See also
| Biofuels | |
|---|---|
| Energy from foodstock | |
| Non-food energy crops | |
| Technology | |
| Concepts | |
Thermal depolymerization (TDP) is the process of converting a polymer into a monomer or a mixture of monomers, [1] by predominantly thermal means. It may be catalysed or un-catalysed and is distinct from other forms of depolymerisation which may rely on the use of chemicals or biological action. This process is associated with an increase in entropy.
For most polymers thermal depolymerisation is chaotic process, giving a mixture of volatile compounds. Materials may be depolymerised in this way during waste management, with the volatile components produced being burnt as a form of synthetic fuel in a waste-to-energy process. For other polymers thermal depolymerisation is an ordered process giving a single product, or limited range of products, these transformations are usually more valuable and form the basis of some plastic recycling technologies. [2]
For most polymeric materials thermal depolymerisation proceeds in a disordered manner, with random chain scission giving a mixture of volatile compounds. The result is broadly akin to pyrolysis, although at higher temperatures gasification takes place. These reactions can be seen during waste management, with the products being burnt as synthetic fuel in a waste-to-energy process. In comparison to simply incinerating the starting polymer, depolymerisation gives a material with a higher heating value which can be burnt more efficiently and may also be sold. Incineration can also produce harmful dioxins and dioxin-like compounds and requires specially designed reactors and emission control systems in order to be performed safely. As the depolymerisation step requires heat it is energy-consuming, thus the ultimate balance of energy efficiency compared to straight incineration can be very tight and has been the subject of criticism. [3]
Many agricultural and animal wastes can be processed, but these are often already used as fertilizer, animal feed, and, in some cases, as feedstocks for paper mills or as low-quality boiler fuel. Thermal depolymerisation can convert these into more economically valuable materials. Numerous biomass to liquid technologies have been developed. In general, biochemicals contain oxygen atoms which are retained during pyrolysis, giving liquid products rich in phenols and furans. [4] These can be viewed as partially oxidised and make for low-grade fuels. Hydrothermal liquefaction technologies dehydrate the biomass during thermal processing to produce a more energy rich product stream. [5] Similarly, gasification produces hydrogen, a very high energy fuel.
Plastic waste consists mostly of commodity plastics and may be actively sorted from municipal waste. Pyrolysis of mixed plastics can give a fairly broad mix of chemical products (between about 1 and 15 carbon atoms) including gases and aromatic liquids. [6] Catalysts can give a better defined product with a higher value. [7] Likewise, hydrocracking can be employed to give LPG products. The presence of PVC can be problematic, as its thermal depolymerisation generates large amounts of HCl, which can corrode equipment and cause undesirable chlorination of the products. It must be either excluded or compensated for by installing dechlorination technologies. [8] Polyethylene and polypropylene account for just less than half of global plastic production and being pure hydrocarbons have a higher potential for conversion to fuel. [9] Plastic-to-fuel technologies have historically struggled to be economically viable due to the costs of collecting and sorting the plastic and the relatively low value of the fuel produced. [9] Large plants are seen as being more economical than smaller ones, [10] [11] but require more investment to build.
The method can however, result in a mild net-decrease in greenhouse gas emissions, [12] though other studies dispute this. E.g., a 2020 study released by Renolds on their own Hefty EnergyBag program shows net greenhouse gas emissions. The study showed then when all cradle-to-grave energy costs are tallied, burning in a cement kiln was far superior. Cement kiln fuel scored a -61.1 kg CO2 equivalents compared to +905 kg CO2 eq. It also fared far worse in terms of landfill reduction vs. kiln fuel. [13] Other studies have confirmed that plastics pyrolysis to fuel programs are also more energy intensive. [14] [15]
For tire waste management, tire pyrolysis is also an option. Oil derived from tire rubber pyrolysis contains high sulfur content, which gives it high potential as a pollutant and requires hydrodesulfurization before use. [16] [17] The area faces legislative, economic, and marketing obstacles. [18] In most cases tires are simply incinerated as tire-derived fuel.
Thermal treatment of municipal waste can involve the depolymerisation of a very wide range of compounds, including plastics and biomass. Technologies can include simple incineration as well as pyrolysis, gasification and plasma gasification. All of these are able to accommodate mixed and contaminated feedstocks. The main advantage is the reduction in volume of the waste, particularly in densely populated area lacking suitable sites for new landfills. In many countries incineration with energy recovery remains the most common method, with more advanced technologies being hindered by technical and cost hurdles. [19] [20]
Some materials thermally decompose in an ordered manner to give a single or limited range of products. By virtue of being pure materials they are usually more valuable than the mixtures produced by disordered thermal depolymerisation. For plastics this is usually the starting monomer and when this is recycled back into fresh polymer it is called feedstock recycling. In practice, not all depolymerisation reactions are completely efficient and some competitive pyrolysis is often observed.
