Pyrolysis - Wikipedia

Thermal decomposition of materials Not to be confused with Pyrrolysine. "KALLA" redirects here. For other uses, see Kalla (disambiguation).

Pyrolysis is a process involving the separation of covalent bonds in organic matter by thermal decomposition within an inert environment without oxygen.[1][2][3]

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Etymology

[edit]

The word pyrolysis is coined from the Greek-derived elements pyro- (from Ancient Greek πῦρ : pûr - "fire, heat, fever") and lysis (λύσις : lúsis - "separation, loosening").

Applications

[edit]

Pyrolysis is most commonly used in the treatment of organic materials. It is one of the processes involved in the charring of wood[4] or pyrolysis of biomass. In general, pyrolysis of organic substances produces volatile products and leaves char, a carbon-rich solid residue. Extreme pyrolysis, which leaves mostly carbon as the residue, is called carbonization. Pyrolysis is considered one of the steps in the processes of gasification or combustion.[5][6] Laypeople often confuse pyrolysis gas with syngas.[why?] Pyrolysis gas has a high percentage of heavy tar fractions, which condense at relatively high temperatures, preventing its direct use in gas burners and internal combustion engines, unlike syngas.

The process is used heavily in the chemical industry, for example, to produce ethylene, many forms of carbon, and other chemicals from petroleum, coal, and even wood, or to produce coke from coal. It is used also in the conversion of natural gas (primarily methane) into hydrogen gas and solid carbon char, recently introduced on an industrial scale.[7] Aspirational applications of pyrolysis would convert biomass into syngas and biochar, waste plastics back into usable oil, or waste into safely disposable substances.

Terminology

[edit]

Pyrolysis is one of the various types of chemical degradation processes that occur at higher temperatures (above the boiling point of water or other solvents). It differs from other processes like combustion and hydrolysis in that it usually does not involve the addition of other reagents such as oxygen (O
2, in combustion) or water (in hydrolysis).[8] Pyrolysis produces solids (char), condensable liquids, (light and heavy oils and tar), and non-condensable gasses.[9][10][11][12]

Pyrolysis is different from gasification. In the chemical process industry, pyrolysis refers to a partial thermal degradation of carbonaceous materials that takes place in an inert (oxygen free) atmosphere and produces both gases, liquids and solids. The pyrolysis can be extended to full gasification that produces mainly gaseous output,[13] often with the addition of e.g. water steam to gasify residual carbonic solids, see Steam reforming.

Types

[edit]

Specific types of pyrolysis include:

• Carbonization, the complete pyrolysis of organic matter, which usually leaves a solid residue that consists mostly of elemental carbon.
• Methane pyrolysis, the direct conversion of methane to hydrogen fuel and separable solid carbon, sometimes using molten metal catalysts.
• Hydrous pyrolysis, in the presence of superheated water or steam, producing hydrogen and substantial atmospheric carbon dioxide.
• Dry distillation, as in the original production of sulfuric acid from sulfates.
• Destructive distillation, as in the manufacture of charcoal, coke and activated carbon.
  • Charcoal burning, the production of charcoal.
  • Tar production by destructive distillation of wood in tar kilns.
• Caramelization of sugars.
• High-temperature cooking processes such as roasting, frying, toasting, and grilling.
• Cracking of heavier hydrocarbons into lighter ones, as in oil refining.
• Thermal depolymerization, which breaks down plastics and other polymers into monomers and oligomers.
• Ceramization[14] involving the formation of polymer derived ceramics from preceramic polymers under an inert atmosphere.
• Catagenesis, the natural conversion of buried organic matter to fossil fuels.
• Flash vacuum pyrolysis, used in organic synthesis.

Other pyrolysis types come from a different classification that focuses on the pyrolysis operating conditions and heating system used, which have an impact on the yield of the pyrolysis products.

History

[edit]

Pyrolysis has been used for turning wood into charcoal since ancient times. The ancient Egyptians used the liquid fraction obtained from the pyrolysis of cedar wood, in their embalming process.[17]

The dry distillation of wood remained the major source of methanol into the early 20th century.[18] Pyrolysis was instrumental in the discovery of many chemical substances, such as phosphorus from ammonium sodium hydrogen phosphate NH4NaHPO4 in concentrated urine, oxygen from mercuric oxide, and various nitrates.[citation needed]

General processes and mechanisms

[edit]

Pyrolysis generally consists in heating the material above its decomposition temperature, breaking chemical bonds in its molecules. The fragments usually become smaller molecules, but may combine to produce residues with larger molecular mass, even amorphous covalent solids.[citation needed]

In many settings, some amounts of oxygen, water, or other substances may be present, so that combustion, hydrolysis, or other chemical processes may occur besides pyrolysis proper. Sometimes those chemicals are added intentionally, as in the burning of firewood, in the traditional manufacture of charcoal, and in the steam cracking of crude oil.[citation needed]

Conversely, the starting material may be heated in a vacuum or in an inert atmosphere to avoid chemical side reactions (such as combustion or hydrolysis). Pyrolysis in a vacuum also lowers the boiling point of the byproducts, improving their recovery.

