difficulty, however, commonly encountered during MAP is the low capacity of microwave absorption by the biomass feedstock due to its poor dielectric properties [19] Thus, in the MAP process, several methods have been investigated to improve the quality of both bio-oil and biochar. One of the most effective methods is using the additives in the MAP process [8, 20, 21]. Carbon-based additives are most effective in microwave pyrolysis, thus activated carbon (AC) from the process itself may be considered as an effective additive to improve the heating process and quality of the products [8–20–25]

Based on the literature, most of the MAP systems emphasize batch lab-scale with little attention to scaling-up the process in adopting the technology among the producers to make it economically viable [26]. In the agricultural sector, where a huge quantity of agro-residues are generated annually, the scaling-up of the MAP plants may be considered economically viable [27]. However, the initial high operating costs, as well as the economic benefits, are the most important aspects for crop growers to adopt this technology in a scale-up system.

One of the major constraints is the reliability the availability of electrical power for operating the system in rural and off-grid areas. Moreover, increased use of grid-electricity results in the indirect emission of harmful pollutants because its generation is mostly from coal-based thermal power plants, where the burning of coal produces greenhouse gases into the environment. Therefore, the integration of other sources of renewable energy, especially solar energy, as an input power for the MAP process has been found to be more feasible [28, 29].

From the above discussions, an attempt has therefore been made in this study to provide appropriate insight for valorizing one of the most underutilized agro-residues (i.e., CS) in an agrarian society from economical, environmental, and societal standpoints. Efforts have also been made to experimentally investigate the potential of extracting more useful and value-added products such as bio-oil and biochar from raw CS in a convenient manner by following MAP, integrated with reliable solar PV electricity, with the goals of safely disposing of them, mitigating greenhouse gas emissions by preventing improper uses, and creating avenues for earning extra income, particularly among resource poor crop growers. The process was firstly optimized then the products under optimal condition have been evaluated (quantity and quality). Techno-economic analysis for a 0.75 tonne/h capacity pyrolysis facility has been done based on our experimental investigation and evaluation of the quality and quantity of both products (biochar and bio-oil) by using an additive (AC). Also, the techno-economic and environmental assessment of the process in the scale-up system using grid electricity and solar PV have been evaluated.

2.1. Biomass feedstock and pyrolysis additives

Corn stover (CS) was collected from corn field after the corn crop was harvested. The CS was sun-dried until the excess moisture was removed, then mechanically shredded to reduce the particle size approximately of 10 mm. Table 1 shows the results of the investigation into the CS characterizations. The additive used in this study was activated carbon (AC) in granular form with particle sizes of 2 mm, specific surface area of 1050 m²/g, micropore volume of 0.35 cm³/g, and pore width of 1.8 nm.

Table 1

The composition and characteristics of raw corn stover.
Property	Value
Proximate analysis (wt.%)^a
Moisture content	5.12 ± 3%
Volatile matter	77.49 ± 5%
Ash	6.16 ± 2%
Fixed carbon^b	11.32 ± 3%
Ultimate analysis (wt.%)^c
C	44.47 ± 2%
N	0.97 ± 1%
H	6.19 ± 2%
S	0.48 ± 1%
O^b	47.88 ± 3%
Bulk density (kg/m³)	67.33 ± 2%
Higher heat value (MJ/kg)	16.71 ± 1%
a Dry basis.
^b Calculated by difference.
^c Dry and ash-free basis.

2.2. Experimental Set-up and Procedure

The current study used a modified domestic microwave oven (LG, 28 liter convection oven) with a 1-liter cylindrical quartz reactor. The diagram of the experimental set-up, as well as other essential information on the experimental set-up and procedures followed, are detailed in our previous work [28–30]. Based on our prior work [4], the experiments were conducted under different microwave power of 500, 700, and 900 W and different ratios of AC additive (10, 20, and 30%) with a constant reaction time of 15 min.