Biorefineries convert low-value agricultural and animal waste into useful chemicals. The industrial production of furfural by the acid catalysed thermal treatment of hemicellulose has been in operation for over a century. Lignin has been the subject of significant research for the potential production of BTX and other aromatics compounds, [21] although such processes have not yet been commercialised with any lasting success. [22]
Certain polymers like PTFE, Nylon 6, polystyrene and PMMA [23] undergo depolymerization to give their starting monomers. These can be converted back into new plastic, a process called chemical or feedstock recycling. [24] [25] [26] In theory this offers infinite recyclability but it is also more expensive and has a higher carbon footprint than other forms of plastic recycling, however in practice this still yields an inferior product at higher energy costs than virgin polymer production in the real world because of contamination.
Although rarely employed presently, coal gasification has historically been performed on a large scale. Thermal depolymerisation is similar to other processes which use superheated water as a major phase to produce fuels, such as direct hydrothermal liquefaction. [27] These are distinct from processes using dry materials to depolymerize, such as pyrolysis. The term Thermochemical Conversion (TCC) has also been used for conversion of biomass to oils, using superheated water, although it is more usually applied to fuel production via pyrolysis. [28] [29] A demonstration plant due to start up in The Netherlands is said to be capable of processing 64 tons of biomass (dry basis) per day into oil. [30] Thermal depolymerisation differs in that it contains a hydrous process followed by an anhydrous cracking / distillation process.
Condensation polymers baring cleavable groups such as esters and amides can also be completely depolymerised by hydrolysis or solvolysis, this can be a purely chemical process but may also be promoted by enzymes. [31] Such technologies are less well developed than those of thermal depolymerisation but have the potential for lower energy costs. Thus far polyethylene terephthalate has been the most heavily studied polymer. [32] It has been suggested that waste plastic could be converted into other valuable chemicals (not necessarily monomers) by microbial action, [33] [34] such technology is still in its infancy.
Waste management or waste disposal includes the processes and actions required to manage waste from its inception to its final disposal. This includes the collection, transport, treatment, and disposal of waste, together with monitoring and regulation of the waste management process and waste-related laws, technologies, and economic mechanisms.
The pyrolysis process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere.
Gasification is a process that converts biomass- or fossil fuel-based carbonaceous materials into gases, including as the largest fractions: nitrogen (N2), carbon monoxide (CO), hydrogen (H2), and carbon dioxide (CO2). This is achieved by reacting the feedstock material at high temperatures (typically >700 °C), without combustion, via controlling the amount of oxygen and/or steam present in the reaction. The resulting gas mixture is called syngas (from synthesis gas) or producer gas and is itself a fuel due to the flammability of the H2 and CO of which the gas is largely composed. Power can be derived from the subsequent combustion of the resultant gas, and is considered to be a source of renewable energy if the gasified compounds were obtained from biomass feedstock.
Polymer degradation is the reduction in the physical properties of a polymer, such as strength, caused by changes in its chemical composition. Polymers and particularly plastics are subject to degradation at all stages of their product life cycle, including during their initial processing, use, disposal into the environment and recycling. The rate of this degradation varies significantly; biodegradation can take decades, whereas some industrial processes can completely decompose a polymer in hours.
Although PET is used in several applications,, as of 2022 only bottles are collected at a substantial scale. The main motivations have been either cost reduction or recycle content of retail goods. An increasing amount is recycled back into bottles, the rest goes into fibres, film, thermoformed packaging and strapping. After sorting, cleaning and grinding, 'bottle flake' is obtained, which is then processed by either:
A biorefinery is a refinery that converts biomass to energy and other beneficial byproducts. The International Energy Agency Bioenergy Task 42 defined biorefining as "the sustainable processing of biomass into a spectrum of bio-based products and bioenergy ". As refineries, biorefineries can provide multiple chemicals by fractioning an initial raw material (biomass) into multiple intermediates that can be further converted into value-added products. Each refining phase is also referred to as a "cascading phase". The use of biomass as feedstock can provide a benefit by reducing the impacts on the environment, as lower pollutants emissions and reduction in the emissions of hazard products. In addition, biorefineries are intended to achieve the following goals:
Plastic recycling is the processing of plastic waste into other products. Recycling can reduce dependence on landfill, conserve resources and protect the environment from plastic pollution and greenhouse gas emissions. Recycling rates lag those of other recoverable materials, such as aluminium, glass and paper. Through 2015, the world produced some 6.3 billion tonnes of plastic waste, only 9% of which has been recycled, and only ~1% has been recycled more than once. Additionally, 12% was incinerated and the remaining 79% sent to landfill or to the environment including the ocean.
Bioplastics are plastic materials produced from renewable biomass sources, such as vegetable fats and oils, corn starch, straw, woodchips, sawdust, recycled food waste, etc. Some bioplastics are obtained by processing directly from natural biopolymers including polysaccharides and proteins, while others are chemically synthesised from sugar derivatives and lipids from either plants or animals, or biologically generated by fermentation of sugars or lipids. In contrast, common plastics, such as fossil-fuel plastics are derived from petroleum or natural gas.
