Advancements in Waste to Fuel Technology for Sustainable Growth

The energy landscape is undergoing a significant shift, with environmental and energy-related concerns becoming more prominent and garnering more attention. A very promising clean and sustainable energy source, biomass energy has steadily attracted a lot of attention in recent years. According to literature reviews, pyrolysis plays a critical role in turning biomass into energy by achieving the thermal breakdown of biomass materials in an argon and nitrogen environment at varying temperatures (300–800C). Solid char, bio-oil, and non-condensable gases—all of which have high energy densities and are therefore easier to store and transport—are produced from biomass. Among the many drawbacks of the Bio-oil/Fuel produced by Biomass pyrolysis are its high oxygen concentration and complex chemical composition. These difficulties hinder bio-oil's marketability and competitiveness, underscoring the pressing need for efficient catalysis to resolve them. 

There is a global attention toward the use of Hydrogen (H₂) as a clean fuel, as Hydrogen H₂ combustion generates water vapour as the only emission product. Hydrogen (H₂) provides a promising near-future solution for the decarbonization of the currently fossil-fuel-dominated transport and industrial sectors. Hydrogen (H₂) is produced from fossil-fuel-derived natural gas and coal via two main pathways. 

1. Steam reforming of methane, which accounts for up to 76% of worldwide production.

2. Gasification of coal, which covers the remaining portion. It is desirable to adopt a more sustainable alternative to fossil fuel as the feedstock for hydrogen. 

As a third-generation biofuel, biodiesel has drawn interest as an environmentally friendly alternative. The reasons are high cetane number, oxidative stability, high heat capacity, and full combustion with reduced CO₂ emissions. Because they are considered carbon neutral and devoid of fossil fuel inputs, biofuels are often viewed as more environmentally benign than fossil fuels. The development of biofuel may lead to pesticide effects that endanger ecosystems and biodiversity, soil deterioration, and water contamination from overuse of fertilizers. 

Figure 1: Illustration of Advancements in Waste-to-Fuels Technology for Sustainable Growth. 

Figure 2: Survey on Various Processes for Biofuel Production 

Glycerol is a byproduct of this method, which uses a catalyst and a short-chain alcohol to convert it into biodiesel. Notwithstanding the numerous benefits of microalgae, there are still major obstacles to the commercialisation of biodiesel, mostly because of the high cost of production and technological difficulties. Therefore, the lipid content of microalgae is crucial for improving the biodiesel production's economic feasibility. Biochemical engineering was still a conventional but successful method of increasing lipid production in microalgae when I was in my fifth semester of chemical engineering. I referred to the book of Biochemical Engineering. The regulation of several parameters, such as temperature, pH, nutrient supply, salinity, and illumination, is necessary to achieve growth rates and particular component yields in grown microalgae. Depending on the species, different values of these factors must be maintained for microalgal culture to be successful. stress conditions and the crucial elements that will influence microalgae growth. Carbon, nitrogen, and phosphorus are the primary nutrients needed for microalgae growth. Since carbon makes up organic components necessary for the creation of biofuel, such as proteins, lipids, and carbs, it should make up the majority of the nutrient mix. Research articles provide the acceptable concentration C:N:P ratio. Wastewater has been suggested as a possible nutrient source for microalgae cultivation because of the needs of microalgae. While animal wastewater, primarily from manure, has substantial phosphorus and nitrogen content in the form of ammonia, the wastewater's composition is known to contain low levels of these nutrients but high levels of heavy contaminants, making it less suitable as a source of nutrients.

I'll now talk about the upstream and downstream processes, which are two different phases in the creation of biofuel and microalgae. All systems for producing microalgae biofuel generally use upstream processes, where stress is applied to encourage the buildup of certain chemicals. Over a specific period, it was the first to be grown in an open pond system using the culture medium. The contents of the sedimented tank are used to thicken the biomass until it is appropriate for centrifugation once the microalgae are ready to be harvested. Alternatively, the biomass may be separated from the culture medium by filtering, flocculating, electro-coagulating, or floating. The remaining biomass is then concentrated to 20% w/w in a centrifuge. Following both procedures, the desired components were extracted from the biomass. 