When organic matter is heated at increasing temperatures in open containers, the following processes generally occur, in successive or overlapping stages:[citation needed]

• Below about 100 °C, volatiles, including some water, evaporate. Heat-sensitive substances, such as vitamin C and proteins, may partially change or decompose already at this stage.
• At about 100 °C or slightly higher, any remaining water that is merely absorbed in the material is driven off. This process consumes a lot of energy, so the temperature may stop rising until all water has evaporated. Water trapped in crystal structure of hydrates may come off at somewhat higher temperatures.
• Some solid substances, like fats, waxes, and sugars, may melt and separate.
• Between 100 and 500 °C, many common organic molecules break down. Most sugars start decomposing at 160–180 °C. Cellulose, a major component of wood, paper, and cotton fabrics, decomposes at about 350 °C.[5] Lignin, another major wood component, starts decomposing at about 350 °C, but continues releasing volatile products up to 500 °C.[5] The decomposition products usually include water, carbon monoxide CO and/or carbon dioxide CO2, as well as a large number of organic compounds.[6][19] Gases and volatile products leave the sample, and some of them may condense again as smoke. Generally, this process also absorbs energy. Some volatiles may ignite and burn, creating a visible flame. The non-volatile residues typically become richer in carbon and form large disordered molecules, with colors ranging between brown and black. At this point the matter is said to have been "charred" or "carbonized".
• At 200–300 °C, if oxygen has not been excluded, the carbonaceous residue may start to burn, in a highly exothermic reaction, often with no or little visible flame. Once carbon combustion starts, the temperature rises spontaneously, turning the residue into a glowing ember and releasing carbon dioxide and/or monoxide. At this stage, some of the nitrogen still remaining in the residue may be oxidized into nitrogen oxides like NO2 and N2O3. Sulfur and other elements like chlorine and arsenic may be oxidized and volatilized at this stage.
• Once combustion of the carbonaceous residue is complete, a powdery or solid mineral residue (ash) is often left behind, consisting of inorganic oxidized materials of high melting point. Some of the ash may have left during combustion, entrained by the gases as fly ash or particulate emissions. Metals present in the original matter usually remain in the ash as oxides or carbonates, such as potash. Phosphorus, from materials such as bone, phospholipids, and nucleic acids, usually remains as phosphates.

Safety challenges

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Because pyrolysis takes place at high temperatures which exceed the autoignition temperature of the produced gases, an explosion risk exists if oxygen is present. To control the temperature of pyrolysis systems careful temperature control is needed and can be accomplished with an open source pyrolysis controller.[20] Pyrolysis also produces various toxic gases, mainly carbon monoxide. The greatest risk of fire, explosion and release of toxic gases comes when the system is starting up and shutting down, operating intermittently, or during operational upsets.[21]

Inert gas purging is essential to manage inherent explosion risks. The procedure is not trivial and failure to keep oxygen out has led to accidents.[22]

Occurrence and uses

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Clandestine chemistry

[edit] See also: Clandestine chemistry § Pyrolysis

Conversion of CBD to THC can be brought about by pyrolysis.[23][24]

Cooking

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Pyrolysis has many applications in food preparation.[25] Caramelization is the pyrolysis of sugars in food (often after the sugars have been produced by the breakdown of polysaccharides). The food goes brown and changes flavor. The distinctive flavors are used in many dishes; for instance, caramelized onion is used in French onion soup.[26][27] The temperatures needed for caramelization lie above the boiling point of water.[26] Frying oil can easily rise above the boiling point. Putting a lid on the frying pan keeps the water in, and some of it re-condenses, keeping the temperature too cool to brown for longer time.

Pyrolysis of food can also be undesirable, as in the charring of burnt food (at temperatures too low for the oxidative combustion of carbon to produce flames and burn the food to ash).

Coke, carbon, charcoals, and chars

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Carbon and carbon-rich materials have desirable properties but are nonvolatile, even at high temperatures. Consequently, pyrolysis is used to produce many kinds of carbon; these can be used for fuel, as reagents in steelmaking (coke), and as structural materials.

Charcoal is a less smoky fuel than pyrolyzed wood.[28] Some cities ban, or used to ban, wood fires; when residents only use charcoal (and similarly treated rock coal, called coke) air pollution is significantly reduced. In cities where people do not generally cook or heat with fires, this is not needed. In the mid-20th century, "smokeless" legislation in Europe required cleaner-burning techniques, such as coke fuel[29] and smoke-burning incinerators[30] as an effective measure to reduce air pollution[29]

The coke-making or "coking" process consists of heating the material in "coking ovens" to very high temperatures (up to 900 °C or 1,700 °F) so that the molecules are broken down into lighter volatile substances, which leave the vessel, and a porous but hard residue that is mostly carbon and inorganic ash. The amount of volatiles varies with the source material, but is typically 25–30% of it by weight. High temperature pyrolysis is used on an industrial scale to convert coal into coke. This is useful in metallurgy, where the higher temperatures are necessary for many processes, such as steelmaking. Volatile by-products of this process are also often useful, including benzene and pyridine.[31] Coke can also be produced from the solid residue left from petroleum refining.

The original vascular structure of the wood and the pores created by escaping gases combine to produce a light and porous material. By starting with a dense wood-like material, such as nutshells or peach stones, one obtains a form of charcoal with particularly fine pores (and hence a much larger pore surface area), called activated carbon, which is used as an adsorbent for a wide range of chemical substances.

Biochar is the residue of incomplete organic pyrolysis, e.g., from cooking fires. It is a key component of the terra preta soils associated with ancient indigenous communities of the Amazon basin.[32] Terra preta is much sought by local farmers for its superior fertility and capacity to promote and retain an enhanced suite of beneficial microbiota, compared to the typical red soil of the region. Efforts are underway to recreate these soils through biochar, the solid residue of pyrolysis of various materials, mostly organic waste.

Carbon fibers are filaments of carbon that can be used to make very strong yarns and textiles. Carbon fiber items are often produced by spinning and weaving the desired item from fibers of a suitable polymer, and then pyrolyzing the material at a high temperature (from 1,500–3,000 °C or 2,730–5,430 °F). The first carbon fibers were made from rayon, but polyacrylonitrile has become the most common starting material. For their first workable electric lamps, Joseph Wilson Swan and Thomas Edison used carbon filaments made by pyrolysis of cotton yarns and bamboo splinters, respectively.

