Thermochemical conversion technology - Biomass pyrolysis
Presentation Outlines Introduction - Operating conditions of the thermal processes What is pyrolysis? Classification of pyrolysis methods Some Advantages of Pyrolysis of Biomass Pyrolysis of Biomass: reaction and products Pyrolysis Technology Variant Types of pyrolytic reactors Parameters influencing pyrolysis process Mechanism and products of biomass pyrolysis Mobile pyrolysis unit
Introduction - thermochemical conversion technologies
Introduction - Operating conditions of the thermal processes Temperature Atmosphere Products Mean overall Yield Combustion > 900°C O2 (air) CO2 + H2O + N2 + ashes to be treated ~ 65 % Pyrolysis < 500°C Inert gas or Low pressure char + tars + gas, which proportions are related to the pyrolysis parameters ~ 45 % Gasification by Fast pyrolysis > 700°C Mainly gas (CO, H2, CH4, C2H4 …) with low quantity of char used ~ 75 % Gasification > 800°C Air or H2O vapour Gas (H2, CO, CO2, CH4, N2) 50-60 % Liquefaction by Fast Pyrolysis < 550°C High viscosity liquid (phenols) Direct Liquefaction 300°C- 350°C Slurry in water CO High pressure High viscosity liquid (phenols) non soluble in water ~ 80 %
Heat introduced, O2 excluded What is pyrolysis? There are several definitions depending on the source, but essentially it is a thermochemical process, conducted at 400-600°C in the absence of oxygen. Pyrolysis is thermal cracking in the absence of oxygen. Pyrolysis is the thermal decomposition of organic material at elevated temperatures, in the absence of gases such as air or oxygen. Pyrolysis is the irreversible chemical composition and physical phase change brought about by heat in the absence of oxygen. In practice, it is not possible to achieve a completely oxygen-free atmosphere. Because some oxygen is present in any pyrolysis system, a small amount of oxidation occurs. Heat introduced, O2 excluded An endothermic reaction
Pyrolysis is the key reaction of all the thermal processes What is pyrolysis? Pyrolysis is the key reaction of all the thermal processes WOOD Cutting or Grinding Drying Pyrolysis Gasification Combustion Liquefaction Charcoal making Heated Wood Pyro = heat Lysis = break down
What is pyrolysis? Because no oxygen is present the material does not combust but pyrolyzed biomass undergoes a sequence of changes and normally yields a mixture of gases, liquids and solid. The solid is called charcoal while the condensable liquid is variously referred to as pyroligneous liquid, pyroligneous liquor, pyroligneous acid or pyrolysis oil. The gas is called producer gas or wood gas. The proportion of these products depends on several factors including the composition of the feedstock and process parameters. Generally low temperatures and slow heating rates results in high yield of charcoal. This type of pyrolysis is called carbonization. Char (containing carbon & inorganic ash) can be separated. Vapours can be rapidly quenched yielding Bio-Oil & uncondensable gases. Gases contain light hydrocarbons (CO2, H20, CO, C2H2, C2H4, C2H6, C6H6,etc.).
What is pyrolysis? In a now obsolete process for production of methanol, acetic acid and acetone, wood is heated in a retort in absence of air and the liquid vapours are condensed. This type of pyrolysis is generally called destructive distillation. In a relatively recent development it is found that yields of volatiles (gas and liquid) increase with the rate of heating. At sufficiently high heating rates all biomass can be nearly converted to volatiles. This is known as fast pyrolysis. Although pyrolysis (as a process for charcoal making) has been known to man since time immemorial, the complex pattern of series and parallel reactions involved in the process is not yet fully understood.
What is pyrolysis? Charcoal Retort Retort - a vessel in which substances are distilled or decomposed by heat. The Exeter charcoal retort is particularly useful for small farming operations, small forest enterprises, compost and greenhouse operations, and livestock processors.