Sources of Biomass

Wood Waste: This is easily procured, and I have worked on in my research work too. Wood and wood residues are important biomass sources because they are widely available and renewable. It is a primary energy source for humanity and remains one of the most utilised renewable energy sources today. In contrast to fossil fuels, wood biomass is renewable through sustainable forestry practices, presenting a more environmentally favourable alternative. But one problem is that wood emits CO2, much of which is counterbalanced by the CO2 taken in during the tree's growth, helping to keep the carbon cycle in balance. Now, the question is how to convert it into fuel. Thereby, technologies such as gasification, torrefaction, and pyrolysis have emerged to convert wood biomass into more efficient forms of energy. While this can yield benefits, harvesting wood at unsustainable rates could lead to deforestation and biodiversity loss, underscoring the need for responsible forest management. Moreover, the inefficient combustion of wood can contribute to air pollution by releasing particulate matter and harmful gases.

One of the best catalytic processes evolved using Sawdust. The process implement by utilizing reactor employed in the experiments. 

Figure 3: Catalytic process diagram for the catalytic pyrolysis process 

The catalytic pyrolysis of sawdust is a rather simple process. A wool-covered porous quartz cylinder was positioned centrically inside the reactor to support the catalyst material. Unlike upgrading pyrolysis reactors, ex-situ catalysis is a two-stage process that involves heating biomass quickly to produce vapours, which are then passed over a separate vapours with a separate catalyst bed for upgrading. To ensure that the catalyst and both were separated, a sawdust was suspended above to hold the biomass sawdust powder. The catalyst and sawdust were heated and kept at a constant temperature using a pyrolysis work station. A mass flow rate was used to regulate the flow rate of the pure nitrogen gas used as a protective carrier gas. Following this process, the liquid products were first condensed by cooling them at a low temperature in a spiral tube. The final gas sample was then collected in a gas pocket after the gas was passed through a gas scrubber that was filled with silica gel and DI water.  

Temperatures of 500, 600, and 700C can be tolerated by catalytic pyrolysis. At a mass ratio of 0.5, the catalyst and powdered pine sawdust were preloaded into the centre of the reactor. The reactor and piping were purged with nitrogen flowrate for a predetermined amount of time in order to remove tapping air during setup and airtightness inspection. The nitrogen flow rate should decrease after the purge is finished. After reaching the desired temperature, the reactor continued to heat for 20 minutes. The products can be stored in the tanks once the reaction is finished and the reactor has been cooled to room temperature under a nitrogen environment. The residue that remained in the quartz cylinder served as a symbol to complete the process. The liquid component was kept in a separate tank, while the gas could be collected in a gas storage.

Yield Calculation for Prepared Products - 

Y(solid) = M (solid) / Mp * 100%

Y(liquid) = M (liquid) / Mp * 100%

Y(gas) = 100 % - Y (liquid) - Y (solid)

Where Mp denotes the mass of sawdust dust weighed before the experiment in g; M(solid) denotes the quality of the solid residue in the quartz after the experiment, g; M(liquid) denotes the quality of the liquid product gathered after the experiment, g. Ysolid, Yliquid, and Ygas represent yields of the solid, fluid, and gaseous products. 

Food Waste: Yes, this garbage is also the best feature, as it can be fermented and utilised to turn food waste into biofuels like biogas and biodiesel, which is something I have seen a lot in kitchens, messes, and other environments. Why? Food waste is a rich source of organic materials, including fibres, proteins, and carbohydrate polymers such as cellulose and starch. This biomass is challenging to dispose of by landfilling and incineration because of its high moisture content and calorific value. This biomass is therefore a valuable source for the production of bioethanol and a solution for waste management problems. However, be aware that transporting food waste could result in higher greenhouse gas emissions while ignoring certain environmental advantages.

