Exergy Efficiency and Energy Analysis of Silicon Production Using Coffee Husks as a Carbon Material

The silicon industry faces new challenges to increase sustainability and competitiveness as it is an energy-intensive sector with high energy running costs.The mixture of different carbon materials plays a very important role in the unit power consumption and exergy efficiency of the silicon production process. As an ideal reducing agent for industrial silicon, the demand for charcoal is increasing, posing a serious threat to the ecological environment. It is therefore urgent to study carbon-reducing agents which can replace charcoal, or to find energy-saving and cost-saving strategies for silicon production.In this study, we used waste biomass (coffee husks) as an additive during the smelting of industrial silicon. The influence of different carbon material mixtures on the silicon furnace performance was evaluated using the cover fire data, carbon excess factor, and exergy efficiency evaluation index. Explore its possibility as a reducing agent for silicon smelting.The results showed that TG increased by 47.75% on average after adding coffee husks. When semi-coke was used in the silicon furnace, the carbon excess factor was 1.29, the non-recovery exergy efficiency was 0.342, and the recovery exergy efficiency was 0.435. After adding coffee husks, the carbon excess factor decreased to 1.28, and the exergy efficiency increased to 0.343 and 0.437. When coffee husks were used without semi-coke, the carbon excess factor was reduced to 1.25, and the exergy efficiency increased to 0.348 and 0.441. The results indicate that using coffee husks as a reducing agent stabilizes the furnace conditions, reduces carbon loss, and improves the exergy efficiency of the silicon furnace. This study provides new insights into using raw biomass materials as reducing agents for silicon smelting.