Why pyrolysis? Pyrolysis technology offers the following unique selling points: Pyrolysis oil is produced from non food biomass and is therefore a second generation biofuel which does not compete with the food chain. The local decentralized production of pyrolysis oil seperates out the minerals in the biomass so they can be recycled to maintain the soil quality. Due to its energy density and liquid form existing infrastructure can be used for transporting pyrolysis oil. GHG savings (GreenHouse Gas) of raw pyrolysis oil are well above that of other biofuels. (85-95% for heat and power application) Pyrolysis oil can be stored for long periods of time, and is therefore available when necessary. It can substitute fossil fuels in heat and power applications and thereby provide peak renewable power to complement other (intermittent) renewable power sources such as wind and solar.
Classification of pyrolysis methods There are three types of pyrolysis: 1) conventional/slow pyrolysis, 2) fast pyrolysis, and 3) ultra-fast/flash pyrolysis. Classification of pyrolysis methods with differences in temperature, residence time, heating rate, and major products. The major products are listed in decreasing importance. Method Temperature (°C) Residence Time Heating rate (°C/s) Major products Conventional/slow pyrolysis Med-high 400-500 Long 5-30 min Low 10 Gases Char Bio-oil (tar) Fast pyrolysis Med-high 400-650 Short 0.5-2 s High 100 Bio-oil (thinner) Gases Char Ultra-fast/flash pyrolysis High 700-1000 Very short < 0.5 s Very high >500 Gases Bio-oil
Classification of pyrolysis methods conventional/slow pyrolysis, fast pyrolysis, and ultra-fast/flash pyrolysis. Most significant difference is the residence time of the solid phase within the reactor –seconds, minutes, up to hours and correlated energy transfer and temperature distribution Gas phase residence times for fast and intermediate pyrolysis are usually below 2s
Classification of pyrolysis methods Flash Pyrolysis achieve up to 75% of bio-oil yield. rapid de-volatilization in an inert atmosphere, high heating rate of the particles, High reaction temperatures between 450 °C and 1000 °C. Limitations: poor thermal stability and corrosiveness of the oil, solids in the oil Slow Pyrolysis takes several hours to complete results in biochar as the main product Fast Pyrolysis takes seconds for complete pyrolysis. yields 60% bio-oil In addition, it gives 20% biochar and 20% syngas.
Classification of pyrolysis methods Slow pyrolysis Primarily to produce Char through Carbonization. Utilizes low temperatures around 400 ⁰C over a long period of time to maximize char formation Oldest form of “man-made” pyrolysis, Fast pyrolysis Primarily to produce Bio-Oil and Gas Biomass is very rapidly heated (~1000-10,000 ⁰C/s) to a temperature around 650⁰C- 1,000⁰C depending if bio-oil or gas products are desired. Product gases are quickly removed and quenched (t<2s) Pyrolysis in a Medium –Usually water or Hydrogen Hydrogen is used because the hydrogen molecules bind to the decomposed hydrocarbons in a manner that increases the volatile (gaseous) yield of light hydrocarbons. Water is used to crack biomass in order to produce bio-oil with reduced oxygen content.
Some Advantages of Pyrolysis of Biomass Carbon neutrality Utilizes otherwise waste biomass Potential to be self-sustaining energy-wise Increases bulk and energy density of biomass Source of valuable chemicals Biomass source can be decoupled from the energy utilization
Economical and environmental advantages Utilization of renewable resources through carbon neutral route – environmental potential Utilization of waste materials such as lumber processing waste (barks, sawdust, forest thinnings, etc.), agricultural residues (straws, manure, etc.)– economic potential Self-sustaining energy – economic potential Conversion of low energy in biomass into high energy density liquid fuels – environmental & economic potentials; Potential to produce chemicals from bio-based resources – environmental & economic potentials
