Prospective of biochar material production and process optimization using co-pyrolysis approach-A mini-review

This mini-review explores the perspective of biochar material production using the co-pyrolysis approach, which involves the thermal decomposition of biomass and other carbonaceous materials in the absence of oxygen at low temperatures (300-500°C). The study investigates the co-pyrolysis of biomass with different materials such as plastics, tires, municipal solid waste, and other organic waste to produce a high biochar yield. The review focuses on the benefits of co-pyrolysis, including higher yield and better quality of biochar, as well as reduced environmental impact by using different waste materials as feedstock. The review also highlights co-pyrolysis challenges, such as process optimization, feedstock preparation, and product characterization. The study concludes that co-pyrolysis of biomass with different materials can be a promising approach for producing high-quality biochar with multiple applications. However, more research is needed to optimize the co-pyrolysis process and evaluate the economic feasibility of biochar production using a computation approach.


Introduction
Historically, fossil fuels have been central to energy provision, driving transportation, electricity generation, and climate control in buildings.However, their use has led to severe environmental issues, from habitat destruction to greenhouse gas emissions, as well as posing risks like oil spills [1].The rising costs of fossil fuels have also impacted the global economy [2].In contrast, renewable energy sources like solar, wind, and hydro offer cleaner alternatives, essential for reducing environmental damage, estimated at $2360 billion annually [3].Thus, A planned transition to cleaner energy forms, such as natural gas and hydrogen, is crucial for both economic and environmental well-being.Collaboration is essential to accelerate the shift towards renewables, which have been shown to negatively correlate with fossil fuel consumption, emphasizing their role in resource conservation [4].This aligns with broader goals of sustainable development and the Circular Economy, underlining the urgency of adopting renewable energy solutions to mitigate the harmful impacts of fossil fuels [5].
Renewable energy technologies like solar, bioenergy, and wind power are increasingly recognized for their environmental benefits, including reduced greenhouse gas emissions [6].Despite their promise, challenges such as market limitations, information gaps, and raw material constraints need to be addressed through well-crafted policies [7].Research has illuminated multiple benefits of renewables, ranging from energy security to climate change mitigation, prompting national governments to prioritize their adoption [8].The EU, for instance, has ambitious targets to cut greenhouse gas emissions by up to 95% by 2050 [9].In order to evaluate the 3E objectives of renewable energy progression in Taiwan, the fuzzy analytic hierarchy process (FAHP) is employed to address the complex, multi-objective challenge [10].Developments in sustainable energy were also discussed at the SEEP2014 conference in Dubai, including the Energy Quadr concept [9].
Historically, biomass has been a foundational energy source for various applications and currently accounts for 10-14% of global energy with the potential to increase substantially [11].Its abundant availability, particularly in the form of lignocellulosic materials, makes it a promising candidate for sustainable energy production [12].While traditional fuels still meet the energy demands of rural and underprivileged urban areas, alternative solutions like biogas and energy-efficient appliances could offer more sustainable uses of biomass [13].Harnessing waste for electricity is also under exploration, but the growth of biomass energy must be balanced with environmental concerns like habitat conservation and food security [14].Challenges specific to Indonesia, such as industry competition and environmental hazards, as well as the impact of the Covid-19 pandemic on renewable energy targets, further complicate its deployment [15].In summary, while biomass presents a renewable and environmentally-friendly alternative to fossil fuels, its development must be approached with careful consideration of long-term sustainability and socio-economic impacts [16].
The conversion of biomass into biofuels can be achieved through three main types of processes: thermochemical, biochemical, and physical.Thermochemical conversion involves heating biomass through processes like combustion, gasification, and pyrolysis to release energy.The biochemical conversion uses microorganisms like bacteria and enzymes to break down the biomass into valuable products like ethanol and biogas.Physical transformation involves changing the physical form of the biomass to make it easier to transport and store, like through densification [17].The three primary pathways for biofuel production are examined, and Table 1 presents a standard categorization of the procedures involved in the transformation of biomass into energy.Thermochemical conversion encompasses combustion, pyrolysis, gasification, and liquefaction processes.Meanwhile, biochemical conversion involves anaerobic digestion and fermentation techniques.Lastly, physicochemical conversion facilitates the creation of high-density biofuels such as biodiesel via esterification and transesterification processes [18]; [19].

