Pyrolysis: Thermochemical Conversion

Thermochemical Conversion Processes: Pyrolysis

• Pyrolysis involves thermal decomposition or chemical change induced by heat.
• Torrefaction, carbonization, and pyrolysis are all types of pyrolysis.
• Torrefaction occurs without oxygen within a narrow temperature range.
• Carbonization uses partial oxygen for combustion to provide heat for the process.
• Pyrolysis typically occurs without oxygen, or with partial oxygen to supply thermal energy.
• The primary goal of pyrolysis is to produce liquid extract from biomass, in addition to char and gas.
• Pyrolysis occurs within a temperature range of 300 to 650°C.
• Catalysts are often used to increase energy density by removing oxygen from biomass.
• Pyrolysis breaks down large molecular weight hydrocarbon compounds into lower molecular weight compounds, mainly condensable gases.
• Condensable gases are transformed into a liquid product called bio-oil.

Historical Milestones in Pyrolysis Development

• Char production through wood carbonization dates back to recorded human history.
• Initially, char was the sole product of wood carbonization.
• Tar, acetic acid, acetone, and methanol were later produced as civilization progressed.
• Reactor and bio-oil recovery systems were designed to obtain acetic acid, acetone, methanol, and tar.
• By the 18th century, technologies were developed to recover and utilize condensable gases as pyrolysis products.
• Brick kilns were used to recover condensable gases.
• Iron retorts followed brick kilns.
• In the 19th century, the acid wood industry produced charcoal and liquid by-products like acetic acid, methanol, and acetone.
• The hardwood distillation industry is a precursor to the petrochemical industry.
• The rise of the petroleum industry in the 20th century led to the decline of the pyrolysis industry due to cheaper products.
• The oil crisis in the 1970s prompted reconsideration of biomass pyrolysis to reduce dependence on fossil fuels.

Pyrolysis Process Details

• Pyrolysis, unlike combustion, is carried out in the total absence of oxygen, except for some cases where partial combustion provides thermal energy.
• Thermal energy can be supplied by partially combusting char and gases.
• Pyrolysis involves breaking down large hydrocarbon components of biomass into smaller molecules through thermal decomposition and chemical changes.
• It is a promising technique for converting waste biomass into liquid fuel.
• Product composition depends on temperature, pressure, and heating rate.
• Gas composition during devolatilization affects the final product.
• Biomass is thermally decomposed to produce solid, liquid, and gas by rapidly heating it above 300 to 400°C.
• Fast pyrolysis mainly produces liquid fuel with small fractions of gas and tar.
• Slow pyrolysis mainly forms gases and solid biochar.
• The heating rate differentiates slow and fast pyrolysis.

Ternary Diagram Representation

• Ternary diagrams represent biomass conversion processes, including pyrolysis, gasification, and combustion.
• The corners of the triangle represent carbon, hydrogen, and oxygen in the biomass sample.
• Points within the triangle represent the ternary mixture of these components.
• The side opposite a corner represents zero concentration of that component.
• For example, the side opposite to C represents the zero concentration of C.
• Slow pyrolysis moves the product toward C via solid char formation during carbonization, which gives solid char as a product.
• Fast pyrolysis moves the product toward the edge, away from oxygen, leading to higher liquid product of gas and char.

Classical Pyrolysis Operation

• Carbonaceous material is fed into a reactor and undergoes thermal cracking, forming solid char, gaseous fuel, and liquid organic compounds.
• Partial combustion of char and volatile gases can supply the heat energy required for pyrolysis.
• The combustion of char and volatile gases heats the pyrolysis chamber.
• The carbonaceous material is broken down inside the pyrolyzer to produce the product of interest.
• Pyrolysis is an endothermic reaction, requiring an external heat source.
• Char and volatile gasses can be partially combusted to provide the thermal energy.

Fluidized Bed Pyrolyzer

• Biomass is fed into the pyrolysis chamber containing hot solids using a screw feeder.
• Heat from the hot solids is transferred to the biomass causing the pyrolysis temperature to rise which leads to thermal cracking.
• Condensable and non-condensable gases leave the chamber.
• Solid char partially remains inside the chamber and partly leaves with the gas and is collected to cool.
• Condensable gases are condensed into liquid bio-oil, and non-condensable gases are released as a gaseous product.
• Char can be sold as a commercial product or partially combusted to provide thermal energy.
• Product gas (non-condensable gas) can be a heat source or fluidizing medium inside the pyrolysis chamber.

Types of Pyrolysis Processes

• Pyrolysis processes are classified into four categories based on heating rate: slow, fast, flash, and isothermal.
• Based on the environment/medium, they are classified into high-pressure pyrolysis and hydrous pyrolysis.

Slow Pyrolysis

• Biomass is heated slowly in the absence of oxygen to a relatively low temperature.
• Low-temperature heating is preferred over an extended period.
• Slow pyrolysis is the oldest form of pyrolysis.
• It is primarily used for char production.
• Carbonization and torrefaction are types of slow pyrolysis processes.