2.3. Optimization of pyrolysis conditions for bio-oil and biochar yields

Response surface methodology (RSM) based on central composite design (CCD) is one of the most successful experimental designs for optimization of process parameters with the less possible number of experiments [31]. The interrelation between independent and dependent variables can be clearly known with this method. In this study, the optimized levels of microwave power (500 W, 700 W, and 900 W) and the ratios of AC as an additive (10%, 20%, 30%) were considered for investigation

The effects of these parameters were investigated on the yields of bio-oil and biochar, the desirable products for this study. After knowing the optimal condition of pyrolysis for maximum yield of bio-oil followed by biochar, the physical and thermochemical properties of both products were evaluated and characterized. The independent variables were coded as microwave power (X₁), and additive to biomass ratios (X₂). Table 2 shows the details of the coded and actual values of the experimental design. The product yields of bio-oil and biochar were chosen as dependent variables. For statistical calculations, the variables X_i are coded as x_i according to Eq. (1).

$${x}_{i}=\left({X}_{i}-\frac{{x}_{0}}{\varDelta x}\right)$$

Where ${x}_{i}$, ${X}_{i}$, ${x}_{0}$, and $\varDelta x$ are respectively the coded value for any independent variable, actual value, central value of the independent variable and step change. A 2² factorial CCD and 5 (4 axials and one central) points and 3 replications were employed to optimize the pyrolysis conditions for maximum yields of bio-oil and biochar. The second-degree polynomials as per the Eq. (2) were calculated using the Design-Expert® 13 software statistical package (DOE 13, Stat-Ease, Minneapolis, MN, USA) to estimate the response of the dependent variables.

Table 2

Coded and actual values for the experimental ranges of independent variables.
Variables	Coded	Range and levels
Variables	Coded	-1	0	+ 1
Microwave power (W)	X₁	500	700	900
Additive to biomass ratio (%)	X₂	10	20	30

$${Y}_{i}= {a}_{1}+ {a}_{2}{X}_{1}+{a}_{3}{X}_{2}+{a}_{5}{X}_{1}{X}_{2}+{a}_{4}{X}_{1}^{2}+{a}_{6}{X}_{2}^{2} \left(2\right)$$

Where the parameters ${a}_{1}, {a}_{2},{a}_{3},{a}_{4},{a}_{5}, \text{a}\text{n}\text{d} {a}_{6}$ are regression coefficients, while ${Y}_{i}$, ${X}_{1}, and {X}_{2}$are predicted response and the independent variables microwave power and additive to biomass ratios respectively. The experimental data from the results of the model were analyzed statistically and the fitness of the predicted model was checked by the R² value for providing the validation percentage of the model. Furthermore, the statistical significance of the independent parameters, considered in the study, was performed by the F-test method using a one-way analysis of variance (ANOVA) table at a significance level of 0.05 (p-value ≤ 0.05).

2.4. Product characteristics

The proximate analysis determines the components such as fixed carbon, volatile matter, moisture content and ash content present in feedstock and biochar for assessing the combustible and non-combustible constituents present among them. Ultimate analysis was done to determine the elemental composition such as CHNS of biomass or its pyrolytic products. The elemental analyses were conducted using Elementar (Germany), model UNICUBE CHNS/O analyser as per standards ASTM D 5291 and ASTM D1552. The Higher heat value (HHV) for raw CS and its biochar were measured by bomb calorimeter, while for bio-oil, it was calculated with the help of standard equation [32].

The water content in the bio-oil was measured using VEEGO/MATIC-D Volumetric Karl Fischer titration according to ASTM E203. The acidity, dynamic viscosity, and density of bio-oil were also measured at 25 ℃. The chemical compositions of bio-oil samples were identified by gas chromatography-mass spectrometer (GC-MS) (Agilent 5890–5973). There is the provision of HP-5 capillary column in the system. The carrier gas was helium (99.999% purity) with constant flow rate of 1.2 mL/min. The volume of injected sample (1:20 split ratio) was 1 µL. The temperature of the oven was initially set at 40 ℃ for 40 min in the system program, then it was raised by 5 ℃/min to 250 ℃ and held for 5 min. The injected and detector temperatures were maintained at 250 ℃ and 230 ℃, respectively. Different compounds present in the bio-oil sample were identified by comparing their mass spectra with standard mass spectra data library of National Institute of Standards and Technology (NIST).