Liquid fuels, Glycerol
Pyrolysis is a thermal decomposition process that involves heating organic matter, such as biomass, to high temperatures without oxygen.This results in the breakdown of the material into simpler components, including biochar, bio-oil, and syngas.These products have various uses in fields such as energy production, agriculture, and waste management.Pyrolysis represents a significant advancement in the conversion of biomass to valuable resources, while simultaneously reducing waste [21].It has the potential to serve as an eco-friendly and sustainable alternative to conventional fossil fuel-based energy production.The pyrolysis procedure comprises four primary stages, each occurring at distinct temperature levels.The initial phase entails dehydration of the incoming biomass at temperatures from 100-120°C, succeeded by the gas outlet distillation at 275°C.Exothermic reactions transpire within the 280°C to 350°C temperature range, extracting intricate combinations of chemical compounds, CO2, carbon monoxide, methane, C2H6, and H2 by severing the most fragile chemical bonds.The final stage involves eliminating all volatile compounds through evaporation at 350°C, leading to increased concentrations of H2, CO, and carbon.Liquids condensed during the pyrolysis procedure can be isolated, and the estimated output from 100 kg of dry wood encompasses products such as charcoal, gas, wood oil, light tar, methyl alcohol, pitch, acetic acid, and esters.
Various methods of executing the pyrolytic process exist, which influence the generation of bio-oil, syngas, and carbon-rich byproducts.Carbonization, slow pyrolysis, fast pyrolysis, and flash pyrolysis all produce varying amounts of these products [22].The two primary pyrolysis methods are slow and fast, with slow pyrolysis characterized by a low heating rate and a temperature range of 300°C to 700°C, resulting in high-quality biochar as the main product.In contrast, fast pyrolysis utilizes rapid heating rates and extremely high temperatures, typically between 600°C and 1000°C, resulting in the production of bio-oil as the main output and biochar as a secondary output.Slow pyrolysis is energy-efficient and suitable for small-scale production, on the other hand, fast pyrolysis is a widely recognized and established technique for the production of bio-oil, which is also used for waste treatment.However, it is crucial to maintain careful conditions during the process to obtain high-quality bio-oil [23].The differences in the heating rate and maximum reaction temperature influence the quality of biochar produced, with slow pyrolysis reducing secondary pyrolysis and thermal cracking processes [24].The two methods differ in terms of their output yields of biochar and bio-oil, with fast pyrolysis favoring bio-oil production.The vapor residence time in hot zones can be minimized to maximize biooil yield.Upgrading the liquid product and adjusting the applications to accommodate its unique characteristics and behavior poses a significant challenge in fast pyrolysis technology [25].Table 2 provides an overview of the distinctions between slow and fast pyrolysis for biomass.
Co-pyrolysis of multiple feedstocks, including biomass, coal, plastics, tires, and sludge, has gained attention as an alternative approach for enhancing pyrolysis outputs [27].Co-pyrolysis produces a unique product with combined properties, and the synergistic effect between different feedstocks significantly influences the process.This method has shown the potential to produce high-quality biochar and bio-oil, reduce production costs, and provide waste management alternatives [28]; [29].It can be used to convert various biomass, plastics, and waste materials into biofuels, chemicals, and other value-added products, Promoting the advancement of a circular and sustainable economy [26].
Studies on plastic pyrolysis have primarily concentrated on converting plastic to pyrolysis oil and gas, with PET being a common target [30]; [31].Both co-pyrolysis and traditional pyrolysis employ similar mechanisms, wherein the process takes place in a sealed reactor system under high temperatures and in the absence of oxygen.However, co-pyrolysis holds a distinct advantage in terms of waste volume reduction, cost savings for waste management, and environmental problem-solving.Table 2. Slow and fast pyrolysis for biomass.[24] ; [26]; [27].
Biomass, which is obtained from organic materials derived from plants and animals, can be classified into four categories, including agricultural residues, wood residues, municipal solid waste, and dedicated energy crops.Co-pyrolysis has successfully utilized these waste products, with plastic waste being a common source [30].Although pyrolysis is a promising technology for generating renewable energy from lignocellulosic biomass, its profitability depends on the cost of the feedstock.Greenhouse gas emissions of pyrolysis products are also affected by the source and production of feedstock [32]; [33].Furthermore, the physicochemical properties of biochar produced through pyrolysis of different feedstocks under varying conditions can influence its agricultural and environmental performance [34].Algae are a potential source of biomass for sustainable fuels, chemicals, and materials due to their diverse composition and low land footprint [35].Co-pyrolysis, which combines different feedstocks, is a more effective method for bio-oil production than simple pyrolysis, and its outcome depends on the synergistic effects of different feedstocks [33].Although slow pyrolysis, fast pyrolysis, and carbonization have been used to produce biochar, data gaps limit accurate predictions of its properties and performance [34].The yield of biochar is minimal, and the use of finely ground biomass feed with a particle size of 1-2mm is necessary.Furthermore, it is preferable to use biomass with low moisture content, ideally below 10%.