Pyrolysis is the reaction used to coat a preformed substrate with a layer of pyrolytic carbon. This is typically done in a fluidized bed reactor heated to 1,000–2,000 °C or 1,830–3,630 °F. Pyrolytic carbon coatings are used in many applications, including artificial heart valves.[33]

Liquid and gaseous biofuels

[edit] See also: Biofuel

Pyrolysis is the basis of several methods for producing fuel from biomass, i.e. lignocellulosic biomass.[34] Crops studied as biomass feedstock for pyrolysis include native North American prairie grasses such as switchgrass and bred versions of other grasses such as Miscantheus giganteus. Other sources of organic matter as feedstock for pyrolysis include greenwaste, sawdust, waste wood, leaves, vegetables, nut shells, straw, cotton trash, rice hulls, and orange peels.[5] Animal waste including poultry litter, dairy manure, and potentially other manures are also under evaluation. Some industrial byproducts are also suitable feedstock including paper sludge, distillers grain,[35] and sewage sludge.[36]

In the biomass components, the pyrolysis of hemicellulose happens between 210 and 310 °C.[5] The pyrolysis of cellulose starts from 300 to 315 °C and ends at 360–380 °C, with a peak at 342–354 °C.[5] Lignin starts to decompose at about 200 °C and continues until  °C.[37]

Synthetic diesel fuel by pyrolysis of organic materials is not yet economically competitive.[38] Higher efficiency is sometimes achieved by flash pyrolysis, in which finely divided feedstock is quickly heated to between 350 and 500 °C (660 and 930 °F) for less than two seconds.

Syngas is usually produced by pyrolysis.[25]

The low quality of oils produced through pyrolysis can be improved by physical and chemical processes,[39] which might drive up production costs, but may make sense economically as circumstances change.

There is also the possibility of integrating with other processes such as mechanical biological treatment and anaerobic digestion.[40] Fast pyrolysis is also investigated for biomass conversion.[41] Fuel bio-oil can also be produced by hydrous pyrolysis.

Methane pyrolysis for hydrogen

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Methane pyrolysis[42] is an industrial process for "turquoise" hydrogen production from methane by removing solid carbon from natural gas.[43] This one-step process produces hydrogen in high volume at low cost (less than steam reforming with carbon sequestration).[44] No greenhouse gas is released. No deep well injection of carbon dioxide is needed. Only water is released when hydrogen is used as the fuel for fuel-cell electric heavy truck transportation, [45][46][47][48][49] gas turbine electric power generation,[50][51] and hydrogen for industrial processes including producing ammonia fertilizer and cement.[52][53] Methane pyrolysis is the process operating around  °C for producing hydrogen from natural gas that allows removal of carbon easily (solid carbon is a byproduct of the process).[54][55] The industrial quality solid carbon can then be sold or landfilled and is not released into the atmosphere, avoiding emission of greenhouse gas (GHG) or ground water pollution from a landfill.

In , a company called Monolith Materials built a pilot plant in Redwood City, CA to study scaling Methane Pyrolysis using renewable power in the process.[56] A successful pilot project then led to a larger commercial-scale demonstration plant in Hallam, Nebraska in .[57] As of , this plant is operational and can produce around 14 metric tons of hydrogen per day.  In , the US Department of Energy backed Monolith Materials' plans for major expansion with a $1B loan guarantee.[58] The funding will help produce a plant capable of generating 164 metric tons of hydrogen per day by . Pilots with gas utilities and biogas plants are underway with companies like Modern Hydrogen.[59][60] Volume production is also being evaluated in the BASF "methane pyrolysis at scale" pilot plant,[7] the chemical engineering team at University of California - Santa Barbara[61] and in such research laboratories as Karlsruhe Liquid-metal Laboratory (KALLA).[62] Power for process heat consumed is only one-seventh of the power consumed in the water electrolysis method for producing hydrogen.[63]

The Australian company Hazer Group was founded in to commercialise technology originally developed at the University of Western Australia.  The company was listed on the ASX in December . It is completing a commercial demonstration project to produce renewable hydrogen and graphite from wastewater and iron ore as a process catalyst use technology created by the University of Western Australia (UWA). The Commercial Demonstration Plant project is an Australian first, and expected to produce around 100 tonnes of fuel-grade hydrogen and 380 tonnes of graphite each year starting in .[citation needed] It was scheduled to commence in . "10 December : Hazer Group (ASX: HZR) regret to advise that there has been a delay to the completion of the fabrication of the reactor for the Hazer Commercial Demonstration Project (CDP). This is expected to delay the planned commissioning of the Hazer CDP, with commissioning now expected to occur after our current target date of 1Q ."[64] The Hazer Group has collaboration agreements with Engie for a facility in France in May ,[65] A Memorandum of Understanding with Chubu Electric & Chiyoda in Japan April [66] and an agreement with Suncor Energy and FortisBC to develop 2,500 tonnes per Annum Burrard-Hazer Hydrogen Production Plant in Canada April [67][68]

The American company C-Zero's technology converts natural gas into hydrogen and solid carbon. The hydrogen provides clean, low-cost energy on demand, while the carbon can be permanently sequestered.[69] C-Zero announced in June that it closed a $34 million financing round led by SK Gas, a subsidiary of South Korea's second-largest conglomerate, the SK Group. SK Gas was joined by two other new investors, Engie New Ventures and Trafigura, one of the world's largest physical commodities trading companies, in addition to participation from existing investors including Breakthrough Energy Ventures, Eni Next, Mitsubishi Heavy Industries, and AP Ventures. Funding was for C-Zero's first pilot plant, which was expected to be online in Q1 . The plant may be capable of producing up to 400 kg of hydrogen per day from natural gas with no CO2 emissions.[70]

One of the world's largest chemical companies, BASF, has been researching hydrogen pyrolysis for more than 10 years.[71]