The parameters, considered for estimating the techno-economic and environmental assessment of the practice followed in this study include initial cost of the experimental set-up, components used under solar photovoltaic (PV) system, cost for life cycle analysis of the system, scope for mitigating the emissions of CO₂ and earning of the carbon credit potential through it. This assessment is evaluated in the scale-up system using grid electricity and using solar PV power. The scale-up system is considered to process about 1125 tonnes feedstock annually (750 kg per batch, 5 batches/day, and 300 working days/year) [33]. Four laborers are considered to be engaged per day to prepare the feedstock and to operate the system as seen in Table 3.

Table 3

Description of operating conditions of scale-up microwave pyrolysis production system.
Description	Unit	Scale-up production system	References
Operating conditions
1- System capacity^a	kg/batch	750
2- Operation time	h/day	10	[13]
3- Total duration per batch^b	h	2	[13]
4- Number of batches per day	Batch/day	5
5- Annual operating time^c	h/year	300
6- Feedstock	tonnes/year	1125
7- Number of labors	-	4
8- Operating time of the labors	h/year	12000
Operating cost measurements
1- Transportation of feedstock to project site and pyrolytic products to market	%	2% of capital cost
2- Maintenance	%	4% of capital cost	[33]
3- Insurance and taxes	%	1.5% of capital cost
4- Consumables	%	4.5% of capital cost
5- Other miscellaneous expenses	%	2% of capital cost
Labour cost^d	USD/year	Number of operating hours per year × wage cost per hour (UAD1.2/h)
Feedstock cost^e	USD/year	Amount of feedstock (tonne) per year × cost of the feedstock per unit (USD40/tonne)
^a System capacity for scale-up system it assumed to be 750 kg/batch.
^b Total duration of the batch includes preparing the sample, reaction time, cooling down, and collect the products.
^c Annual operating time considering working days is 300 days per year and operating time per day for both systems.
^d Based on the average wage in Egypt (https://tradingeconomics.com/egypt/minimum-wages).
^e Based on the market survey of corn stover in Egypt [1].

3.1. Response surface analysis

Figure 1 shows the influence of quadratic terms of the dependent parameters on bio-oil and biochar yields along with predicted versus actual responses for each product when using AC as an additive. The quadratic model equations for response Y_Bio−oil and response Y_biochar were obtained using the results of the experiments and presented in equations 3 and 4 respectively. The relation between predicted versus actual responses for bio-oil and biochar when using AC as an additive were presented in Fig. 1a (right) and 1b (right) respectively.

$${Y}_{Bio-oil}=-35.24+0.221735 {X}_{1}+0.835176 {X}_{2}-0.000095 {X}_{1}{X}_{2}-0.000168 {X}_{1}^{2}-0.020694 {X}_{2}^{2} \left(3\right)$$

$${Y}_{Biochar}=57.18-0.049284 {X}_{1}+0.056719 {X}_{2}+0.000071 {X}_{1}{X}_{2}+0.00001 {X}_{1}^{2}-0.005037 {X}_{2}^{2} \left(4\right)$$

From Fig. 1, it can be seen that the actual values are relatively distributed close to the linear line, indicating a high predictive value for the dependent variable under the studied range. Table 4 presents the analysis of variance (ANOVA) for optimization of bio-oil and biochar yields using AC additive.

From Table 4, the ANOVA results, the P-values for bio-oil (< 0.0001) and for biochar (0.0004) were less than 0.05, suggesting that these equations for the resulted models were statistically significant at a 95.0% confidence interval (p < 0.05) to describe the yields of bio-oil and biochar. The coefficients of determination (R²) for the bio-oil and biochar response (equations 3 and 4) were found to be 0.97 and 0.95 respectively. This implies that microwave power and different AC additive to biomass ratios under the studied ranges had a significant catalytic influence on bio-oil and biochar distribution yields during MAP of CS.