Fast or intermediate? Fast pyrolysis seems to be a good approach for wood Non-woody biomass: Liquids itself can be bituminuous, depending on feedstock The separation from solid phase from liquid and gas phase is not yet solved In using slurry all (most) solid ingredients persist in the mixture which limits their application for different gasifier types Feedstock has to be well prepared, usually fine particles Feedstock with elevated moisture content is not suitable Scale for a single pyrolysis units of several hundred to power gasifiers up to 5 GW are described to have up to 220000 t/a throughput for economic reasons –logistic and technical challenge
Fast pyrolysis Advantages Operates at atmospheric pressure and modest temperatures (450 C) Yields of bio-oil can exceed 70 wt-% Disadvantages High oxygen and water content of pyrolysis liquids makes them inferior to conventional hydrocarbon fuels Phase-separation and polymerization of the liquids and corrosion of containers make storage of these liquids difficult
Pyrolysis Technology Variant Pyrolysis processes classified based on heating rates and residence time Process Residence Time Heating Rate Temp (C) Products Carbonization Days Very low 400 Charcoal Conventional 5 – 30 min Low 600 Oil, Gas, Char Fast 0.5 – 5 sec Very high 650 Bio-oil Flash-liquid < 1 sec High <650 Flash-gas Chemicals, Gas Ultra < 0.5 sec 1000 Vacuum 2- 30 sec Medium Hydro-pyrolysis < 10 sec <500 Methano-pyrolysis >700 Chemicals
Pyrolysis of Biomass Flash pyrolysis of biomass Gas Biomass Bio-oil REACTOR Biomass Heat condenser Char Separation Gas Bio-oil
The Organic Chemistry Pyrolytic reaction using cellulose: Liquid Bio-Oil Combustible Gas Biochar Water of Pyrolysis HEAT 3C6H10O5 C6H8O +8H2O +CH4+2CO+2CO2+ 7C
Reaction Scheme Gas + Bio-oil 425-600 oC Gas Char Rapid heating Biomass Rapid heating (1000 K s-1) Rapid cooling Gas Bio-oil Char Heat transfer by conduction, convection and radiation Pressure gradients inside the degrading solid Surface regression, crack formation, swelling, shrinkage Physical Processes Primary degradation to form char and gases Chemical Processes Secondary reactions, polymerization, cracking
Pyrolysis: reaction and products CO CO2 H2 C1-C3 GAS (10-30 wt.%) CxHyOz BIOMASS CHAR (10-35 wt.%) BIO-OIL (10-75 wt.%) Cellulose Hemicellulose Lignin Reaction Conditions: Temperature Heating rate Vapors residence time … (≈ 500 ºC) (103-104 K/s) (≈ 1-2 sg) Fast-Pyrolysis
Pyrolysis Products Gases (10-20%) (Mainly CO, CO2, CH4, H2, H2O) Tar (Bio-oil) (60-75%) (High molecular weight liquids, volatile at reaction temperature) Char (10-20%) (Solid carbonaceous residue) Solid Biomass
Pyrolysis Products Hydrocarbon Fuel from Bio-Oil Bio-oil properties: – Can be acidic, corrosive – Chemically unstable (forms tars) – Low energy content (18-25 MJ/kg) – High water content (15-30%) Molecular hydrogen (H2) used in upgrading methods to date (centralized H2 production)
Fast Pyrolysis and Electrocatalytic Upgrading
Different pyrolysis conditions and products Figure summarizing different pyrolysis conditions and the effect on product distribution
Product of pyrolysis Depending on the thermal environment and the final temperature, pyrolysis will yield: mainly biochar at low temperatures, less than 450 0C, when the heating rate is quite slow, mainly gases at high temperatures, greater than 800 0C, with rapid heating rates. at an intermediate temperature and under relatively high heating rates, the main product is bio-oil.
Product yields for pyrolysis Percentage composition of liquid, solid and gaseous products of different pyrolysis modes
Pyrolysis processes & yields Operating Conditions Bio-Oil Yields Char Gas Slow Pyrolysis T ≈ 400°C Time ≈ hours <5% ~50% ~45% Intermediate T ≈ 300-600°C Time ≈ 10-30 seconds ~40% ~20% Fast Time < 1 second ~75% ~15% ~10%
Pyrolysis Technology Variant It is important to differentiate pyrolysis from gasification. Unlike pyrolysis, gasification of solid biomass is carried out in the presence of oxygen, where the material is decomposed into producer gas or syngas by carefully controlling the amount of oxygen present. Moreover, gasification aims to produce gaseous products while pyrolysis aims to produce bio-oils and biochar. Liquefaction can also be confused with pyrolysis. The two processes differ in operating parameters, requirement of catalyst, and final products. Liquefaction produces mainly liquid.