Ethylene

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Pyrolysis is used to produce ethylene, the chemical compound produced on the largest scale industrially (>110 million tons/year in ). In this process, hydrocarbons from petroleum are heated to around 600 °C (1,112 °F) in the presence of steam; this is called steam cracking. The resulting ethylene is used to make antifreeze (ethylene glycol), PVC (via vinyl chloride), and many other polymers, such as polyethylene and polystyrene.[72]

Semiconductors

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The process of metalorganic vapour-phase epitaxy (MOCVD) entails pyrolysis of volatile organometallic compounds to give semiconductors, hard coatings, and other applicable materials. The reactions entail thermal degradation of precursors, with deposition of the inorganic component and release of the hydrocarbons as gaseous waste. Since it is an atom-by-atom deposition, these atoms organize themselves into crystals to form the bulk semiconductor. Raw polycrystalline silicon is produced by the chemical vapor deposition of silane gases:

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SiH4 → Si + 2 H2

Gallium arsenide, another semiconductor, forms upon co-pyrolysis of trimethylgallium and arsine.

Waste management

[edit] See also: Thermal depolymerization

Pyrolysis can also be used to treat municipal solid waste and plastic waste.[6][19][73] The main advantage is the reduction in volume of the waste. In principle, pyrolysis will regenerate the monomers (precursors) to the polymers that are treated, but in practice the process is neither a clean nor an economically competitive source of monomers.[74][75][76]

In tire waste management, tire pyrolysis is a well-developed technology.[77] Other products from car tire pyrolysis include steel wires, carbon black and bitumen.[78] The area faces legislative, economic, and marketing obstacles.[79] Oil derived from tire rubber pyrolysis has a high sulfur content, which gives it high potential as a pollutant; consequently it should be desulfurized.[80][81]

Alkaline pyrolysis of sewage sludge at low temperature of 500 °C can enhance H
2 production with in-situ carbon capture. The use of NaOH (sodium hydroxide) has the potential to produce H
2-rich gas that can be used for fuels cells directly.[36][82]

In early November , the U.S. State of Georgia announced a joint effort with Igneo Technologies to build an $85 million large electronics recycling plant in the Port of Savannah. The project will focus on lower-value, plastics-heavy devices in the waste stream using multiple shredders and furnaces using pyrolysis technology.[83]

Waste from pyrolysis itself can also be used for useful products. For example, contaminant-rich retentate from liquid-fed pyrolysis of postconsumer multilayer packaging waste can be used as novel building composite materials, which have higher compression strengths (10-12 MPa) than construction bricks and brickworks (7 MPa), as well as 57% lower density, 0.77 g/cm3 .[84]

One-stepwise pyrolysis and Two-stepwise pyrolysis for Tobacco Waste

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Pyrolysis has also been used for trying to mitigate tobacco waste. One method was done where tobacco waste was separated into two categories TLW (Tobacco Leaf Waste) and TSW (Tobacco Stick Waste). TLW was determined to be any waste from cigarettes and TSW was determined to be any waste from electronic cigarettes. Both TLW and TSW were dried at 80 °C for 24 hours and stored in a desiccator.[85] Samples were grounded so that the contents were uniform. Tobacco Waste (TW) also contains inorganic (metal) contents, which was determined using an inductively coupled plasma-optical spectrometer.[85] Thermo-gravimetric analysis was used to thermally degrade four samples (TLW, TSW, glycerol, and guar gum) and monitored under specific dynamic temperature conditions.[85] About one gram of both TLW and TSW were used in the pyrolysis tests. During these analysis tests, CO
2 and N
2 were used as atmospheres inside of a tubular reactor that was built using quartz tubing. For both CO
2 and N
2 atmospheres the flow rate was 100 mL min−1.[85] External heating was created via a tubular furnace. The pyrogenic products were classified into three phases. The first phase was biochar, a solid residue produced by the reactor at 650 °C. The second phase liquid hydrocarbons were collected by a cold solvent trap and sorted by using chromatography. The third and final phase was analyzed using an online micro GC unit and those pyrolysates were gases.

Two different types of experiments were conducted: one-stepwise pyrolysis and two-stepwise pyrolysis. One-stepwise pyrolysis consisted of a constant heating rate (10 °C min−1) from 30 to 720 °C.[85] In the second step of the two-stepwise pyrolysis test the pyrolysates from the one-stepwise pyrolysis were pyrolyzed in the second heating zone which was controlled isothermally at 650 °C.[85] The two-stepwise pyrolysis was used to focus primarily on how well CO
2 affects carbon redistribution when adding heat through the second heating zone.[85]

First noted was the thermolytic behaviors of TLW and TSW in both the CO
2 and N
2 environments. For both TLW and TSW the thermolytic behaviors were identical at less than or equal to 660 °C in the CO
2 and N
2 environments. The differences between the environments start to occur when temperatures increase above 660 °C and the residual mass percentages significantly decrease in the CO
2 environment compared to that in the N
2 environment.[85] This observation is likely due to the Boudouard reaction, where we see spontaneous gasification happening when temperatures exceed 710 °C.[86][87] Although these observations were seen at temperatures lower than 710 °C it is most likely due to the catalytic capabilities of inorganics in TLW.[85] It was further investigated by doing ICP-OES measurements and found that a fifth of the residual mass percentage was Ca species. CaCO
3 is used in cigarette papers and filter material, leading to the explanation that degradation of CaCO
3 causes pure CO
2 reacting with CaO in a dynamic equilibrium state.[85] This being the reason for seeing mass decay between 660 °C and 710 °C. Differences in differential thermogram (DTG) peaks for TLW were compared to TSW. TLW had four distinctive peaks at 87, 195, 265, and 306 °C whereas TSW had two major drop offs at 200 and 306 °C with one spike in between.[85] The four peaks indicated that TLW contains more diverse types of additives than TSW.[85] The residual mass percentage between TLW and TSW was further compared, where the residual mass in TSW was less than that of TLW for both CO
2 and N
2 environments concluding that TSW has higher quantities of additives than TLW. 