MSW (Municipal Solid Waste): Homes, workplaces, and educational establishments produce municipal solid waste, a promising energy source. Municipal solid waste includes a variety of materials such as paper, metals, plastics, and food leftovers, which I like to transform into innovative products. Climate change impacts other biomass types, but MSW provides a more reliable energy source. Therefore, it is important to carefully consider whether employing MSW for energy production is sustainable and cost-effective for waste management and energy generation. 

Energy Crops: Energy crops are cultivated primarily for energy production, aiming to reduce reliance on conventional energy and enhance renewable energy generation from high-potential biomass. 

Alkaline Batteries for Biofuels: You are aware that one of the battery's waste black granules can be used to make biodiesel. How? Use that, the powder was completely dried for six hours at 105°C to eliminate any remaining moisture. After drying, the powder was heated to 500°C for two hours at a regulated rate of 5°C/min while being circulated with nitrogen gas at a flow rate of 80 mL/min. In addition to preparing the material for use as a catalyst, this heat treatment made it easier for potassium hydroxide and zinc hydroxide to transform into their catalytically active oxides, ZnO and K₂O. Previous research demonstrating the catalytic capability of these oxide compounds provided guidance for the calcination procedure.

Figure 4: Procedure for the utilisation of scrap material into useful products 

The initial step in turning WCO into biodiesel was filtering the oil with a cloth to get rid of any solid contaminants. To get rid of any moisture, it was then heated to 85°C for two hours. In a 250 mL glass container with a cooling system, temperature control, and a magnetic mixer working at 500 rpm, biodiesel was made from WCO using a special catalyst made from used batteries. First, the WCO's free fatty acid (FFA) level was reduced to less than 2%. The link for further procedures has been given. Kindly verify it. So, biodiesel-based production is high, particularly in areas where the local economy is focused on tourism.

Table 1: As of the latest data list from 2023-2024, the Top producers of Biodiesel are

In terms of production technology, supercritical methanol and enzymatic transesterification provide faster rates and greater yields, but they are costly. Solids-based heterogeneous catalysis is becoming more and more popular due to its industrial scalability and reusability. Animal fats and WCO (waste cooking oil) are becoming more popular since they are sustainable, but their supply is constrained. Ultrasound-assisted transesterification or microreactor-based systems could achieve >95% yield in 10–30 minutes in a small lab-scale unit. This process uses KOH or NaOH catalysts with methanol under ideal conditions of 60–65°C.

Parameters

Now, we are going to discuss parameters on Bio-Oil Production: 

Bio-Oil Production: Temperature

In the process of pyrolysing biomass (remember that I worked with sawdust biomass), temperature is also critical. For a decent yield and quality of Bio-Oil with Bio-Char, the temperature range should be between 200 and 1000°C. Temperatures less than 550°C encourage devolatilization and the breakdown of proteins and carbohydrates, while temperatures above this threshold tend to decompose acidic compounds such as carboxylic acids. The acidity of Bio-Oil decreases as the temperature rises, which is advantageous for some applications where less acidity is required. Increase the bio-coke's condensation and polymerisation at temperatures between 500 and 800 degrees Celsius. By comprehending these impacts, pyrolysis settings can be optimised to create bio-oil with the appropriate characterization methods for specific uses. 

Bio-Oil Production: Residence Time

One crucial factor in figuring out the makeup and caliber of bio-oil is the residence time during pyrolysis. Shorter residence times tend to favor the generation of bio-oil with higher yields and better quality because they reduce secondary processes that degrade bio-oil, such as cracking and repolymerization. Data from some Books, I would like to explain that different Bio-oil outputs are produced by different pyrolysis processes (slow, quick, flash, and catalytic) based on the biomass composition, operating conditions, and the employment of catalysts to improve the quality of the bio-oil by lowering its oxygen and nitrogen content. Furthermore, longer residence periods may result in the formation of heavier and more complex molecules, which could compromise the bio-oil's stability and usability.