Introduction
Energy is critical for the survival and development of society.As a developing country, China is in the stage of industrialization, and an increase in energy consumption is inevitable for economic and social development.The utilization and development of renewable energy have become the trend of global resources to reduce environmental pollution caused by non-renewable fossil fuels in industrial production [1].Solar energy is ubiquitous and the most promising renewable energy [2,3].Developed countries have focused primarily on solar energy for long-term planning.The photovoltaic industry ranks third in the world after the IT and microelectronics industries.Therefore, the demand for silicon has increased significantly as a material for photovoltaic devices and semiconductors [4].
Silicon is formed by the carbothermal reduction of silicon dioxide using carbon as the reducing agent.Charcoal, coal, and petroleum coke are mixed with silica and react in a submerged arc furnace (SAF) equipped with a threephase graphite electrode at high temperatures (> 1800 ℃) [5][6][7].Therefore, silicon smelting is energy-intensive [1].To better comprehend the influence of various process parameters and further reduce the energy consumption of the high energy-consuming industry, an exergy analysis method was proposed.High utilization energy efficiency is widely considered necessary for sustainable development and effective management in all aspects of life [8,9].Exergy is the highest beneficial energy that can be obtained based on the reversible change of the thermodynamic system from an arbitrary state to a state that is in equilibrium with a given environment [10,11].Exergy technology can locate, quantify losses, and identify systems with high potential for improvement.Compared with exergy, the energy or conservation process cannot quantify the loss of the final state of the thermodynamic process.Up to now,exergy analysis of industrial processes [12][13][14][15][16][17][18][19][20], the national economy [21][22][23] and the human body [24] have been conducted.The most energyconsuming part of silicon production is the electric furnace smelting process, and many studies have focused on this process to understand the specific exergy kinetics of different parts of the smelting process.Suetens et al. [25] analyzed dust treatments of a SAF and proposed methods to improve energy efficiency.Morris et al. [26] used exergy analysis to investigate the energy efficiency during lead smelting and determined the main cause of energy loss.Hjartarson [27] found that the total exergy efficiency of a ferrosilicon furnace was 0.3, which means that 70% of the total exergy input in the furnace is wasted.Børset et al. [28] used a new silicon yield index to evaluate the silicon furnace with a mixture of carbon-containing reducing agents for two operating cycles with and without charcoal, and found that the exergy efficiency was 0.30.Wen et al. [29] used statistical techniques to analyze and evaluate the performance indicators of silicon furnaces from a large number of industrial data.It is concluded that the exergy efficiency of different furnaces when using charcoal, coal and petroleum coke as carbon raw materials in silicon furnace is as follows: 12.5 MVA furnace, the heat energy in unrecovered waste gas is 0.32, and the recovery is 0.40; 22.5 MVA furnace, 0.33 ( no recovery) and 0.42 ( recovery).All of these means that there is still much room for improvement in exergy efficiency.Gajic et al. [30] reported that the chemical composition of the charge material combination affects power consumption and energy efficiency.Therefore, the best furnace charge combination can reduce power consumption, improve energy efficiency and reduce the energy cost of electric arc furnace production.Takla et al. [31] calculated that about 55% of the energy was provided by the carbonaceous reductant, and the rest was provided by electric energy.Therefore,the properties and combinations of the carbonaceous reductant are critical to the energy efficiency of silicon smelting.
Charcoal is the preferred carbonaceous reductant for industrial silicon smelting [32].However, charcoal is derived from forest resources, and excessive clearcutting has adverse environmental consequences and intensifies the greenhouse effect.Petroleum coke is a by-product of crude oil cracking [5].It has a high fixed carbon content, low ash content, and porosity second only to charcoal; thus, it can replace charcoal as a reducing agent for silicon smelting.However, petroleum coke has the disadvantages of strong conductivity [5], low specific resistance, and low reaction performance, resulting in significant energy loss during silicon production.Coal has high specific resistance and does not readily graphitize at high temperatures.It can replace charcoal as a reducing agent for producing industrial silicon.However, the ash content of coal [33] is very high, and the thermal stability of high-temperature combustion is low, resulting in a low utilization rate of raw materials.Therefore, China 's industrial silicon plants began to adopt the method of cross-combination of petroleum coke, bituminous coal, wood and other raw materials in order to achieve the same reduction effect as charcoal, but the production practice has proved that the reduction process has greater energy consumption, low efficiency and high production cost.Therefore, exploring the development and application of new reducing agents is the only way to solve the sustainable development of the industry [4].
Coffee, cocoa, and tea are the three major drinks consumed globally.They were planted and consumed in Arabia as early as 2,500 years ago [34].The coffee planting area in Yunnan Province is 12,000 square kilometers, accounting for 98% of the national coffee planting area.The annual coffee production is about 139,000 tons, accounting for 99% of China's total output [35].Coffee husks are a by-product of coffee production and account for about 20% of the dry weight of coffee [35].Most of this material is discarded as waste, polluting the environment.Thus, it is urgent to use these resources efficiently.Coffee husks contain many compounds, such as cellulose, hemicellulose, protein, and fat.They are an organic substance composed of carbon, hydrogen, and oxygen [36,37].One kg of coffee husks contains 3.5 kcal of heat energy; thus, the material can be used as fuel to replace fossil energy [38][39][40].Coffee husks contain alkali metals (including potassium and sodium), which are synergistic catalysts, and small amounts of nitrogen and phosphorus, which can be used as fertilizer [41].Adhering to the principle of waste reuse and the characteristics of coffee husks, we considered using coffee husks as fuel in the smelting of industrial silicon to provide high heat energy, improve chemical reactivity and reduce energy consumption.
No studies have investigated the use of coffee husks in industrial silicon production.We use a cover fire index and carbon excess factor index to evaluate the influence of coffee husks on the furnace condition in a 12.5 MVA silicon furnace and on carbon consumption, respectively.The effect of three cases (semi-coke, coffee husks, and semi-coke and coffee husks) on energy consumption and energy utilization is analyzed.The purpose of this study is to investigate the potential possibility of using biomass as an additive or charcoal substitute in silicon production to improve energy efficiency and reduce specific power consumption, and to lay the foundation for the strategy of biomass raw materials as alternative reducing agents.

The Silicon Production Process
Silicon is commercially prepared from carbon-reduced silica (SiO 2 ) in a large SAF. Figure 1 shows a diagram of a typical silicon production process.This process includes an SAF, raw materials, power supply systems,off-gas purification devices, and heat recovery, tapping, casting, and crushing equipment.
The furnace burden of a SAF is quartz and a carbonaceous reducing agent ( charcoal, coal, petroleum coke, wood, etc.).The high temperature (> 1800 °C) required for the carbothermic reduction reaction in the furnace requires electricity, which is provided by three electrodes.After hightemperature smelting, liquid silicon is discharged from the bottom of the furnace.After slag treatment and gas purge refining, the liquid silicon is poured into a suitable mold, cooled, and broken to the required particle size.
The off-gas (SiO + CO) generated by the reaction in the furnace is mixed with excess air under the furnace hood and burned.The off-gas temperature is controlled by the amount of excess air.In addition to the gas, the off-gas also contains solid particles (dust), which are removed by a gas purification system.The particles are primarily composed of amorphous SiO 2 , which can be used as filling materials for concrete, ceramics, rubber, etc.Thus, they are valuable by-products of the process.