Fast Pyrolysis

• Biomass is heated rapidly, reaching peak temperature before decomposing.
• Vapor residence time is in the order of seconds or milliseconds.
• The primary goal is to maximize liquid fuel production.
• Heating rates can be as high as 1000 to 10,000°C per second.
• Peak temperature should be below 650°C if bio-oil is the desired product.
• Peak temperatures of about 1000°C are preferred if gas production is the primary objective.
• Flash and ultra-rapid pyrolysis are types of fast pyrolysis used primarily for bio-oil and gas production.

Flash Pyrolysis

• Biomass is heated rapidly in the absence of oxygen, but to a relatively modest temperature of around 450 to 600°C.
• Product gases lose the pyrolyzer within a short residence time of around 30 to 1500 milliseconds.
• Gases are not allowed to stay for a long time inside the reactor and can be condensed outside in the downstream reactor to collect liquid.
• Short vapor time allows production of liquid fuel.
• This increases liquid yield and reduces char production.
• A typical oil yield is around 70 to 75% of total pyrolysis product.

Hydro and Hydrous Pyrolysis

• Hydro pyrolysis takes place in an atmosphere of high-pressure hydrogen.
• Hydro pyrolysis can increase volatile yield and the proportion of low molar mass hydrocarbons.
• This process can produce bio-oil with reduced oxygen content.
• Bio-oil can be effectively upgraded using hydro-processing techniques.
• Hydrous pyrolysis involves the thermal cracking of biomass in high-temperature water.
• It could convert a raw material into the light hydrocarbon for the production of the fuel as well as fertilizer and chemical as a product.
• In the first stage it is carried out in water at around 200 to 300 degree C under pressure.

Pyrolysis Processes

• Hydrocarbons from the first pyrolysis stage are cracked into lighter hydrocarbons around 500°C.
• High oxygen content in bio-oil is a limitation of hydrous pyrolysis, unlike hydro-pyrolysis, which has lower oxygen content.
• Pyrolysis conditions can be adjusted to maximize char, liquid, or gas production based on the desired output.

Slow Pyrolysis

• Slow pyrolysis uses a slow heating rate (0.01-2°C per second) and a low final temperature to maximize char production.
• Long residence time in the reactor during slow pyrolysis enhances char yield.

Fast Pyrolysis

• Fast pyrolysis uses a high heating rate with a moderate final temperature (450-600°C) and short gas residence time.
• Fast pyrolysis maximizes liquid yield (bio-oil) due to the short residence time of gas in the reactor.

Gas Production

• Moderate to slow heating rates, high final temperatures (700-900°C) and long vapor residence times maximize gas production during pyrolysis.

Physical Aspects of Pyrolysis

• From a thermal standpoint, pyrolysis is divided into drying, initial, primary, and secondary cracking zones.
• The drying operation, occurring around 100°C, releases free and loosely bound moisture from biomass.
• The initial stage (100-300°C) involves exothermic dehydration, releasing non-condensable gases like CO and CO2.
• Primary pyrolysis (200-600°C) produces vapor, which can be condensed into bio-oil if quickly removed and cooled.
• Secondary cracking occurs if vapors remain in the reactor, leading to the thermal cracking of long hydrocarbon compounds.
• Secondary cracking reduces liquid product and produces secondary char and non-condensable gases.

Chemical Aspects of Pyrolysis

• Pre-pyrolysis is followed by competing dehydration and depolymerization reactions, using cellulose as an example.
• Reaction 2 involves dehydration, decarboxylation, and carbonization, yielding char and non-condensable gases.
• Reaction 3 involves depolymerization, producing condensable gases and vapors, including tar.
• Quick removal of condensable vapors maximizes liquid bio-oil yield, whereas prolonged exposure leads to secondary cracking.

Comparison of Thermo-Chemical Conversion Processes

• Slow pyrolysis has a moderate peak temperature (450-500°C) and a long residence time, yielding 35% char, 30% liquid, and 35% gases.
• Intermediate pyrolysis operates at similar temperatures but with shorter residence times to maximize liquid product.
• Fast pyrolysis uses moderate peak temperatures and very short residence times (under 2 seconds) to maximize liquid yield.
• Gasification requires high peak temperatures (over 800°C) and longer residence times (10-20 seconds) to maximize gas production.

Hydrothermal Liquefaction (HTL)

• HTL involves direct liquefaction of biomass into liquid oil at temperatures below 400°C, using water as a reactant and catalyst.
• HTL bio-oil has higher energy content than syngas and alcohol from biochemical conversion.
• HTL does not require drying feedstock, unlike gasification and pyrolysis allowing energy recovery from heating.
• Complex reactions in HTL are due to the complex composition of feedstock.