Features
This review evaluates the effectiveness of co-pyrolysis for biochar production by focusing on critical variables like feedstock preparation, end-product characterization, and process optimization.It investigates how varying pyrolysis temperatures and mixing ratios affect biochar yield and examines the synergistic effects of co-pyrolysis on different types of biomasses.The paper also contrasts slow and fast pyrolysis methods, assessing their respective merits and drawbacks.Finally, it anticipates future challenges and opportunities in waste pyrolysis.In doing so, this review offers a comprehensive overview of recent advancements in co-pyrolysis technologies and their applications.

Co-pyrolysis of Biomass
Biochar production utilizing an eco-friendly co-pyrolysis process has been investigated, to remove water pollutants from water such as the emoves phosphate and ammonium from water sources by mixing sewage sludge with walnut husk to produce biochar [36].The optimal proportion of sewage sludge to the walnut shell was determined to be 3:1, yielding biochar possessing an elevated adsorption capacity for ammonium under neutral or slightly alkaline aquatic conditions.The addition of a walnut shell enhanced the pore structure of the biochars made from sewage waste, leading to enhanced adsorption performance due to the presence of multiple metal oxides, functional groups, and surface adsorption active sites [37].Furthermore, the potential of sewage sludge-derived biochar in agriculture has been explored, despite challenges posed by heavy metal content.Tube furnace pyrolysis experiments were conducted to generate various types of biochars, including SB, ISB, KSB, and IKSB [38].
In another study, It was discovered that co-pyrolyzing lignin and sewage sludge improved the quality and output of biochar.The experiment explored the influence of pyrolysis temperature and mixing proportion, demonstrating that co-pyrolysis promoted mixture decomposition at lower reaction temperatures.At 800°C, alkali and alkaline earth metals (AAEMs) were shown to have a dominant catalytic effect, leading to significant pore development [39].Additionally, the co-pyrolysis of sludge and biomass, including industrial sludge and rice straw, produced high-quality biochar with reduced heavy metal risks, offering a comprehensive utilization method for waste materials.The co-pyrolytic biochar exhibited improved pore structure and functional group abundance, with a 102.8% increase in specific surface area [40].Moreover, sewage sludge and cotton stalks were co-pyrolyzed at different mixing ratios to produce biochars with both agronomic and environmental benefits as soil amendments.
The mixing ratios were found to substantially impact biochar properties and their potential for improving soil quality [41].
Recent research has revealed that the co-pyrolysis of various waste materials holds promise for generating valuable outputs, including bio-oil, biochar, and syngas while addressing environmental concerns.One investigation examined co-pyrolyzing plastic waste and date seeds, revealing that increased date seed proportions and lower temperatures contributed to enhanced biochar production [42].Another study explored co-pyrolyzing agricultural waste with minor plastic contamination, demonstrating the possibility of producing stable carbon forms while avoiding labor-intensive separation processes [43].Furthermore, research on pyrolyzing