Cover Fire Index in Silicon Production
Due to differences in the performance of raw materials, large amounts of burden accumulate in the silicon furnace.This material has low permeability, resulting in anomalous furnace conditions.The fire must be covered to increase the permeability of the material surface, expand the crucible, and prevent a stabbing fire, which increases the reaction rate in the furnace.Therefore, we propose the cover fire index to assess the reaction condition of the material in the furnace and reflect the advantages and disadvantages of the carbonaceous reductant.
The cover fire data include three evaluation indices: the number of cover fires (Gh), the average duration of the cover fires (Tg), and the average interval of cover fires (T G ). Gh refers to the number of cover fires in one cycle of industrial silicon smelting.Tg is the duration of the cover fire in one cycle divided by the total number of cover fires.T G refers to the time interval from the first to the second cover fire in one cycle of industrial silicon smelting.By analogy, the sum of the intervals from the nth cover fire to the n + 1st cover fire is divided by the number of cover fire intervals (n), where n = Gh-1.

Carbon Excess Factor Index in Silicon Production
The carbon consumption and carbon utilization rate during the carbothermal reduction of silica are analyzed.We assume that 1 ton of industrial silicon is produced.The theoretical carbon consumption calculated using the theoretical reaction equation SiO 2 + 2C → Si + 2CO is 0.857 t.The actual fixed carbon consumption refers to the consumption of all carbon materials during the production of one ton of industrial silicon.Φ is the carbon excess factor: where W theory and W actual are the theoretical and actual fixed carbon consumption (t) for producing 1 ton of silicon; W theory = 0.857t.The carbon efficiency is calculated as follows: The value is reciprocal to the carbon excess coefficient, and the product of the two is 1.