Table 4

Analysis of variance (ANOVA) for optimization of bio-oil and biochar yields using AC additive.
Source	SS^a	DF^b	MS^c	F-value	p-value	Remarks
Bio-oil
Model	399.18	5	79.84	35.35	< 0.0001	significant
X₁- Microwave power	72.77	1	72.77	32.22	0.0008	significant
X₂- Additive ratios	2.79	1	2.79	1.24	0.3029
X₁X₂	0.14	1	0.14	0.0639	0.8076
${X}_{1}^{2}$	313.39	1	313.39	138.76	< 0.0001	significant
${X}_{2}^{2}$	29.79	1	29.79	13.19	0.0084	significant
Residual	15.81	7	2.26
Lack of Fit	15.81	3	5.27
Pure Error	0.0000	4	0.0000
Total	414.99	12
Biochar
Model	367.17	5	73.43	21.06	0.0004	significant
X₁- Microwave power	356.50	1	356.50	102.25	< 0.0001	significant
X₂- Additive ratios	7.21	1	7.21	2.07	0.1937
X₁X₂	0.08	1	0.08	0.0233	0.8830
${X}_{1}^{2}$	1.19	1	1.19	0.3416	0.5773
${X}_{2}^{2}$	1.77	1	1.77	0.5063	0.4998
Residual	24.41	7	3.49
Lack of Fit	24.41	3	8.14
Pure Error	0.0000	4	0.0000
Total	391.58	12
^a Sum of squares.
^b Degree of freedom.
^c Mean square.

3.2. Products yield and their characteristics

Table 5 shows the main properties of biochar and bio-oil from microwave pyrolysis of CS using 10% of the AC additive. As can be seen, from Table 5, the yields of both bio-oil and biochar, under optimal pyrolysis condition, represent about 74 wt.% (29 wt.% biochar and 45 wt.% bio-oil) of the total weight of raw CS used in the experiments. This clearly indicates that the major percentage of raw CS feedstock is converted into solid and liquid products as compared to the gas product (26 wt.%). The biochar with HHV of 26 MJ/kg was achieved compared to 16.7 MJ/kg for raw CS and can be used as a solid biofuel in many applications.

Table 5

Main properties of biochar and bio-oil from microwave pyrolysis of corn stover with activated carbon.
Biochar characteristics		Bio-oil characteristics
Property	Value	Property	Value
Proximate analysis (wt.%)^a
Volatile matter	30.79 ± 5%	pH value	5.27 ± 3%
Ash	13.18 ± 3%	Dynamic viscosity (mPa.s)^d	2.89 ± 5%
Fixed carbon^b	56.03 ± 3%	Water content (%)	38.92 ± 6%
Ultimate analysis (wt.%)^c		Ultimate analysis (wt.%)^c
C	71.10 ± 7%	C	31.15 ± 1%
N	0.2 ± 3%	N	0
H	2.7 ± 7%	H	11.24 ± 2%
S	0	S	0
O^b	26 ± 9%	O^b	57.61 ± 2%
Bulk density (kg/m³)	33 ± 3%	Density (g/mL)^d	1.036 ± 1%
HHV (MJ/kg)	25.79 ± 3%	HHV (MJ/kg)^e	16.36 ± 3%
Yield (wt.%)	29 ± 2%	Yield (wt.%)	45 ± 4%
Energy yield (%)	43.45 ± 2%	Energy yield (%)	43.29 ± 3%
^a Dry basis.
^b Calculated by difference.
^c Dry and ash-free basis.
^d Density and viscosity measured at 25 ℃.
^e Calculated by the equation HHV (MJ/kg) = 0.3382 C% + 1.4428 (H% − 0.125 O%).

In addition, the energy yield from biochar with respect to the energy content in the raw CS was about 44%. Also, the biochar produced from the present study contains low content of nitrogen and was free of sulphur compared with raw CS biomass. This indicates that the biochar produced can be utilised as an environmentally friendly and clean solid biofuel to replace conventional coal, reducing greenhouse gas emissions such as CO and CO₂, as well as harmful gas emissions such as SOx and NOx into the atmosphere.

For bio-oil, Table 5 shows that the HHV and energy yield from bio-oil reached 16.4 MJ/kg and 44% respectively which is due to the higher yield of bio-oil. Also, the bio-oil free from sulphur and nitrogen contents represents a beneficial feature for its use as a cleaner biofuel with respect to reduced emissions of greenhouse gases to the atmosphere. The moderate values of pH (5.3), viscosity (2.89 mPa.s), and water content of 38.92% of the bio-oil were also achieved. The GC-MS analysis of the bio-oil revealed that the chemical composition was classified into 9 groups as well as the unidentified compounds (as per a match factor of 85%). These groups are acids, phenols, furans, guaiacols, esters, alcohols, hydrocarbons, ketones/aldehydes, and nitrogenous compounds. The contents of hydrocarbons and phenols in the bio-oil were up to 36% and 28% respectively with low oxygen-containing compounds (2%), low acids (2%), esters (1.2%), and alcohols (2%), in addition to furans content of 6% and ketones/aldehydes of 7%. As a result of the increased quality of both bio-oil and biochar, produced from this study, both the products can be considered as a source of biofuel in many applications.