Pyrolysis Technology Variant - Oxidizing Pyrolysis It’s impossible to achieve a completely oxygen-free atmosphere. Thus, a small amount of oxidation occurs. If volatile or semi- volatile materials are present in the waste, thermal desorption will also occur. Thermal decomposition of industrial waste by its partial burning or direct contact with end product of fuel combustion This method is used for neutralization of most wastes including “inconvenient” ones for burning
Pyrolysis chart
Fast pyrolysis cycle Schematic of the Fast Pyrolysis Process The pyrolysis process can be self-sustained, as combustion of the syngas and a portion of bio-oil or bio-char can provide all the necessary energy to drive the reaction.
Types of pyrolytic reactors System Configuration A pyrolysis system unit typically consists of the equipment for biomass pre-processing, the pyrolysis reactor, and equipment for downstream processing. Can be classified as units that produce heat and biochar (using slow pyrolysis) or units that produce biochar and bio-oils (using fast pyrolysis).
Types of pyrolytic reactors Classification based on solid movement Reactors used for biomass pyrolysis is most commonly classified depending on the way the solids move through the reactor during pyrolysis. Type A: No solid movement through the reactor during pyrolysis (Batch reactors) Type B: Moving bed (Shaft furnaces) Type C: Movement caused by mechanical forces (e.g. rotary kiln, rotating screw etc.) Type D: Movement caused by fluid flow (e.g., fluidized bed, spouted bed, entrained bed etc.)
Types of pyrolytic reactors Classification based on method of heat supplied Pyrolytic reactor can also be classified depending the way heat is supplied to biomass: Type 1: Part of the material burnt inside the reactor to provide the heat to carbonize the remainder Type 2: Direct heat transfer from hot gases produced by combustion of the pyrolysis products or any other fuel outside the reactor. Type 3: Direct heat transfer from inert hot material (hot gases or sand introduced into the reactor). Type 4: Indirect heat transfer through the reactor walls (i.e. external heat source due to combustion of one or more pyrolysis products or any other fuel).
Types of pyrolytic reactors Variations in the Process and Reactors Many different combinations of modes of solid movement and modes of heat transfer are possible in practice. Accordingly, the type of a pyrolytic reactor can probably be best specified by denoting it as type XI where X stands for type of solid movement and I indicates the type of heat transfer. Different names are often used to describe specific type of pyrolytic reactors. The term “kiln” is used for devices producing only charcoal. The terms “retort” and “converter” are used for equipments capable of recovering by-products. The term “converter” normally refers to devices used for pyrolysing biomass of small particle size and the term “retort” refers to equipment for pyrolysing log reduced in size to about 30 cm length and 18 cm diameter.
Types of pyrolytic reactors Fast Pyrolysis Reactor Designs A number of different pyrolysis reactor designs are available. These include Fluidized bed, Re-circulating fluidized bed, Ablative, Rotating cone, Auger (or screw), Vacuum, Transported bed, and Entrained flow. Fluidized bed Rotating cone
Types of pyrolytic reactors Fast Pyrolysis Reactor Designs Fluidized Bed These types of reactors utilize vessels containing a mass of heated particles, such as inert sand or catalyst particles, that are “fluidized” by passing inert gas or recycled product gas through the particle bed. Biomass residue particles are injected into or above the hot sand bed by a solids feeder, such as a screw feeder or intermittent solid slug feeder. Re-circulating fluidized bed
Types of pyrolytic reactors Vacuum Pyrolysis Biomass moved by gravity and rotating scrappers through multiple hearth pyrolyzer with temperature increasing from 200 C to 400 C Can use larger particles and employs little carrier gas Expensive vacuum pump and difficult to scale-up Advantages Drawbacks Feed particle size flexibility Low Bio-oil yield, Increased pyrolytic water generation Fewer aerosols formed (easier quenching) Low heating efficiency Bio-oil free of char Absorption of liquid effluents in the liquid ring compressor pump No additional carrier gas/product dilution High capital cost, maintenance cost and high sealing/gasket requirements
Types of pyrolytic reactors Types of Pyrolysis Reactor Designs As pyrolysis is a precursor to gasification and combustion, the same reactors used for gasification can be used for pyrolysis. Bubbling fluidized bed reactors are simpler to design and construct than other reactor designs, and have good gas to solids contact, good heat transfer, good temperature control, and a large heat storage capacity. Circulating fluidized bed pyrolysis reactors are similar to bubbling fluidized bed reactors but have shorter residence times for chars and vapors which results in higher gas velocities, faster vapor and char escape, and higher char content in the bio-oil. They have higher processing capacity, better gas-solid contact, and improved ability to handle solids that are difficult to fluidize.