The one-stepwise pyrolysis experiment showed different results for the CO
2 and N
2 environments. During this process the evolution of 5 different notable gases were observed. Hydrogen, Methane, Ethane, Carbon Dioxide, and Ethylene all are produced when the thermolytic rate of TLW began to be retarded at greater than or equal to 500 °C. Thermolytic rate begins at the same temperatures for both the CO
2 and N
2 environment but there is higher concentration of the production of Hydrogen, Ethane, Ethylene, and Methane in the N
2 environment than that in the CO
2 environment. The concentration of CO in the CO
2 environment is significantly greater as temperatures increase past 600 °C and this is due to CO
2 being liberated from CaCO
3 in TLW.[85] This significant increase in CO concentration is why there is lower concentrations of other gases produced in the CO
2 environment due to a dilution effect.[85] Since pyrolysis is the re-distribution of carbons in carbon substrates into three pyrogenic products.[85] The CO
2 environment is going to be more effective because the CO
2 reduction into CO allows for the oxidation of pyrolysates to form CO. In conclusion the CO
2 environment allows a higher yield of gases than oil and biochar. When the same process is done for TSW the trends are almost identical therefore the same explanations can be applied to the pyrolysis of TSW.[85]

Harmful chemicals were reduced in the CO
2 environment due to CO formation causing tar to be reduced. One-stepwise pyrolysis was not that effective on activating CO
2 on carbon rearrangement due to the high quantities of liquid pyrolysates (tar). Two-stepwise pyrolysis for the CO
2 environment allowed for greater concentrations of gases due to the second heating zone. The second heating zone was at a consistent temperature of 650 °C isothermally.[85] More reactions between CO
2 and gaseous pyrolysates with longer residence time meant that CO
2 could further convert pyrolysates into CO.[85] The results showed that the two-stepwise pyrolysis was an effective way to decrease tar content and increase gas concentration by about 10 wt.% for both TLW (64.20 wt.%) and TSW (73.71%).[85]

Thermal cleaning

[edit] See also: Thermal cleaning

Pyrolysis is also used for thermal cleaning, an industrial application to remove organic substances such as polymers, plastics and coatings from parts, products or production components like extruder screws, spinnerets[88] and static mixers. During the thermal cleaning process, at temperatures from 310 to 540 °C (600 to 1,000 °F),[89] organic material is converted by pyrolysis and oxidation into volatile organic compounds, hydrocarbons and carbonized gas.[90] Inorganic elements remain.[91]

Several types of thermal cleaning systems use pyrolysis:

• Molten Salt Baths belong to the oldest thermal cleaning systems; cleaning with a molten salt bath is very fast but implies the risk of dangerous splatters, or other potential hazards connected with the use of salt baths, like explosions or highly toxic hydrogen cyanide gas.[89]
• Fluidized Bed Systems[92] use sand or aluminium oxide as heating medium;[93] these systems also clean very fast but the medium does not melt or boil, nor emit any vapors or odors;[89] the cleaning process takes one to two hours.[90]
• Vacuum Ovens use pyrolysis in a vacuum[94] avoiding uncontrolled combustion inside the cleaning chamber;[89] the cleaning process takes 8[90] to 30 hours.[95]
• Burn-Off Ovens, also known as Heat-Cleaning Ovens, are gas-fired and used in the painting, coatings, electric motors and plastics industries for removing organics from heavy and large metal parts.[96]

Fine chemical synthesis

[edit]

Pyrolysis is used in the production of chemical compounds, mainly, but not only, in the research laboratory.

The area of boron-hydride clusters started with the study of the pyrolysis of diborane (B
2H
6) at ca. 200 °C. Products include the clusters pentaborane and decaborane. These pyrolyses involve not only cracking (to give H
2), but also recondensation.[97]

The synthesis of nanoparticles,[98] zirconia[99] and oxides[100] utilizing an ultrasonic nozzle in a process called ultrasonic spray pyrolysis (USP).

Other uses and occurrences

[edit]

• Pyrolysis is used to turn organic materials into carbon for the purpose of carbon-14 dating.
• Pyrolysis liquids from slow pyrolysis of bark and hemp have been tested for their antifungal activity against wood decaying fungi, showing potential to substitute the current wood preservatives[101] while further tests are still required. However, their ecotoxicity is very variable and while some are less toxic than current wood preservatives, other pyrolysis liquids have shown high ecotoxicity, what may cause detrimental effects in the environment.[102]
• Pyrolysis of tobacco, paper, and additives, in cigarettes and other products, generates many volatile products (including nicotine, carbon monoxide, and tar) that are responsible for the aroma and negative health effects of smoking. Similar considerations apply to the smoking of marijuana and the burning of incense products and mosquito coils.
• Pyrolysis occurs during the incineration of trash, potentially generating volatiles that are toxic or contribute to air pollution if not completely burned.
• Laboratory or industrial equipment sometimes gets fouled by carbonaceous residues that result from coking, the pyrolysis of organic products that come into contact with hot surfaces.