Techno-economic environmental assessment

For scale-up system, it is reported that a 150 kW microwave reactor can process 750 kg of feedstock per batch [34] with 1 h duration for completion [33] of the process and performing 5 batches for 10 working hours in a day [13]. The pyrolysis activities can be undertaken for 300 days per year. The electricity required to operate the system was calculated to be 780 kWh per day [(150 kW microwave reactor + 4 kW water pump + 2 kW temperature monitoring set-up) × reaction time per batch (1 h) × number of batches per day (5)]. In the case of operating the system by solar PV system, the required PV system should be sized accordingly. The sizing and cost estimation of solar PV system for both the systems have been presented in the Table 6 as per the calculations and explained in Appendix A.

Table 6

Sizing and cost estimation of solar PV system.
Component	Cost per unit (USD/unit)^a	Scale-up solar-powered production system
Component	Cost per unit (USD/unit)^a	Number	Sizing		Total cost (USD)
1- Solar module	0.47/W_p	780	300 W_p		109980
2- Charge controller	0.86/A	195	50 A x 24 V		8385
3- Batteries	0.59/Ah	2490	300 Ah x 12 V		440730
4- Inverter	0.14/W	26	10,000 W		36400
5- Wiring and connections ^b					29775
Total				625270
^a The cost was calculated based on Egyptian market survey exchange rate.
^b Wiring and connections was assumed to be 5% of the sum of solar module, charge controller, batteries and inverter [35].

Table 7 illustrates the estimation of total expenditures with respect to capital and operating costs as well as the revenues to be earned from both the production systems. The capital cost has been estimated based on the studies carried out by the previous researcher in microwave pyrolysis [33] considering a 3% escalation factor per year for the price of the components. Operating costs, such as feedstock and pyrolytic product transportation, maintenance, and other miscellaneous costs, are estimated as percentages of the capital cost (see Table 3). The cost of the electricity required to operate the system (in case of grid electricity used) has also been calculated.