Types of pyrolytic reactors Bubbling Fluidized Bed Bubbling fluidized bed reactors utilize fluidized bed reactors with gas passing through the reactor so that the solids fluidization is in the “bubbling” regime, i.e. the bed has fully expanded and is bubbling aggressively, but without reaching the turbulent flow regime. Heat supplied externally to bed Good mass & heat transfer Requires small biomass particles (2-3 mm) Advantages Drawbacks Good temperature control and mixing Product dilution from fluidization gas Easy to scale up Easy to scale up Condensation train & separation challenges Well-established technology Particle size restricted Intense heat and mass transfer Char traps some sand
Types of pyrolytic reactors Circulating Fluidized Bed/Transport Reactor The heat transfer medium is the bed of particles (sand, catalyst, etc.) which is circulated, using high flowrates of gas, from the reactor vessel into a burner. In the burner, the particles are exposed to oxygen and recycled product gas or solid reaction products are burned to heat the particles and then they are circulated back into the reactor vessel Hot sand circulated between combustor and pyrolyzer Heat supplied from burning char High throughputs but more char attrition
Types of pyrolytic reactors Ablative Pyrolyzer Ablative pyrolysis processes involve the contact between the biomass residue and a hot reaction surface, which also performs mechanical ablation of the biomass surface removing the char layers formed. Heat transfer limitations do not exist within the particle in these processes as a result of the ablation, and therefore relatively large particles can be used (< 20 mm) heat transfer limitations are often present in delivering the heat required to the reaction surface High pressure of particle on hot reactor wall achieved by centrifugal or mechanical motion Can use large particles and does not require carrier gas Complex and does not scale well
Types of pyrolytic reactors Advantages and Drawbacks of the Circulating Fluidized Bed Advantages Drawbacks Well-established technology Challenging to operate/condensation/ separation Very large processing capacity Smaller biomass particles required Controllable residence time High gas flow and product dilution High heating rate Char attrition, Char contains some sand Good heat and mass transfer High separation and quenching requirements Advantages and Drawbacks of the Ablative Reactor Advantages Drawbacks Large particle sizes can be used Reaction rates limited by heat transfer to the reactor Inert gas is not required Process is surface area controlled, high cost to scale up Controllable residence time High gas flow and product dilution System is more intensive Good heat transfer
Types of pyrolytic reactors Rotating Cone Pyrolyzer Sand and biomass brought into contact within rotating cone Compact design and does not need carrier gas Requires very small biomass particles and is hard to scale-up Advantages Drawbacks Centrifugal forces moves heated sand and biomass Complex process No carrier gas needed Difficult to scale up Easy quenching High capital costs Small particle size needed
Types of pyrolytic reactors Auger Reactor Hot sand and biomass mixed by auger By using high thermal conductivity heat carriers, the energy required for fast pyrolysis is rapidly transferred to the biomass. Suitable for small scale Requires hot sand heating and circulation system Advantages Drawbacks Low pyrolysis temperature (400°C) Plugging risk Compact, flexible design Lower bio-oil yield No carrier gas/dilution Moving parts in the hot zone Quality bio-char produced Heat transfer limitations at large scale