PAHs generation

[edit]

Polycyclic aromatic hydrocarbons (PAHs) can be generated from the pyrolysis of different solid waste fractions,[12] such as hemicellulose, cellulose, lignin, pectin, starch, polyethylene (PE), polystyrene (PS), polyvinyl chloride (PVC), and polyethylene terephthalate (PET). PS, PVC, and lignin generate significant amount of PAHs. Naphthalene is the most abundant PAH among all the polycyclic aromatic hydrocarbons.[103]

When the temperature is increased from 500 to 900 °C, most PAHs increase. With increasing temperature, the percentage of light PAHs decreases and the percentage of heavy PAHs increases.[104][105]

Thermogravimetric analysis

[edit]

Thermogravimetric analysis (TGA) is one of the most common techniques to investigate pyrolysis with no limitations of heat and mass transfer. The results can be used to determine mass loss kinetics.[5][19][6][37][73] Activation energies can be calculated using the Kissinger method or peak analysis-least square method (PA-LSM).[6][37]

TGA can couple with Fourier-transform infrared spectroscopy (FTIR) and mass spectrometry. As the temperature increases, the volatiles generated from pyrolysis can be measured.[106][82]

Macro-TGA

[edit]

In TGA, the sample is loaded first before the increase of temperature, and the heating rate is low (less than 100 °C min−1). Macro-TGA can use gram-scale samples to investigate the effects of pyrolysis with mass and heat transfer.[6][107]

Pyrolysis–gas chromatography–mass spectrometry

[edit]

Pyrolysis mass spectrometry (Py-GC-MS) is an important laboratory procedure to determine the structure of compounds.[108][109]

Machine learning

[edit]

In recent years, machine learning has attracted significant research interest in predicting yields, optimizing parameters, and monitoring pyrolytic processes.[110][111]

See also

[edit]

References

[edit]

Effect of process on the basic property of chars

Effect of HTC and LTP on char yield, HHV and energy yield

The main objective of this work was to compare the difference of chars produced via LTP and HTC in terms of their product yield and properties. For simplifying, char samples were named as following the format such as “Process Temperature” (e.g., H220 stands for hydrothermal carbonization conducted at 220 °C, whereas L220 stands for low temperature pyrolysis conducted at 220 °C). The proximate analysis, HHV, product yield and fuel rate are summarized in Fig. 1(a,b).

Although processed under the same reaction temperature and holding time, a higher yield of chars was observsevd with LTP. The downtrend of hydrochar yields lowed down once the reaction temperature of HTC exceeded 260 °C, while this phenomenon occurred near 340 °C in the LTP process. The results indicate that cellulose degraded significantly before 260 °C by HTC and 340 °C by LTP. It is similar to the previous study9. Regardless of the processes, the fixed carbon content increased, and the volatiles content declined with the increments of reaction temperature. For example, H320 has 52.10% fixed carbon and 47.35% volatiles. L440 has 71.25% fixed carbon and 25.00% volatiles. Compared to raw materials (13.56% fixed carbon, 85.47% volatiles), they both changed significantly.

In the LTP process, the ash content increased with increase in temperature as a result of the release of volatiles. Interestingly, the ash content of HTC did not show the same growth trend. Specifically, under the same reaction temperature, the ash content in HTC experiments was significantly lower than that of the LTP and fluctuated between 0.13% and 0.61%. For instance, even maximum ash content (0.61%) of HTC which was observed with H200 was much smaller than that of the L200 (1.05%). It could be explained by the dual effects by elution and accumulation of the ash. As is reported in Fig. 1(a), regarding the ash content, the ash removed by subcritical water is greater than its accumulation in chars.

The change of HHV versus temperature was similar to that of the fixed carbon. It proves that the content of fixed carbon is a major factor determining the HHV. In this study, the maximum HHV of the chars from HTC reached 29.61 MJ/kg (H320), and the maximum HHV of the chars from LTP reached 31.40 MJ/kg (L440). In the temperature range of 180–320 °C, the fuel rate of both HTC and LTP significantly increased from 0.16 to 1.10 and 0.84, respectively. The fuel rate of hydrothermal carbon has been higher at lower temperatures, such as H260 which had a fuel rate of 0.74, while the L260 has fuel rate of 0.24. In comparison, at the higher reaction temperature, chars from LTP had a higher fuel rate. For example, the fuel rate of L440 reached 2.85.

With the results from the proximate analysis, HHV as well as char yield, it was found that regardless the process of HTV or LTP, the rising tendency of fixed carbon content was highly consistent with that of HHV, both the tendencies of volatiles and char yield were similar. In addition, the char yields of H200 and L300 were quite similar (66.50% and 66.74%, respectively). While the chars from L300 has better basic fuel characteristics with 30.59% fixed carbon content (which was 15.71% of H200), 23.43 MJ/kg HHV (which was 20.54 MJ/kg of H200) and 0.45 fuel rate (which was 0.19 of H200). The HHV of H240 was closed to that of the L300 (23.99 MJ/kg and 23.43 MJ/kg), and both were higher than that of the standard lignite (23.00 MJ/kg). As was shown in Table 1, H240 and L300 have extremely similar composition of elements which could explain the similarity of HHV. However, H240 has lower fixed carbon content (26.07% to 30.59% of L300) and higher volatiles content (73.70% to 67.80% of L300). It shows that compared with L300, H240 has stronger ignition performance and the higher heat release in the same heat production condition. The HHV of H300 and L340 were similar (28.52 MJ/kg and 28.73 MJ/kg) and reached the HHV of bituminous which was 27.17 MJ/kg29. In the comparison of elements, H300 has more C content which was 73.03% (65.59% of L340) and less O content which was 20.61% (28.05% of L340). In addition, a group of chars samples which were H220 and L220 under the same reaction temperature were selected for process comparison. It was found that H220 has slightly higher HHV and fuel rate but has lower chars yield. The characteristics compared to the latter are based on these groups.

SEM analysis

A close inspection of the raw material and selected samples was performed using scanning electron microscopy (SEM) technique. Images shown in Fig. 2 reveals different transformations in the morphologies of the hydrochars versus the pyrolytic carbon. As the figure shows, the structure of the raw material is less porous than the char samples of HTC and LTP. In the image of H200, H220 and H240, the hydrochars contained aggregates of spherical microparticles. These microparticles are originated from the decomposition of cellulose, and the subsequent precipitation and growth into spheres30. However, in the image of H300, these spherical microparticles almost disappeared completely. This is most likely due to that they have been broken down into small molecules and was dissolved in the liquid phase under this temperature. Comparing the images of L220, L300, and L340, it was found that higher reaction temperatures of LTP contributed to the formation of pores and resulted in increased roughness on the biomass surface. By the result of char yield and SEM image from Fig. 2 (H220 and L220, H300 and L300), it could be concluded that the lignocellulosic structure of the HTC was decomposed much more significantly than LTP under the same reaction temperature.