Table 7

Techno-economic and environmental assessment for scaling-up microwave pyrolysis production system of corn stover.
Item	Unit	Production system
Item	Unit	Solar-powered system	Grid electricity system
System capacity	kg/batch	750	750
Capital cost
1- Solar PV system	USD	625270^a	-
2- Microwave reactor system^b	USD	272000	272000
3- Total capital cost	USD	897270	272000
Operating cost
1- Feedstock^c	USD/year	45000	45000
2- Labour^d	USD/year	14400	14400
3- Electricity^e	USD/year	-	18720
4- AC additive	USD/year	2160	2160
5- Maintenance^f	USD/year	35890	10880
6- Transportation^f	USD/year	17945	5440
7- Consumables^f	USD/year	40377	12240
8- Insurance and taxes^f	USD/year	13459	4080
9- Other miscellaneous expenses^f	USD/year	17945	5440
10- Total operation cost	USD/year	187176	118360
Annual products yield
1- Biochar^g	Tonne/year	315	315
2- Bio-oil^h	Liter/year	506250	506250
Annual revenues
1- Biocharⁱ	USD/year	157500	157500
2- Bio-oil^j	USD/year	506250	506250
Total annual revenues	USD/year	663750	663750
Net annual income^k	USD/year	476574	545390
Payback period	Year	1.9	0.5
Monthly income	USD/month	39700	45400
Environmental assessment
1- CO₂ mitigation^l	Tonne/life	20549	16875
2- Credit earned from CO₂ mitigation^m	USD/life	51373	42187
Note: The cost was calculated based on Egyptian market survey exchange rate. ^a Based on the estimation in Table 6. ^b Total cost of the microwave pyrolysis system include the reactor, microwave system, condensation system, N₂ gas, and temperature monitoring system was estimated based on previous work in microwave pyrolysis [33]. considering 3% escalation factor per year (https://fred.stlouisfed.org/), [200000 + (200000 × 3/100 ×12 year)] = USD272000. ^c Feedstock cost was estimated to be USD40/ton, the annual cost of the feedstock was USD45000 (1125 ton/year × USD40/ton). ^d Labour cost was estimated to be USD1.2/h (Table 3), the cost was USD14400 (12000 h/year × USD1.2/h). ^eThe cost of electricity when using grid electricity, was estimated to be USD18720 per year (780 kWh per day × USD 0.08/kWh (Rs6/kWh) × 300 working days/year). ^f Maintenance, transportation of the feedstock and pyrolytic products, consumables, insurance and taxes, and other miscellaneous costs were estimated to be 4%, 2%, 4.5 %, 1.5 %, and 2% of capital cost respectively [33]. ^g Annual yield of the biochar was 28 wt.% from annual feedstock consumed (1125 tonne/year × 28 wt.%/100 = 315 tone biochar/year). ^h Annual yield of the bio-oil was 45 wt.% from annual feedstock consumed (1125000 × 45 wt.%/100 = 506250 kg bio-oil/year). Considering the density of the bio-oil is 1.036 g/mL (~1 g/mL) (Table 5), thus the annual bio-oil yield was estimated to be 506250 L/year. ⁱ Selling price of biochar as fertilizer or soil conditioner and fuel in domestic cooking was estimated to be USD500/tonne. ^j Selling price of bio-oil for chemical and phenol extraction was estimated to be USD1/L. ^k Net annual income is equal to total annual revenues - total operation cost. ^l Mitigation of CO₂ emission during 10-year life time of the set-up = [(1.57 kg/kWh/1000 × E_out kWh/year × life of set-up) + (1.5 kg CO₂emitted per kg of agro-residue burning × kg of feedstock pyrolyzed per year × life of set˗ up)]. ^m Credit earned from carbon mitigation = (USD2.5 per tonne CO₂ × CO₂ emission mitigated by set up tons per life) (www.ecx.eu).

The cost of the electricity, when using grid electricity, was estimated to be USD18720 per year (780 kWh per day × USD0.08/kWh (Rs6/kWh) × 300 working days/year). Feedstock cost is estimated to be USD40 per tonne (based on farmer’s survey in Egypt). The proposed scale-up system is assumed to process about 1125 tonnes (750 kg per batch × 5 batches/day × 300 working days/year) as seen in Table 3. Percentages in the yield of pyrolytic products were estimated to be 45 wt.%, 28 wt.%, and 27 wt.% for bio-oil, biochar and pyrolytic gas respectively as per our experimental condition when pyrolyzed at 700 W microwave power and using AC additive (Table 3).

These pyrolysis conditions were selected as it produced the higher yields of bio-oil along with biochar (desirable products for the present study) with high quality. Also, the AC additive can be used as an additive repeatedly used in the MAP process system for 25 days [4, 24]. The cost of AC is USD2.4/kg and the required quantity per batch is 75 kg (750 kg feedstock/batch × 10/100 ratio of AC).

Thus, the total cost of using AC was estimated to be USD180 per 25 days (75 kg AC × USD2.4/kg). The total cost of AC per year is estimated to be USD2160 per year (USD180 per 25 days × 300 working days per year). The revenues to be generated by trading the pyrolytic products i.e., bio-oil and biochar have been estimated, based on the current prevailing prices as per the Egyptian market, considering their quality achieved in this study. The bio-oil produced in this study has a high proportion of hydrocarbons (36%). Phenols compounds accounted for the second-highest proportion (28%). In addition, the bio-oil had a low concentration of guaiacols compounds (2%) and acids (2%). Thus, it is considered to be a good source for phenols and chemical extraction. Therefore, the cost of the bio-oil is estimated to be USD1 per kg and the density of the bio-oil is estimated to be 1.036 g/mL (~ 1 g/mL), thus price is USD1 per liter [33, 36, 37]. The annual bio-oil production is estimated to be 506250 kg (liter) (1125 tonne feedstock per year × 1000 × 45 wt.%/100 bio-oil yield).