Types of pyrolytic reactors Heat Transfer Modes and features of various reactors: Reactor type Mode of heat transfer Typical features Fluidized bed 90% conduction; 9% convection; 1% radiation High heat transfer rates; Heat supply to fluidizing gas or to bed directly; Limited char abrasion; Very good solids mixing; Particle size limit < 2 mm in smallest dimension; Simple reactor configuration Circulating fluidized bed 80% conduction; 19% convection; High heat transfer rates; High char abrasion from biomass and char erosion; Leading to high char in product; Char/solid heat carrier separation required; Solids recycle required; Increased complexity of system; Maximum particle sizes up to 6 mm; Possible liquids cracking by hot solids; Possible catalytic activity from hot char; Greater reactor wear possible Entrained flow 4% conduction; 95% convection; Low heat transfer rates; Particle size limit < 2 mm; Limited gas/solid mixing Ablative 95% conduction; 4% convection; Accepts large size feedstocks; Very high mechanical char abrasion from biomass; Compact design; Heat supply problematical; Heat transfer gas not required; Particulate transport gas not always required
Reactor selection criteria Reactor Type Simple Capital Expense Low Temperature Low Gas/Solid Ratio Operating Expense Easy Scale-up Bubbling Fluidized Bed Very Good Excellent Good Circulating Fair Poor Rotating Cone Vortex Ablative Very Poor Vacuum Processes Entrained Flow
Parameters influencing pyrolysis process The basic phenomena that take place during pyrolysis: Heat transfer from a heat source, leading to an increase in temperature inside the fuel; Initiation of pyrolysis reactions due to this increased temperature, leading to the release of volatiles and the formation of char; Outflow of volatiles, resulting in heat transfer between the hot volatiles and cooler unpyrolysed fuel; Condensation of some of the volatiles in the cooler parts of the fuel to produce tar; and Autocatalytic secondary pyrolysis reactions due to these interactions.
Parameters influencing pyrolysis process Pyrolysis process control parameters: Important pyrolysis process control parameters include: Heat rate (length of heating and intensity), Prevailing temperature and pressure The presence of ambient atmosphere The chemical composition of the fuel (e.g., the biomass resource), Physical properties of the fuel (e.g. particle size, density), Residence time and the existence of catalysts. These parameters can be regulated by selection among different reactor types and heat transfer modes, such as gas–solid convective heat transfer and solid–solid conductive heat transfer.
Overall mechanism of biomass pyrolysis Parameters influencing pyrolysis process Overall Mechanism On heating, the constituents of biomass materials decompose following different pathways and yielding a variety of products, each of which has its own kinetic characteristics. In addition, secondary reaction products result from cross-reactions of primary pyrolysis products and between pyrolysis products and the original feedstock molecules. Overall mechanism of biomass pyrolysis
Parameters influencing pyrolysis process Pyrolysis kinetics Important Factors governing the molecular reactions are: Chemical Kinetics, Heat Transfer and Mass Transfer. Physical aspects of the molecular reactions: Three Stages of Pyrolysis Up to 100⁰C the particle's dry out. At 200-600⁰C molecules of biomass decompose into solid and liquid yield along with non-condensable gases. (This is „Primary Pyrolysis‟) Vapours are extracted and condense as “tar” or “bio-oil” while large molecules continue to crack yielding char at 300-800⁰C. This is called “Secondary Pyrolysis”. Fine particle sizes offer little resistance to the escape of condensable gases and will therefore lead to a higher liquid yield. Whereas larger particle sizes will provide more resistance to the vapours as they try to escape and will therefore provide more opportunities for the vapours to crack resulting in a higher char yield.
Parameters influencing pyrolysis process Pyrolysis kinetics Chemical Aspects The biomass composition (i.e. relative levels of Cellulose, Hemi- cellulose and Lignin) effects the final pyrolysis products. There are several theorized “models” of the reaction processes that describe the interplay of factors like pyrolysis temperature, residence time and heating rate. Kinetic Models of Pyrolysis One-Stage global, single reaction models The pyrolysis is modeled by a one step reaction using experimentally determined factors One-Stage, Multi-reaction models: Several parallel reactions describe the degradation of biomass into the various yields. Two-stage semi-global models: Includes primary and secondary reactions to provide the most detailed results.