Key Reactions in HTL

• Cracking and reduction of polymers (lignin and lipids) are key reactions in HTL.
• Hydrolysis of cellulose and hemicellulose into glucose and simple sugars also occurs in HTL.
• Hydrogenolysis occurs in the presence of hydrogen produced during HTL, along with reduction of amino acids.
• Reformation reactions such as dehydration and decarboxylation remove oxygen from biomass in the form of H2O and CO2.
• C-O and C-C bond cleavage reactions, followed by hydrogenation of functional groups, are involved.

Products of HTL

• HTL products are categorized into bio-crude oil, aqueous phase, gaseous phase, and solid residue.
• Bio-crude oil, making up to 18-67% of feedstock weight, is a dark brown, viscous liquid.
• The quality and yield of bio-oil depend on biomass type, operating conditions, and catalysts or co-solvents used.
• HTL bio-oil contains a large fraction of phenolic compounds and fewer polar compounds.
• HTL bio-oil has high energy content (30-36 MJ/kg), with elemental composition of 64-73% carbon, 8-10% hydrogen, 10-25% oxygen, and 3-5% nitrogen.
• HTL aqueous phase makes up 20-50% of the feedstock weight; its composition depends on operating conditions and biomass type.
• Key chemicals in the aqueous phase are organic acids, alcohols, ketones, and phenolic compounds.
• The HTL aqueous bioproduct contains nitrogen, sulphur, halogens, and minerals, allowing it to be recycled or used for biomass cultivation.
• In HTL the gaseous product makes up 5-10% of the feedstock, mainly made of CO2, along with small fractions of hydrogen, carbon monoxide and methane.
• Solid residue from HTL, termed biochar, has a high fraction of carbon and hydrogen, is used as a soil amendment or enhancer.

Comparison of Bio-Oil Properties

• HTL bio-oil has a higher carbon content, close to that of fossil oil.
• HTL bio-oil has about half the oxygen content of pyrolysis bio-oil.
• Water content is significantly lower in HTL bio-oil than in pyrolysis bio-oil.

Limitations in Bio-Oil

• Low pH, high viscosity, instability, temperature sensitivity, char and solid content, alkali metals, and water content affect the use of bio-oil as fuel.
• Low pH causes corrosion problems, high viscosity causes handling and pumping issues.
• Char and solid content effects combustion-related issues and equipment blockage/erosion.
• Hydro-processing can be used to deoxygenate the bio-oil, improving its quality for use as a high-quality fuel.

Chemical Conversion Processes

• Lignocellulosic biomass, containing cellulose, hemicellulose, and lignin, can produce value-added chemicals through emerging conversion processes.
• Specific biomass types can be used to extract proteins, vitamins and pharmaceutical-grade chemicals using suitable extraction techniques.
• Oilseeds can be transesterified chemically to produce biodiesel and glycerol.
• Glycerol can be further processed by hydrogenolysis to produce propane diol as a chemical.

Additional Chemical Conversions

• Reducing sugars from biomass hydrolysis can be fermented to produce ethanol and lactic acid.
• The thermochemical conversion of reducing sugars can produce bio-SNG, while bio-syngas can be converted into chemicals using catalytic conversion techniques.
• Lignin can be processed into cement and fuel additives.
• Hemicellulose can be hydrolyzed to produce glucose, arabinose, and xylose.
• These platform chemicals can be used to produce ethanol, xylitol, and butane diol.
• Lignocellulose biomass's major component, cellulose, converts into value-added chemicals: high-value polymers, platform chemicals like levulinic acid, biofuels (ethanol), solvents, monomers (diphenolic acid), and pesticides (5-aminolevulinic acid).
• Glucose, a platform chemical, is produced from lignocellulose biomass hydrolysis.
• Glucose converts into lactic acid, succinic acid, 3-hydroxypropionic acid, itaconic acid, and glutamic acid.
• Via oxidation, glucose yields gluconic acid, glucuronic acid, and glucaric acid.
• Lactic acid from glucose produces acetaldehyde, lactide, and propanediol.
• Lactic acid also produces 2,3-pentanediol and pyruvic acid.
• Levulinic acid, another platform chemical, produces 1,4-pentanediol, succinic acid, and the pesticide 5-aminolevulinic acid.
• Hemicellulose, a lignocellulose biomass fraction, can be hydrolyzed to produce pentose sugar.
• Xylose, a pentose sugar from hemicellulose hydrolysis, converts into xylitol and furfural.
• Furfural converts into furfural alcohol and furan.
• Xylitol, a pentose sugar alcohol, is a sugar substitute in the food industry because of its low calorific value and anti-carcinogenic effect.
• Xylitol is a building block for commodity chemicals.
• Furfural is a building block for non-petroleum-based chemicals (bio plastics), biofuels, and fuel additives.
• Furfural and its derivatives are used to make jet and diesel fuel range alkenes, used as a gasoline blend stock.
• Lignocellulosic biomass can be converted into a range of chemicals using emerging process conversion chemistry.