Energy and Exergy Balance During Silicon Production
In any thermodynamic system, energy is conserved, which is an obvious disadvantage for the system to determine the process of reducing energy consumption [43].The exergy analysis can identify and quantify the exergy loss in the thermodynamic process, and identify the location with high exergy destruction and improvement potential [43],which has important guiding significance for energy saving and optimization of the system.The general energy and exergy equilibrium equations of the unsteady flow process are expressed by ( 3-4) [44].The simplified form is shown in the formula (5-6).
In the equation, h, ke and pe are specific enthalpy, kinetic energy and potential energy respectively, m is mass flow heat transfer of system boundary ( b), W is work, and I is exergy consumption.For a closed system, Equation ( 5-6 ) can be simplified as follows: In most thermodynamic processes, energy efficiency is described as the ratio of product energy to total energy input [45].The ratio of product exergy to total input exergy is called exergy efficiency, as shown in the formula: Assuming that it is in a stable state, ignoring the contribution of kinetic energy and potential energy, the energy balance of a control volume is as follows: The first two terms on the right side of Equation ( 14) are the heat and work added to the control volume, and h represents the specific enthalpy of the material flow m.The subscripts e and o represent the input and output flows, respectively.The exergy of the system is defined as the maximum work that can be obtained by the interaction between the system and an idealized system called the reference environment.In this work, the temperature of the reference environment is T 0 = 298 K and the pressure is P 0 = 1 atm.In a stable state, the exergy equilibrium of a control volume is as follows: The first term on the right of Eq. ( 15) is the exergy accompanied by heat transfer.The second term is work (= exergy).e represents the specific exergy of the material flow m, ĖD represents the exergy destruction, an irrevers- ible measure of the control volume.The specific exergy of a material flow can be expressed as the sum of physical exergy and chemical exergy: Physical exergy is the maximum work that the system can obtain from the initial temperature and pressure ( T and P) to the temperature and pressure of the reference environment ( T 0 and P 0 ): Where h and s are the specific enthalpy and specific entropy of the system under temperature and pressure, when h 0 = h ( T 0 , P 0 ), s 0 = s ( T 0 , P 0 ).
The chemical exergy is the maximum work that can be obtained when the system is completely balanced with the reference environment at T 0 and P 0 .According to the exergy equilibrium calculation of the reversible formation reaction: In the formula ( 18), e ch is the molar chemical exergy, ΔG 0 f is the Gibbs energy of formation at temperature T0 and pressure P0, n y is the amount of element y, and e ch y is the standard molar chemical exergy of element y.The standard chemical exergy values based on the standard reference environment are listed in the table [46].The chemical exergy of the ideal mixture is calculated by the following formula: In the formula, x i represents the molar fraction of component i in the solution, and R is the general gas constant.The first term on the right side of Eq. ( 19) is the weighted sum of the chemical exergy of each component, and the second term can be expressed as the minimum work required to separate the components of the mixture.
The chemical exergy of the fuel can be determined by the exergy balance of the combustion reaction, where the fuel reacts with oxygen in the reference environment to form a reference material at a specific ambient temperature and pressure ( T 0 , P 0 ): In the formula, ΔG 0 is the Gibbs energy of the combus- tion reaction, n is the molar number, and the subscripts P and R are the product and reactant, respectively.
Carbon-based materials are carbon-dominated materials containing a variety of substances with complex bonds and unknown thermodynamic properties, which are difficult to calculate exergy by formula (11) [47].Therefore, researchers have developed interrelationships to estimate (16) e = e ph + e ch (17) fuel exergy.Song et al. [48] estimated the unified correlation of the specific chemical exergy of solid and liquid fuels.The statistical results show that the chemical exergy in ash and inorganic matter is very small and negligible compared with the specific chemical exergy in coal and biomass, while the specific chemical exergy of solid or liquid fuel is approximately equal to the chemical exergy of organic matter in fuel.
The exergy content of the waste stream is usually called the exergy loss of the process.Exergy efficiency is the part of exergy recovered in the product provided to the system, which can also be expressed by Equation ( 21): In a reversible ideal process has = 1 , and for all the actual process have 0 <  < 1 .It is used as a parameter to evaluate thermodynamic systems [49].
Combined with the above description, the energy consumed in a silicon furnace comes from the energy and electric energy generated by the oxidation reaction of carbon materials during the carbothermal reduction in silicon production.The energy balance expression of the furnace is as follows: Electricity accounts for 45% of the total energy, and carbon materials account for the rest (55%).Therefore, carbon materials are necessary as reducing agents and supplying energy for silicon smelting.However, carbon materials improve efficiency when used as reducing agents but reduce efficiency when used as fuel.Therefore, balancing the role of carbon materials is critical for optimizing carbon utilization.Contributions to heat loss include heat radiation and heat exchange in the furnace, cooling water, and cooling of liquid silicon (released as gas).The energy loss due to thermal radiation and heat exchange accounts for 5% of the total energy [50], the silicon slag takes accounts for less than 0.5% of the energy [51], and the contribution of quartz, limestone, ash, and water is close to 0.1% of the total exergy supplied to the furnace [48].
The exergy equilibrium of the silicon furnace is determined by three carbon material mixtures as carbonaceous raw materials: semi-coke without coffee husks, coffee husks without semi-coke, and semi-coke and coffee husks.The results are listed in Table 4. Figure 2 shows the Grassmann diagram of the exergy balance during silicon production.The carbon materials and electric energy contribute to exergy during silicon production, and the exergy increase is attributed to the electrodes and volatiles.Thermal radiation, heat exchange, and energy losses due to cooling water and off-gassing are responsible for exergy destruction.
Exergy efficiency is typically evaluated by two indicators: the standard evaluation index in Eq. ( 23) [31] and the exergy efficiency index proposed by Børset et al. [28] in Eq. (24).

where E Ch
Silicon denotes the chemical exergy of silicon, and E Total denotes the total energy used in silicon production.E el represents the power consumption per ton of silicon, and E ch Fc represents the combustion energy supply of fixed carbon by a carbonaceous reducing agent.

Case Studies
The experiment was carried out in a 12.5 MVA SAF in a silicon plant in Yunnan Province, China.Mixtures of raw materials were discontinuously added daily.The weight of the coffee husks added to each feeding was 10 kg.We collected data every 24 h and removed anomalous data caused by power outages, equipment maintenance, and abnormal furnace conditions.Table 1 lists the parameters of the three raw materials used for silicon smelting in the SAF.