cow manure and Amaranthus retroflexus L. stems for biorenewable hydrogen production showed promising results, with high fixed carbon content in the resulting biochar [44].Co-pyrolyzing food waste and lignocellulosic biomass in a continuous-flow reactor has also been identified as an efficient method for the clean disposal and valueadded utilization of biowaste, with temperature and blending ratio impacting product yields [45].The characteristics of biochar are affected by both the nature of feedstocks and pyrolysis circumstances, and blending different feedstocks is crucial for developing versatile biochar suitable for various applications.
A critical review on co-pyrolyzing lignocellulosic and macroalgae biomass for biochar production highlighted the potential of this technique and emphasized the need for exploring more biomass types and conducting comprehensive sensitivity analysis [46].Additionally, co-pyrolyzing pig manure and Japanese knotweed has been investigated to enhance carbon properties, reduce heavy metal risks, and address bio-invasion issues.The resulting biochar demonstrated improved aromaticity, lower heavy metal bioavailability, and increased nutrient retention, indicating its potential as a soil amendment for restoration purposes [47].Table 3 presents an overview of recent investigations into biomass pyrolysis, illustrating the diverse range of feedstocks employed in these studies.Moderate Food waste and biowaste Electric tube furnace [58] final products.Higher pyrolysis temperatures generally promote gas and bio-oil formation while reducing biochar yield, yet they can also improve biochar properties like porosity, surface area, and carbon content [48]; [23].The heating rate, the speed at which feedstocks are heated, affects product distribution, with faster rates favoring bio-oil and gas production and slower rates encouraging biochar yield [49]; [29].Residence time, or the length of pyrolysis exposure, is essential, as longer times increase biochar yield and enhance its physicochemical properties, while shorter times favor bio-oil and gas production [50].The type and mixing ratio of feedstocks also plays a significant role in the resulting products, with synergistic properties improving biochars for specific applications like soil amendment or carbon sequestration [40].Additionally, particle size impacts mass and heat transfer during copyrolysis, with smaller particles generally yielding higher bio-oil and gas production due to increased surface area and reduced mass transfer limitations [49].Biomass pyrolysis typically occurs in an inert environment, although other gases such as steam may be used to alter the process.The heating rate is crucial in determining the pyrolysis type-fast, slow, or flash-with faster rates leading to greater gas production and reduced char due to rapid biomass fragmentation.Fast heating rates also boost bio-oil production by alleviating mass and heat transfer limitations and minimizing secondary reactions, which are further diminished by shorter residence times that enable prompt removal of organic vapors from reactors [31]; [51].Numerous studies have explored the impact of different parameters on the production and characteristics of biochar and bio-oil generated via the pyrolysis of various biomass types, highlighting the substantial effect of elements including temperature, heating rate, and particle size on the final products [52]; [53]; [54].