Effect of HTC and LTP on the physicochemical properties of chars

Elemental analysis and fiber analysis

As shown by the results of the elemental analysis in Table 1, regardless of the process of HTC or LTP, carbonization caused the aggregation of carbon element and the releasing of hydrogen and oxygen. To analysze the variation in elemental composition, the ratios of hydrogen to carbon (H/C) and oxygen to carbon (O/C) of selected samples were ploted in Fig. 3 in the form of “Van Krevelen diagram”31. The diagram confirms that both of the HTC and LTP had a significant effect on the elemental composition of the products. Specifically, in Fig. 3, after the treatment of HTC, when the reaction temperature increased to 300 °C, the atomic H/C and O/C ratios dropped from 1.55 and 0.73 to 0.82 and 0.21, repectively. The atomic H/C and O/C ratios of L340 were 0.77 and 0.22, respectively. Four typical coals such as anthracite, bituminous, sub-bituminous, and lignite were used for comparision with the prepared chars. It was found that the H/C and O/C ratios of H300 and L340 were in the sub-bituminous region. In addition, with the increasing of temperature, the H/C and O/C atomic ratios of the samples decreased and became closed to that of coal.

Comparing the fiber analysis results of H220 and L220 (Table 1), it is obvious that most of the hemicellulose was removed at a lower temperature in the HTC process. However, the hemicellulose content in L220 is very close to that of the feedstock, indicating that most of the hemicellulose in pyrolytic carbon was not degraded in 220 °C via LTP process. This also explains the fact that the yield of hydrothermal carbon is generally higher than that of pyrolytic carbon at the same temperature (Fig. 1b). Therefore, it was considered that the degradation of hemicellose is the critical factor which influenced the results of the proximate analysis, char yield, and HHV in lower temperature region. Regadless of the processes, the content of cellulose was always experienced an initial stage of ascending and a subsequent desending. For example, the content of cellulose (40.92%) in H200 was higher than that of the raw material (34.04%) and H240 (34.21). For the LTP treatment, the content was 43.06% in L300 was higher than that of L220 (36.45%) and L340 (30.28%). Above all, it can be concluded that the increasing of cellolose content in lower temperature region was mainly due to the removal of hemicellulose. The subsequent decrease in the content of cellulose is mainly due to the decomposition of the cellulose.

FT-IR spectra of chars

The FT-IR spectra of the selected samples are shown in Fig. 4. It could be observed that the raw materials and treated samples showed similar spectral patterns, however, the intensities of infrared absorption peaks are differnt. Besides, it is worth noticing that due to the presence of CO2 during the measurement, a peak around – cm−1 was observed. In region 1, due to the vibration of –OH stretching, the peak between  cm−1 and  cm−1 can be found. It is obvious that there was a decrease in the intensity of –OH peak with the increase in the reaction temperature regardless of HTC or LTP32. The peaks between and  cm−1 represent the aliphatic groups of CH, CH2 and CH3, respectively33. The content of aliphatic compounds was increased by the HTC treatment as slight incerase of the peak intensity was found, while it was not obvious with the LTP process. Various functional groups were identified in region 3. The peak around  cm−1 represents the C–H stretching vibration and deforming vibration of cellulose and hemicellulose. With the rising reaction temperature, the dehydration and depolymerization of cellulose and hemicellulose caused the decrease in peak strength. The peaks between  cm−1 and  cm−1 are the characteristics of C–O–C vibrations in the cellulose and hemicellulose. These peaks shrinked with the increase in temperature by both HTC and LTP processes, which proved the decomposing of cellulose and hemicellulose34. In addition, compared with the raw material, new absorption peaks appeared at – cm−1 prove the presence of C–OH in alcohol groups33. The peak appeared at  cm−1 (– cm−1,C–O in methoxy) decreased, because the C–O band was broken due to the decarboxylation reation. It indicated that lignin was degraded during HTC and LTP processes. The peaks appeared at 765 cm−1 could be assigned to the C–H group in substituted aryl, which was observed in the samples treated with higher reaction temperatures such as H300 and L340. It means the aromatization of lignin was more pronounced in H300 and L340. Comparing H300 and L300 which were prepared under the same reaction temperature, it was found that H300 showed weaker absorption peaks of –OH (– cm−1) and C–H which at  cm−1, while had stronger absorption peaks of C–H at 765 cm−1. It shows the HTC process enabled stronger carbonization at the same reaction temperature.

Effect of process operating conditions on the fuel properties of chars

Major ash content related metals analysis

The ash content and composition of feedstock are important indexes which need to be assessed for fuel utilization. Most lignocellulosic biomasses are rich in inorganic elements which are the main components of ash in raw materials and chars. These inorganic elements, including magnesium (Mg), sulfur (S), calcium (Ca), manganese (Mn), copper (Cu), zinc (Zn) and iron (Fe), often existed in the biomass in the oxidized forms, MgO, SO3, CaO, Mn3O4, CuO and Fe2O3, respectively. These can cause slagging, fouling and corrosion of boiler. It has shown in Fig. 1(a) that the ash contents in HTC samples were significantly lower than that of the LTP samples prepared under the same reaction temperature and the values are fairly low (0.13–0.61%), which is due to the leaching of inorganic compositions. The different behavior observed for the LTP process is the consequence of the concentration effect. Table 2 shows the effect of HTC and LTP processes on the inorganic composition yields of hydrochar and pyrolytic carbon. The results suggest that HTC and LTP are different in the mechanism of yielding inorganic elements. It is evident that the HTC process removed the considerate amount of the inorganic components (26–90%) from the raw feedstock, especially magnesium, calcium and manganese. In comparison, the contents of inorganic elements in pyrolytic carbon were higher than that of the raw material. With the increase in reaction temperature, the volatiles was separated from the sample and the inorganic elements were hence concentrated in the produced chars. The observations are in coherence with the finding reported in the literature9,24,25.