Data Collection
Table 2 lists the parameters of several carbon materials used for silicon production: fixed carbon, volatile matter, moisture, ash (dry basis), and caking index (G).Table 3 lists the main elements in the ash of coffee husk.The content of the alkali metal ion K in the ash of coffee husks is relatively high (27.42%).Table 4 lists the total amount and composition of the raw materials and the power consumption in the three cases.All data were based on the production of one ton of industrial silicon.It was assumed that the silicon furnace had 100% operating efficiency and the electrode material was carbon (100% fixed carbon).
We considered the energy input from the fixed carbon of the carbon material and the volatile substances and electrical energy and calculated the total input energy.Fixed carbon is considered to be pure graphite obtained from solid combustion.We only considered pure liquid silicon (1600 °C) as the furnace output.The enthalpy of the off-gas and cooling water and the heat lost by radiation and convection were not considered as outputs.The heat generated by the off-gas was recycled.The offgas temperature was 800 °C [31] for the energy loss calculation: (25)  The enthalpy carried by the cooling water is determined by the measured water flow and the temperature difference between the inlet and outlet.The enthalpy in the off-gas is taken from the difference between the total energy input in the process and the enthalpy in the cooling water, the heat lost by radiation and convection, and the enthalpy in the product.The first two items on the right of the equation ( 25) are the chemical exergy in the off-gas.In the calculation, we ignore this item and estimate the heat exergy in the off-gas through the third item.Since the chemical exergy in the off-gas of this process is lost to the surrounding environment, it is regarded as a part of the chemical exergy destroyed in the furnace.

Thermodynamic Data
The energy source of raw materials is primarily the chemical exergy of fixed carbon and the consumption of volatiles in carbonaceous materials and carbon electrodes.The enthalpy of the carbon material was calculated as follows: We estimate the specific chemical exergy of carbon materials from the exergy-weighted contribution of fixed carbon and volatiles as follows: where Xi refers to the mass fraction of fixed carbon and volatile matter in the carbonaceous reductant.The volatiles (26) e ch fuel = X Carbon e ch Carbon + X Volatiles1 e ch

Volatiles2
in the other carbon materials except wood are defined as class 1 volatiles, and the volatiles in wood are defined as class 2 volatiles.The values of V1 and V2 are 10.6 kWh/kg and 4.03 kWh/kg, respectively [50,52].The enthalpy and entropy values required for other calculations are derived from the thermochemical software HSC Chemistry ® 6.1 [53].We used formula (18) to calculate the chemical exergy of all components except volatiles, using the standard chemistry exergy of the elements given in [46].The Gibbs energy required for the calculation of Eq. ( 18) can be obtained by the thermochemical software HSC Chemistry ® 6.1 [53].
The calculated standard chemical energy can be applied to the standard reference environment with T 0 = 298 K and P 0 = 1 atm [46].We ignored the T 0 ΔS term and the chemical mixing exergies of the reactants and products in the combustion reaction.We also ignored the contribution of the condensed silicon powder (amorphous SiO 2 ) because the proportion of enthalpy and exergy in slag and amorphous SiO 2 is less than 0.5% of the total enthalpy and exergy leaving the furnace.The effects of quartz, limestone, ash, and moisture were not considered because they accounted for only 0.1% of the total exergy in the furnace.Coffee husks were added, and part of the input of the carbon material energy and exergy was not included in the calculation.

Furnace Conditions With and Without Coffee Husks
The feeding and smelting processes of the coffee husks during silicon production are shown in Fig. 3.The particle size of the coffee husks is relatively small, and the fixed carbon content is 20%, which does not meet the requirements of the carbonaceous reducing agent.Therefore, coffee husks were added to analyze their influence on silicon production.We observe that the combustion of the materials in the SAF differs before and after adding coffee husks.Without the coffee husks, the surface of the material in the furnace is black, and a combustion flame exists.After adding the coffee husks, the combustion flame of the mixture becomes yellow, the temperature in the furnace increases, and the reaction rate accelerates.The reason is that coffee husks contain large amounts of volatile matter with high reactivity.In the cogasification process of carbon materials and coffee husks, the latter is rapidly pyrolyzed at a lower temperature, and the weakest covalent bond in the organic matter breaks to release a large amount of volatiles, which rapidly decompose, forming many free radicals.These radicals are effective active intermediates that can destroy the methylene in the carbon material, promoting the cracking and gasification of the carbon material and accelerating the chemical reaction rate.In addition, the coffee husks are rich in alkali, alkaline earth metals, and alkali metal-oxides, acting as catalysts during the co-gasification with carbon materials.

Results of Cover Fire Data During Silicon Production
We analyzed the cover fires data for 3 d and categorized the values into 9 classes.Figure 4 shows the statistical results of the cover fire data.Class 1 shows the data for the semicoke treatment (no coffee husks), and classes 2-9 show the data for the coffee husk treatment (no semi-coke).The Tg of Classes 2-9 ranges from 6 to 11 min, and the T G is significantly higher than that of Class 1.The Gh in Class 2 is relatively low because the data were obtained after the coffee husks were added (starting at 12.00 noon), and there were no data from 8.00 -12.00.After adding coffee husks, the number of cover fires (Gh) stabilizes, the duration of cover fires (Tg) ranges from 6 to 11 min, and the average interval of cover fires (T G ) of each class increases.Compared with class 1, the T G of class 2-9 increased by 9.1% -86.4%, with an average increase of 47.75%.This means that the furnace condition tends to be stable.The smoldering time is prolonged during smelting because the coffee husks contain 77.5% of volatile matter.The porosity of the sample increases during the co-gasification of carbon materials and coffee husks due to the thermal desorption of the volatile matter.The permeability of the furnace surface increases and the stability of the furnace is prolonged.This also reflects from the side that the addition of biomass ( coffee husk) can improve the performance of carbonaceous reductants.