Waste-to-energy (WtE) or energy-from-waste (EfW) is the process of generating energy in the form of electricity and/or heat from the primary treatment of waste, or the processing of waste into a fuel source. WtE is a form of energy recovery. Most WtE processes generate electricity and/or heat directly through combustion, or produce a combustible fuel commodity, such as methane, methanol, ethanol or synthetic fuels, often derived from the product syngas.
Pyrolysis oil, sometimes also known as bio-crude or bio-oil, is a synthetic fuel with limited industrial application and under investigation as substitute for petroleum. It is obtained by heating dried biomass without oxygen in a reactor at a temperature of about 500 °C (900 °F) with subsequent cooling, separation from the aqueous phase and other processes. Pyrolysis oil is a kind of tar and normally contains levels of oxygen too high to be considered a pure hydrocarbon. This high oxygen content results in non-volatility, corrosiveness, partial miscibility with fossil fuels, thermal instability, and a tendency to polymerize when exposed to air. As such, it is distinctly different from petroleum products. Removing oxygen from bio-oil or nitrogen from algal bio-oil is known as upgrading.
Lignocellulose refers to plant dry matter (biomass), so called lignocellulosic biomass. It is the most abundantly available raw material on the Earth for the production of biofuels. It is composed of two kinds of carbohydrate polymers, cellulose and hemicellulose, and an aromatic-rich polymer called lignin. Any biomass rich in cellulose, hemicelluloses, and lignin are commonly referred to as lignocellulosic biomass. Each component has a distinct chemical behavior. Being a composite of three very different components makes the processing of lignocellulose challenging. The evolved resistance to degradation or even separation is referred to as recalcitrance. Overcoming this recalcitrance to produce useful, high value products requires a combination of heat, chemicals, enzymes, and microorganisms. These carbohydrate-containing polymers contain different sugar monomers and they are covalently bound to lignin.
Biodegradable plastics are plastics that can be decomposed by the action of living organisms, usually microbes, into water, carbon dioxide, and biomass. Biodegradable plastics are commonly produced with renewable raw materials, micro-organisms, petrochemicals, or combinations of all three.
Plasma gasification is an extreme thermal process using plasma which converts organic matter into a syngas which is primarily made up of hydrogen and carbon monoxide. A plasma torch powered by an electric arc is used to ionize gas and catalyze organic matter into syngas, with slag remaining as a byproduct. It is used commercially as a form of waste treatment, and has been tested for the gasification of refuse-derived fuel, biomass, industrial waste, hazardous waste, and solid hydrocarbons, such as coal, oil sands, petcoke and oil shale.
Depolymerization is the process of converting a polymer into a monomer or a mixture of monomers. This process is driven by an increase in entropy.
Second-generation biofuels, also known as advanced biofuels, are fuels that can be manufactured from various types of non-food biomass. Biomass in this context means plant materials and animal waste used especially as a source of fuel.
Commodity plastics or commodity polymers are plastics produced in high volumes for applications where exceptional material properties are not needed. In contrast to engineering plastics, commodity plastics tend to be inexpensive to produce and exhibit relatively weak mechanical properties. Some examples of commodity plastics are polyethylene, polypropylene, polystyrene, polyvinyl chloride, and poly(methyl methacrylate) .Globally, the most widely used thermoplastics include both polypropylene and polyethylene. Products made from commodity plastics include disposable plates, disposable cups, photographic and magnetic tape, clothing, reusable bags, medical trays, and seeding trays.
Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptability, plus a wide range of other properties, such as being lightweight, durable, flexible, and inexpensive to produce, has led to its widespread use. Plastics typically are made through human industrial systems. Most modern plastics are derived from fossil fuel-based chemicals like natural gas or petroleum; however, recent industrial methods use variants made from renewable materials, such as corn or cotton derivatives.
Hydrothermal liquefaction (HTL) is a thermal depolymerization process used to convert wet biomass, and other macromolecules, into crude-like oil under moderate temperature and high pressure. The crude-like oil has high energy density with a lower heating value of 33.8-36.9 MJ/kg and 5-20 wt% oxygen and renewable chemicals. The process has also been called hydrous pyrolysis.
The economics of plastics processing is determined by the type of process. Plastics can be processed with the following methods: machining, compression molding, transfer molding, injection molding, extrusion, rotational molding, blow molding, thermoforming, casting, forging, and foam molding. Processing methods are selected based on equipment cost, production rate, tooling cost, and build volume. High equipment and tooling cost methods are typically used for large production volumes whereas low - medium equipment cost and tooling cost methods are used for low production volumes. Compression molding, transfer molding, injection molding, forging, and foam molding have high equipment and tooling cost. Lower cost processes are machining, extruding, rotational molding, blow molding, thermoforming, and casting. A summary of each process and its cost is displayed in figure 1.
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