Thermal depolymerization

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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 catalyzed or un-catalyzed and is distinct from other forms of depolymerization which may rely on the use of chemicals or biological action. This process is associated with an increase in entropy.

Contents

For most polymers, thermal depolymerization is chaotic process, giving a mixture of volatile compounds. Materials may be depolymerized 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 depolymerization 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]

Disordered depolymerization

For most polymeric materials, thermal depolymerization 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, depolymerization 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 depolymerization 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]

Biomass

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 depolymerization 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 oxidized 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.

Plastics

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 depolymerization 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. For example, 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.

Municipal waste

Thermal treatment of municipal waste can involve the depolymerization 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 areas 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]

Ordered depolymerization

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 depolymerization. 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 depolymerization reactions are completely efficient, and some competitive pyrolysis is often observed.

Biomass

Biorefineries convert low-value agricultural and animal waste into useful chemicals. The industrial production of furfural by the acid-catalyzed 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 aromatic compounds, [21] although such processes have not yet been commercialized with any lasting success. [22]

Plastics

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 depolymerization 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 depolymerization differs in that it contains a hydrous process followed by an anhydrous cracking / distillation process.

Condensation polymers bearing cleavable groups such as esters and amides can also be completely depolymerized 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 depolymerization but have the potential for lower energy costs. Thus far,[ as of? ] 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] but such technology is still in its infancy.

See also

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Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a poor barrier to air and water vapor and has a relatively low melting point. Polystyrene is one of the most widely used plastics, with the scale of its production being several million tonnes per year. Polystyrene is naturally transparent, but can be colored with colorants. Uses include protective packaging, containers, lids, bottles, trays, tumblers, disposable cutlery, in the making of models, and as an alternative material for phonograph records.

<span class="mw-page-title-main">Pyrolysis</span> Thermal decomposition of materials

The pyrolysis process is the thermal decomposition of materials at elevated temperatures, often in an inert atmosphere.

<span class="mw-page-title-main">Gasification</span> Form of energy conversion

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.

<span class="mw-page-title-main">Polymer degradation</span> Alteration in the polymer properties under the influence of environmental factors

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.

<span class="mw-page-title-main">PET bottle recycling</span> Recycling of bottles made of polyethylene terephthalate

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:

<span class="mw-page-title-main">Biorefinery</span> Refinery that converts biomass to energy and other beneficial byproducts

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:

  1. Supply the current fuels and chemical building blocks
  2. Supply new building blocks for the production of novel materials with disruptive characteristics
  3. Creation of new jobs, including rural areas
  4. Valorization of waste
  5. Achieve the ultimate goal of reducing GHG emissions
<span class="mw-page-title-main">Plastic recycling</span> Processes which convert waste plastic into new items

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<span class="mw-page-title-main">Polylactic acid</span> Biodegradable polymer

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3H
4O
2)
n or [–C(CH
3)HC(=O)O–]
n, formally obtained by condensation of lactic acid C(CH
3)(OH)HCOOH with loss of water. It can also be prepared by ring-opening polymerization of lactide [–C(CH
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2, the cyclic dimer of the basic repeating unit.

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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 biocrude or bio-oil, is a synthetic fuel with few 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.

<span class="mw-page-title-main">Lignocellulosic biomass</span> Plant dry matter

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.

<span class="mw-page-title-main">Biodegradable plastic</span> Plastics that can be decomposed by the action of living organisms

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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.

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<span class="mw-page-title-main">Plastic</span> Material of a wide range of synthetic or semi-synthetic organic solids

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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.

<span class="mw-page-title-main">Economics of plastics processing</span> Economic aspects of plastic manufacturing


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.

References

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  5. ↑ Kumar, Mayank; Olajire Oyedun, Adetoyese; Kumar, Amit (January 2018). "A review on the current status of various hydrothermal technologies on biomass feedstock". Renewable and Sustainable Energy Reviews. 81: 1742–1770. doi:10.1016/j.rser.2017.05.270.
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