Carbon Excess Factor During Silicon Production
The consumption of the different raw materials for producing one ton of industrial silicon for the three cases is shown in Fig. 5.A significant difference is observed between Case 2 and Case 1 (3) due to the addition of coffee husks and the replacement of semi-coke with a certain proportion of coal as a reducing agent.Figure 6 shows the carbon excess factor of different carbon-reducing materials during industrial silicon production.The fixed carbon consumption is significantly lower in Case 2 than in Case 1 (3), and the fixed carbon consumption is lower in Case 3 than in Case 1.The carbon efficiency is 80% in Case 2, 77.5% in Case 1, and 78.1% in Case 3. The results show that the proportion of fixed carbon in the carbonaceous reductant used in the reduction reaction is the highest in Case 2. The high carbon efficiency of a furnace can reduce greenhouse gas and CO 2 emissions and the accumulation of SiC.Thus, it is desirable to slow down global climate warming and prolong the furnace's life.
The reason for this result is that the biomass additive (coffee husks) and coal have high porosity, a large surface Fig. 3 Feeding and smelting process of coffee husks in silicon production Fig. 4 Statistical results of cover fire data for 9 classes area, and a strong ability to adsorb gas.The porosity of the sample increases during the pyrolysis of coffee husks and carbon material.SiO emissions are captured by the porous reducing agent, forming silicon.Moreover, the specific resistance of coffee husks is high [33], and the ash is rich in alkali metals, improving the permeability of the furnace burden and synergistically catalyzing the carbothermal reduction reaction.This also shows that biomass raw materials as reducing agents can improve carbon efficiency and have a certain potential to reduce carbon dioxide emissions.