Bio-Oil Production: Type of Waste 

Characteristics like water content, oxygen content, viscosity, acidity, and heating value are all impacted by the type of waste, which is crucial for the manufacture of Bio-Oil. The moisture content and lignin, cellulose, and hemicellulose content of different feedstocks vary, which has an immediate effect on the water content and quality of Bio-Oil. Higher water content agricultural leftovers typically provide higher water content Bio-Oil. One of the best things about wood-based biomass is that, in comparison to other heterogeneous feedstocks, it has a lower oxygen concentration than lignocellulosic biomass.

Figure 5: Structure of Wooden Biomass. 

While lignin oil mostly comprises molecules with fewer oxygen atoms, cellulose and hemicellulose oils have a higher proportion of compounds with more than seven oxygen atoms. Cellulose oil and hemicellulose oil contain a higher proportion of compounds with more than seven oxygen atoms, while lignin oil primarily consists of compounds with fewer oxygen atoms. Wood-based biomass has lower acidity compared to that from agricultural residues. Higher lignin content can produce bio-oils with higher heating values due to their higher carbon content. 

Bio-Oil Production: Heating Rate

The polymerization of Bio-Oil components is improved by slow heating rates, especially when temperatures are lower than 500°C. The carbon concentration of the coke series reduces as the temperature rises, while the hydrogen and oxygen contents rise. This study suggests that while oxygen-containing structures are brittle and prone to breaking, nitrogen and sulfur structures may be stable and concentrated in the coke. Substantial decomposition reactions exhibit heating rates, which decrease tar yields and increase lighter chemicals, secondary coke formation, and more condensed aromatic structures as a result of enhanced secondary reactions above 550°C. Reduced heating rates cause more volatiles to form, which increases secondary reactions like repolymerization and the separation of Bio-Oil into oxygenated and non-oxygenated fractions. This results in a Bio-Oil that has more non-oxygenated components and fewer polyaromatic hydrocarbons and other unwanted byproducts.

Utilisation on Treatment Steps

Figure 6: Methods for treating sewage sludge

Bio-Oil, Bio-Char, and pyrolysis gas are the three primary products of sewage sludge pyrolysis. Hydrocarbons, organic acids, phenols, and nitrogen compounds are the main constituents of bio-oil. Some Furan and odorous gases, CO2, CO, H2, and CH4 may be produced during its synthesis. The proportions of the three products that differ are among the important factors that are based on three distinct product features, various raw materials, and various processing techniques. As we previously mentioned, the temperature and heating rate of the pyrolysis process have an impact on the precise region of biochar and heavy metal content. As the temperature rises during the process, more gas will be released; however, NOx, SOx, tar, polycyclic aromatic hydrocarbons, and particulate matter may also increase. 

Let's have a look at Sewage sludge by pyrolysis of thermal degradation of chemical molecules in sewage sludge under high temperature and inert atmosphere, with the reaction temperature ranging we have been discussing stage-wise. The product of sewage sludge pyrolysis is influenced by many factors. Maximising Bio-Oil production, suitable for fine particles. Optimising Biochar and Bio-Oil quality through synergistic effects, enhancing resource efficiency. There are various reactors implemented in Industries for pyrolysis processing.

 

1. Fluidised Bed Reactor - Good for finding processed Sewage Sludge (SS) and highly reactive materials. 

2. Circulating Fluidised Bed Reactor - Large-scale processing and highly reactive Biomass materials. 

3. Vacuum Pyrolysis Reactor - High Quality Bio-Oil for specialised applications. 

4. Ablative Pyrolysis Reactor - Processing High-Value Sewage sludge with rapid pyrolysis. 

5. Rotating Cone Reactor - High-Efficiency rapid pyrolysis and small-scale Bio-Oil. 

6. Screw Reactor - Good for Processing Bio-Char. 

7. Fixed-Bed Reactor - Preparation of Bio-Char and other thermochemical conversion processes. 

Figure 6: Reactor Designs