For biochar, the selling price is estimated to be USD500/tonne [33, 34, 38]. The annual biochar production was estimated to be 315 tonne biochar per year (1125 tonne/year × 28 wt.%/100 = 315 tone biochar/year) as per the optimal pyrolysis conditions. For the calculation of the quantity of CO₂ emission, it has been reported that about 1.5 kg CO₂ is emitted to the environment by burning 1 kg of CS feedstock [1, 2]. The coal based thermal power plant generally produces about 0.98 kg of CO₂/kWh. However, taking into account, the losses during distribution and transmission, the emission of about 1.57 kg of CO₂/kWh is expected from the thermal power plant based on the coal materials [35]. The emission of CO₂ may thus be mitigated by the use solar PV electricity. Thus, the mitigation of CO₂ emission during 10-year lifetime of the set-up was estimated to be 20549 tone/life for solar-powered system [(1.57 kg/kWh/1000 × E_out kWh/year × life of set-up) + (1.5 kg CO₂ emitted per kg of agro-residue burning × kg of feedstock pyrolyzed per year × life of set˗ up)]. Whilst it estimated to be 16875 tonne/life for grid electricity system. Carbon credit earned from the mitigation of CO₂ to the environment is estimated to be USD51373 per life and USD42187 per life for solar-powered and grid electricity respectively by considering the current price of carbon credit to be USD2.5 per tonne of mitigating CO₂ (USD2.5 per tonne × CO₂ emission mitigated by set up tons per life) (www.ecx.eu).

From the techno-economic analysis, the scaling-up of the MAP system for treating the agro-residues can be economically and environmentally beneficial. It is assessed that by using grid electricity, the payback period would be very nominal (0.5 year) compared to using a solar-powered system (1.9 years), resulting into earning more profit from the set-up during the rest of the life period of the system. Similarly, the benefit-cost ratio would be higher, causing the production system to be more cost-effective. This is because of the higher capital cost in the case of the solar-powered system. However, the mitigation of CO₂ emission during the lifetime of the set-up is assessed to be more 20549 tonnes/life when using the solar-powered system compared to 16875 tonnes/life by the grid electricity system. This results into more credit earned from carbon dioxide mitigation i.e. USD51373 per life for solar PV powered system and USD42187 per life for grid electricity powered system. The MAP system could be a serviceable practice for mitigation of emission of CO₂, causing the effective utilization of agro-residues and increased sustenance for the environment.

In the present study, the environmental and techno-economic aspects of biochar and bio-oil produced by pyrolysis of corn stover using solar-powered microwave heating and activated carbon as an additive were investigated. The process firstly optimized for producing maximum bio-oil yield followed by biochar. The yields of both bio-oil and biochar found to be maximum under optimal condition of 700 W microwave power and 10% of AC additive. The result showed that the cost-effective and environmentally sustainable integration of the microwave-assisted pyrolysis process with solar photovoltaic power to produce biofuel and value-added products is viable. This will assist to reduce the power required for the processes and eliminate the usage of fossil-fuel-based electricity, resulting in additional advantages for users and a more environmentally friendly approach to the technology's long-term sustainability. The findings from the present study would ultimately provide appropriate information regarding the technical feasibility and economic viability of the practice for its acceptability among the users. Future research should also pay attention to more investigations on large scale making the process suitable for adoption among the producers and to become more beneficial.

Acknowledgements

Not applicable.

Authors’ contributions

The Authors have equally contributed to the work. A.E.M.F.: Conduct the experiments, Data analysis, and writing-original draft preparation. T.A.M.A.: conceptualization, methodology, and revision. All authors have read and approved the final manuscript.

Funding

Not applicable.

Availability of data and materials

All data generated or analyzed during this study are included in this published article.

Ethics approval and consent to participate

Not applicable.

Competing interests

The authors declare that there are no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Biofuel production by solar-powered microwave pyrolysis of biomass: a case study on process optimization, environmental, and techno-economic aspects

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Highlights

1. Introduction

2. Materials And Methods

2.1. Biomass feedstock and pyrolysis additives

2.2. Experimental Set-up and Procedure

2.3. Optimization of pyrolysis conditions for bio-oil and biochar yields

2.4. Product characteristics

3. Results And Discussion

3.1. Response surface analysis

3.2. Products yield and their characteristics

4. Conclusion

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References

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