Energy Analysis During Silicon Production
The energy flow Sankey diagrams of different raw material combinations ( three cases) were established using the input data of Equation ( 14) and Section 3, as shown in Fig. 7.The energy input includes electricity and raw materials (including carbon electrodes and carbonaceous reductants).The output is the enthalpy of silicon.Most of the energy in the exhaust gas is recovered, but the heat loss from radiation, convection, and cooling water is significant.
Figure 7 shows that about 66% of the total energy provided to the process by raw materials and electricity in Case 1 ( 3) is converted to heat, and about 65% of the total energy in Case 2 is converted to heat.In addition, the offgas contains up to about 51% of the energy input.From the perspective of energy balance, heat recovery is the key to improving energy efficiency.
We analyzed the energy consumption in three cases.The total energy consumption per ton of silicon for Case 1, Case 2, and Case is 24.58, 24.13, and 24.48 MV, respectively.The energy consumption is significantly lower for Case 2 than for Case 1 (3).The difference in the off-gassing loss is the main difference in the energy output of the three cases.Case 2 has the lowest off-gas loss (12.11MW), accounting for 50%.The off-gas losses of Case 1 (12.49MW) and Case 3 (12.41MW) are higher than that of Case 2, accounting for 51%.These results indicate that biomass raw material(coffee husk) as reducing agents can reduce energy consumption in the silicon smelting process.
The reason for these results may be the difference in the ratio of carbon materials.In Case 1, the ash content of semi-coke [32] is higher than that of petroleum coke and washed clean coal, and the caking index is low.These conditions are not conducive to conserving the reaction heat and extending the smoldering time of furnace smelting, resulting in higher energy consumption per ton Fig. 5 The raw materials consumption applied to one ton metric silicon product with the three cases.The t/t represents the raw materials consumption amount of one ton silicon product and the empty represents not exists the materials in case 2 Fig. 6 Carbon excess factor of different reductant combinations in industrial silicon production Fig. 7 Sankey diagram illustrating the energy input and output of the silicon furnace operated with three cases of silicon.In Case 3, the porosity of the raw material was higher during the pyrolysis process due to the addition of coffee husks to the semi-coke.The ability to adsorb gas was enhanced, promoting the reduction reaction.The alkali metals in the biomass ash acted as catalysis in the carbothermal reduction reaction.The lignin in the coffee husks has thermoplasticity, resulting in the furnace burden not loose at a certain temperature.These conditions compensated for the low caking index of the semi-coke and stabilized the furnace conditions.The reason for the better performance in Case 2 can be attributed to the coffee husks and the small amount of high-coking coal and medium-coking coal, which have high contents of volatile matter and high caking indices.Due to the thermal desorption of volatile matter, the sinterability of the material surface is improved, the porosity of the sample is high, and a large number of active sites exist.The alkali metal in the biomass ash penetrates the pores of the carbon material, increasing the contact area between the reactants and the reaction rate.
The power consumption for producing one ton of silicon and the specific energy derived from the raw materials are shown in Fig. 8.The specific power consumption in Case 1, Case 2, and Case 3 is 12.55, 12.29, and 12.49 MW, and the specific energy from the raw materials is 12.03, 11.84, and 11.99 MW, respectively.The ranking of the cases regarding the power consumption and the specific energy of raw materials is Case 1 > Case 3 > Case 2. This shows that biomass raw materials ( coffee husk) as a reducing agent can reduce the electricity consumption in the silicon smelting process.
The reason for the higher electric energy and fuel energy consumption in Case 1 and Case 3 is that semi-coke has high ash and iron contents and a low caking index.These conditions are not conducive to controlling the iron content during silicon products, conserving heat in the silicon furnace, and extending the burning time [32].The resistance of carbon materials is higher in Case 2 due to the addition of coffee husks and coking coal.The reactivity is strong, and the power consumption is reduced.
Figure 9 shows the specific heat energy output during silicon production for the three cases.There are significant differences in the heat output between the three cases.As shown in Fig. 9(a), the specific heat losses for Case 1, Case 2, and Case 3 are 16.18, 15.73, and 16.08 MW, respectively.The specific heat values of the off-gas at 800 °C are 12.49, 12.11, and 12.41 MW, respectively.The total heat loss and the heat value of off-gassing at 800 °C are significantly higher in Case 1 (3) than in Case 2. These results indicate that using semi-coke in Case 1 and Case 3 results in low sinterability of the material surface, low air permeability, Fig. 8 The electric energy consumption and the specific energy from raw materials for one metric ton silicon produced Fig. 9 The thermal exergy output in the process of silicon production with three cases and high heat energy consumption during silicon production.The resistivity of coffee husks and coal is higher in Case 2. The electrode is inserted deeply and stably.The crucible area is expanded, the heat loss is reduced, and the production capacity of the electric furnace is improved.