Optimization for Biochar Production
The pyrolysis process has gained attention as a favorable waste treatment method due to its product generation and energy recovery compared to other techniques.Multi-stage pyrolysis has been introduced, implementing multiple heating stages to maximize processing advantages and optimize operating parameters [59].[60] Utilized response surface methodology (RSM) to refine experimental conditions, aiming to maximize hydrogen and biochar output derived from the microwave pyrolysis of oil palm fronds.In a separate study, the co-pyrolysis of oil shale and rubber seed shell was optimized using RSM and Design-Expert software, revealing that temperature and blending ratio significantly influenced the yield of pyrolytic products, leading to higher heating values and improved grades of pyrolytic oil.
These innovative approaches demonstrate the potential of pyrolysis optimization in waste treatment and product generation, highlighting its versatility and adaptability in various applications [61].Response Surface Methodology (RSM) is a statistical approach commonly used in engineering and science to optimize system and process responses by identifying the optimal conditions that maximize or minimize desired outputs [62].In the specific context of co-pyrolysis, RSM helps to maximize the yield of the desired products while minimizing unwanted byproducts.By varying process parameters such as temperature, reaction duration, heating rate, and feedstock composition, RSM builds mathematical model that describes the relationship between input variables and system response [63].
The model is used to predict responses for any given set of input conditions and to identify optimal conditions using a desirability function that combines multiple variables into a single objective function.RSM offers several advantages, including the efficient identification of optimal process conditions, insight into key process parameters, and evaluation of process robustness.RSM serves as an effective instrument for enhancing co-pyrolysis procedures, thereby augmenting the quantity and caliber of the resulting pyrolysis products [64].
The growing interest in co-pyrolysis as an innovative approach for waste material treatment stems from its potential to mitigate environmental risks and generate valuable products.Several studies have employed response surface methodology (RSM) to optimize co-pyrolysis processes, analyze the resulting products, and explore synergistic effects between various waste materials.For instance, studies investigating the co-pyrolysis of fisheries by-products and forestry residues [65], sewage sludge and lignin [66], and chitin and oyster shell [57]  These results highlight the potential of co-pyrolysis for generating value-added products from waste materials, particularly when RSM is employed to optimize the process.Moreover, other studies have explored the interaction between different waste materials during co-pyrolysis, yielding improved product quality through synergistic effects.Investigations into the co-pyrolysis of lentil husks and Chlorella vulgaris by [67], lignocellulosic biomass and low-quality coal [68], and waste biomass and refinery oily sludge [69], have revealed significant improvements in bio-oil quality, optimal gaseous product distribution, and enhanced energy recovery.Additionally, a study by [70] examining the copyrolysis of waste tires and waste plastics provided foundational knowledge for polymer waste pyrolysis and demonstrated the importance of RSM in guiding the development of co-pyrolysis technologies for waste treatment.Table 4 provides a summary of recent research endeavors focused on the optimization of pyrolysis processes for the generation of biochar.This table delineates the variables examined, the scope of these variables, the ideal conditions, the maximum biochar yield, and the feedstock employed in the studies.The use of RSM in these studies has not only optimized co-pyrolysis processes but has also facilitated a greater understanding of the synergistic effects and resulting product characteristics, emphasizing its significance in advancing waste treatment technologies and producing value-added products from various waste materials.

Challenges & Future Work
The co-pyrolysis process, which relies on a specific energy source for heating-potentially including fossil fuels-demonstrates significant potential in biochar generation.This energy aspect is crucial, particularly at high temperatures, and demands detailed scrutiny.Contemporary studies focus on biochar from sewage sludge, plastic waste, and organic materials, considering parameters like temperature, heating rates, and feedstock ratios.Further research, however, is required to address other vital variables such as moisture content, particle size, vapor residence time, carrier gas velocity, and reactor pressure.
Evaluating the storage of biomass feedstock and interim products like biochar is essential.Co-pyrolysis, particularly when integrating plastic waste with lignocellulosic biomass, shows promise in waste reduction.Thus, rigorous studies are needed to elucidate kinetic parameters and response models for biochar optimization in different reactor setups.

Conclusion
Co-pyrolysis has emerged as a vital strategy for waste management, serving both renewable energy and environmental goals.While it can be an integral part of holistic waste management systems, its limitations as a standalone solution warrant attention.Technological hurdles, including the need for a deeper understanding of reaction kinetics and process parameter optimization, still persist.Nevertheless, co-pyrolysis holds promise for energy security and the sustainable production of biochar, a material gaining interest for its energy and environmental benefits.Utilizing diverse biomass resources in copyrolysis can be part of a broader energy-based biorefinery strategy.However, preconditioning biomass feedstock remains critical to biochar quality, as does the assessment of its physicochemical properties for specific applications. 6.

Table 3 :
[46]lysis of different biomassCo-pyrolysis, the simultaneous thermal decomposition of multiple feedstocks such as biomass and other organic materials, generates biochar, bio-oil, and syngas and has attracted interest for its potential to enhance biochar properties and improve overall biomass pyrolysis efficiency[46].The success of this process hinges on several factors, including temperature, heating rate, residence time, feedstock composition, and particle size, all of which influence the yield, composition, and characteristics of the