Analysis of combustion characteristics

To evaluate the effects of HTC and LTP processes on the combustion properties, the samples were subjected to thermo-gravimetric (TG) analysis under an oxidizing atmosphere. The TG and DTG curves of raw materials, pyrolytic carbon and hydrochar are shown in Fig. 5(a–h), respectively. For clarity, the regions at temperatures lower than 200 °C (L340 and H300 were lower than 150 °C) and higher than 350 °C (L340 and H300 were higher than 300 °C) are excluded. Table 3 presents the corresponding combustion parametres for selected samples obtained from TG and DTG analysis. Ti (ignition temperature) means the lowest temperature of the combustion reaction. Temperature interval was defined as the range where the sample mass decreasing rates are above 1%/°C. Vmax and Tmax were the maximum value of weight loss rate and its corresponding temperature. ΔT was the maximum value of the temperature difference between actual temperature and set temperature due to the burning of the sample. In the pyrolytic carbons, it was observed that the ignition temperature of the sample decreased from 265 °C to 221 °C when the pyrolysis temperature increased. Hydrothermal carbon also showed a similar trend in the change of ignition points. Generally, in a wider pyrolysis temperature range (<800 °C), the ignition point of the fuel shifted to a higher temperature range with the deepening degree of carbonization and decreasing content of volatiles. However, selected samples did not show the similar trend. It implies that the most important factor affecting the ignition is not the content of volatiles in the lower temperature range.

It could be deduced that smaller particle size and less moisture content could effectively promote the combustion of the fuel and lower the ignition temperature. Figure 6 shows the particle size distributions of the selected samples. The results indicate that the percentage of smaller size particles (<100 μm, 100–250 μm and 250–600 μm) increased in both processes. In the LTP process, larger particle size (>600  μm) was reduced from 34.27% (Raw) to 6.14% (L340). It decreased to 9.95% (H300) by the process of HTC. Interestingly, In the range of 100–250 μm, the yields of chars showed an opposite trend which increased from 61.81% (Raw) to 78.11% (L340) in LTP while decreased to 47.68% in HTC. It is mainly due to the fact that a large number of particles was received in the range of 0–250 μm by HTC. At the same reaction temperature, the HTC samples showed higher pulverization than those obtained via LTP process. For example, L300 produced 12.84% pyrolytic carbons in range of 100–250 μm while the H300 produced 32.94% hydrothermal carbon. In the range of less than 100 μm, L300 received almost no product while H300 had 9.42% hydrothermal carbon. Altogether, through the particle size distribution analysis, it was found that the percentage of smaller size particles increased in both of processes, and it was considered to be one of the main reasons for the decreasing of the ignition temperature.

Moisture resistance is one of the important indicators of biomass fuels. Higher moisture resistance can reduce transportation costs and improve the combustion characteristics of the fuel. Figure 7(a,b) show the moisture uptake profiles of all samples under the storage conditions of 30 °C and 70% relative humidity within 10 h. It is evident that the moisture of the chars reached saturation in 10 h. Compared with the raw material, the treatments of LTP and HTC both improved the moisture resistance of the chars. In Fig. 7(a), it was found that the hydrophobicity of the samples increased with the increase in reaction temperature by LTP before 280 °C. For example, the saturated moisture content of the chars decreased from 8.73 wt.% to 3.88wt.%. Interestingly, this decreasing trend did not continue in the higher temperature region (280 °C–440 °C). Instead, the saturated moisture content was increased to around 5 wt.% (4.90 wt.% for L420 and 5.22 wt.% for L440). In Fig. 7(b), it was evident that saturated moisture content decreased from 8.73 wt.% to around 2 wt.%. In addition, it is worth mentioning that the hydrophobicity of H260, H280, H300, and H320 were quite consistent. It was also found that the change of fixed carbon, volatiles and HHV slowed down. The reason might be that large amount of cellulose and hemicellulose degraded before 260 °C by HTC. Above all, H260 is a better choice as a fuel candidate in terms of energy consumption, char yield, fuel rate, and hydrophobicity. Comparing L200 and H200, it was found that H200 had lower saturated moisture content (4.83%) than that of L200 (7.26%). Comparing L300 and H300, it was also found that H300 had lower saturated moisture content (2.14% for H300, 4.14% for L300). Based on the results, under the same reaction temperature, the chars produced by HTC have stronger hydrophobicity than that of the chars prepared by LTP. Therefore, through both the analyses of particle size distribution and moisture resistance, particle size distribution and moisture resistance have more serious effects on the ignition temperature than the degree of carbonization.

Similar to the change of the ignition point, Tmax also decreased with the increase in reaction temperature and reduced from 268 °C to 224 °C by LTP. While the Vmax had the opposite trend which increased from −1.43E-02%/s to −1.00E-02%/s. With the deepening of reaction degree by LTP process, the temperature interval of the resulting charcoal was increased from 20 °C (261 °C–281 °C for Raw) to 27 °C (250 °C–277 °C for L340 and 221 °C–248 °C for L300). In selected HTC samples, a similar pattern of Tmax, Vmax and temperature interval was not shown. However, ΔT of carbon samples of HTC and LTP showed consistent trends. Temperature differences of pyrolytic carbon samples and hydrothermal carbon samples increased from 45 °C (Raw) to 63 °C (L340) and 62 °C (H300), respectively. It was considered that ΔT has a close relationship with HHV.

Integrated combustion characteristics index (S) was used to further evaluate the combustion performance of HTC and LTP in the same reaction temperature. It could be described by the following formula:

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