Exergy Analysis During Silicon Production
It can be seen from the energy analysis that the composition of the total energy varies with the different raw materials used.Therefore, in order to solve the energy quality problem, we will check the exergy analysis results.Table 5 lists the exergy input and output for the three cases for producing one ton of silicon.The energy input of the raw carbon material in Case 1, Case 2, and Case 3 accounts for 49%, and the electrical energy input accounts for 51%.The silicon output in Case 2 accounts for 35% of the total energy, and the heat during off-gassing accounts for 50% of the total energy.The silicon output in Case 1 and Case 3 accounts for 34% of the total energy, and the heat during off-gassing accounts for 51% of the energy.These results are consistent with the above results.Comparing these results with the results in Reference [32], we found a similar loss distribution.In a standard furnace operation, a total exergy efficiency of about 0.34 or a loss of 66%.After the use of biomass feedstock ( coffee husk) as an additive, the total exergy efficiency was about 0.35 or a loss of 65%.This indicates that biomass raw materials as reducing agents have the potential to improve the exergy efficiency.
The total energy was considered (Eq.( 23)) to compare the performance of three carbonaceous reductant mixtures in the silicon furnace.Figure 10 shows the exergy efficiency during silicon production for the three cases.It is observed that 34.2% of the total energy input is used for processing semi-coke products, similar to the results in Reference [32].The proportion of the total energy input used after adding the coffee husks is 34.3%.In addition, 34.8% of the total energy input is used to process the coffee husks without semi-coke.The combustion efficiency in the three cases is higher (0.33 ± 0.02) than that of coke, coal, charcoal, and wood reported in the literature [31] in the absence of electricity production.This finding demonstrates that semicoke as an industrial silicon-reducing agent can improve the exergy efficiency of the silicon furnace, but the effect is improved after adding coffee husks.
We used two indicators (Eq.( 23) and Eq. ( 24)) to evaluate the exergy efficiency of the silicon furnace in the three cases with or without heat recovery from off-gassing for one ton of silicon production (Fig. 10).By comparing the difference of recovery in standard furnace operation, the exergy efficiency is 0.342, 0.348 and 0.343, respectively, corresponding to Case 1, Case 2 and Case 3.Case 2 has the highest potential energy in the off-gas, followed by Case 3 and Case 1.The reason is that the volatiles of the coffee husks and carbon-containing  As shown in Table 2, the results correspond to the proportion of volatiles in the carbon-containing raw material after adding coffee husk.The order is Case 2 > Case 3 > Case 1.
The calculated exergy efficiencies (excluding the recovery) during silicon production using a mixture of carbon-containing materials with semi-coke (Case 1) are 0.342 and 0.403, respectively.After adding coffee husk (Case 3), the two exergy efficiencies increase to 0.343 and 0.404, respectively.After recovering the heat energy during off-gassing, the values are 0.437 and 0.516.The exergy efficiencies (excluding the recovery) during silicon production using a mixture of no semi-coke and coffee husks (Case 2) are 0.348 and 0.412, respectively.After recovering the heat energy during off-gassing, the values are 0.441 and 0.522, respectively.Reference [31] obtained an upper limit of the exergy efficiency of 0.51 in an ideal process.This value increased to 0.71 after the heat energy in the waste gas was recovered.Therefore, we believe that our results have an acceptable error range.
Our study indicates that semi-coke and coffee husks can be used for silicon smelting to improve energy efficiency, and reduce the use of charcoal to protect the environment.However, the high ash content and low caking index of semi-coke are not conducive to ensuring high product quality and the stability of furnace conditions.Some of the reducing agents could be replaced by semi-coke by equilibrium of impurity elements during silicon smelting.The addition of carbon materials to coffee husks can synergistically catalyze the carbothermal reduction reaction to reduce energy consumption.However, the coffee husks have small particle sizes.They could be pressed into pellets to improve their utilization during silicon smelting.Waste biomass (coffee husk pellet) is a suitable reducing agent to replace charcoal, petroleum coke, coal, and other reducing agents.This will have a broad application prospect in the silicon smelting industry.

Conclusions
We used cover fire data to evaluate the impact of using coffee husks on the reaction rate in a silicon furnace.The carbon excess factor was used to assess the effect of coffee husks on fixed carbon consumption and the utilization rate.Two exergy efficiency indices were used to analyze the energy consumption and energy utilization of the silicon furnace in three cases (semi-coke (Case 1), coffee husks (Case 2), and semi-coke and coffee husks (Case 3)).The results from the study are summarised as follows: (1) The average value of T G increased by 47.75% after adding coffee husks, indicating that the furnace conditions were stable and the smoldering time was prolonged during smelting.
(2) The carbon excess factor decreased by 0.1-0.4,demonstrating that the fixed carbon utilization rate increased, and the carbon loss decreased.(3) The non-recovery exergy efficiency of the silicon furnace was 0.342 using semi-coke without coffee husks and 0.435 after recovery.This value increased to 0.343 and 0.437 after adding coffee husks.The non-recovery efficiency was 0.348 when using coffee husks without semi-coke and 0.441 after recovery.According to the research results of Takla et al., the exergetic efficiency to 0.33 and 0.41 with no recovery and with recovery of thermal exergy in the off-gas, respectively.Our results demonstrated that the exergy efficiency of the silicon furnace was improved after adding coffee husk to the mixture of carbon-containing materials.
Therefore, waste biomass resources can be utilized to replace part of the charcoal, petroleum coke, and coal as a reducing agent to produce silicon materials, saving energy and reducing costs.

Fig. 2
Fig. 2 A grassman diagram illustrating the characteristics of the exergy balance for the silicon furnace

Fig. 10
Fig. 10 Exergetic efficiencies for three cases recovery and without recovery power from thermal enegy in the off-gas in silicon furnace

Table 1
The cases of different carbonaceous reductants

Table 3
Analysis of main elements in ash of coffee husk(wt.%)

Table 4
The total amount and composition of the raw materials and the amount of electric power supplied to the silicon furnace per metric ton produced silicon for the three cases.We defined the volatile matter in coal, charcoal, semi-coke and petroleum coke as type 1 volatiles

Table 5
The exergy input and output within the silicon furnace per metric silicon produced for three cases of raw materials Case1:with semi-coke and no coffee husk Case2:no semi-coke with coffee husk Case3:with semi-coke and coffee husk