Parameters influencing pyrolysis process Heat Transfer in a Pyrolyzer During the Pyrolysis process heat is transferred to the particles primarily through radiation and convection, though some heating techniques use conduction. The primary heat transfer mechanisms are suggested: Conduction inside the particle Convection inside the particle pores Convection and radiation from the particle’s surface Heat transfer considerations are especially important in the design of the pryolyzer as heating rate plays a large role in the determination of the final products. All pyrolyzers will heat up the heating medium before operation though the medium's vary Reactor wall (Ablative Reactor) Gas (Entrained bed reactor) Heat Carrier Solids (Such as those in a fluidized bed) Though most units require heat initially, once the required temperature is reached several exothermic reactions proceed that provide enough heat for the process (Autothermal Reaction).
Mechanism and products of biomass pyrolysis Pyrolysis of main constituents To understand pyrolysis of wood, it is interesting to consider first the pyrolysis of the main wood constituents - cellulose, hemicellulose and lignin. Cellulose: C6H10O5; Lignin: C9H10O3(OCH3)0.9-1.7; Hemicellulose: C5H8O4 On an average hardwood contains 43% cellulose, 35% hemicellulose and 23% lignin while softwood contains 43% cellulose, 28% hemicellulose and 29% lignin. On heating, the constituents of wood decompose following different pathways and yielding a variety of products. Cellulose (150-350⁰C) turns into condensable vapour (tar) Hemi-cellulose (275-350⁰C) yields primarily non-condensable vapour Lignin (250-500⁰C) degrades slowly into char and liquid yield
Mechanism and products of biomass pyrolysis Pyrolysis of main constituents Thermal stability regimes for cellulose, hemi-cellulose and lignin
Mechanism and products of biomass pyrolysis Cellulose Pyrolysis Upon heating to temperatures below 250C cellulose undergoes a drop in the degree of polymerization and pyrolysis takes place slowly, the major products being H2O, CO2, CO and a carbonaceous residue. At temperatures above 250C cellulose begins to pyrolyse rapidly producing condensable “tar” along with gases and leaves a charred residue. The pyrolysis of cellulose proceeds very rapidly at around 350C and above 500C the volatile products begin to undergo gas-phase pyrolysis.
Mechanism and products of biomass pyrolysis Cellulose Pyrolysis Following figure provides a simplified reaction scheme of cellulose pyrolysis.
Mechanism and products of biomass pyrolysis Hemicellulose Pyrolysis Compared to cellulose, hemicellulose pyrolysis begins at a lower temperature but takes place over a much wider temperature range and produces less char. Lignin Pyrolysis Lignin is regarded as the most stable of the major biomass components. Below 200C its rate of thermal degradation is very slow. Lignin decomposes between 280C and 500C and produces more char compared to cellulose. At low heating rates the char yield from lignin exceeds 50% by weight.
Mechanism and products of biomass pyrolysis Behavior of biomass during pyrolysis depends on the behavior of its major components. Products of biomass pyrolysis can be regarded as a linear combination of products expected from the separated pyrolysis of the three major components. Cellulose and hemicellulose are the major sources of volatiles and tar while lignin is the major source of char. The biomass is decomposed by a number of parallel primary reactions into primary products, which are acted upon by a number of secondary reactions. Char is formed as a product of the primary reactions and as solid material deposited due to the secondary reactions.
Mechanism and products of biomass pyrolysis Overview of the thermal fractionation of biomass by a step-wise pyrolysis approach.
Mechanism and products of biomass pyrolysis The chemistry and products of biomass pyrolysis are summarized in the following table.
Mechanism and products of biomass pyrolysis The chemistry and products of biomass pyrolysis are summarized in the following table.
Mechanism and products of biomass pyrolysis Pyrolysis of large biomass is a complicate process and involves following steps: Transfer of heat to the surface of the particle from its surrounding usually by convection and radiation Conduction of heat through the carbonized layer of the particle Carbonization of the virgin biomass over a range of temperature inside the particle Diffusion of the volatile products from inside to the surface of the particles, and Transfer of the volatile products from the surface of the particle to the surrounding inert gas.
Mechanism and products of biomass pyrolysis Thus the rate of expression for pyrolysis in this case will incorporate heat and mass transfer terms in addition to kinetics terms of biomass decomposition reactions. The overall pyrolysis process is further complicated by Secondary pyrolysis of the volatile products while diffusing out through the particle, Heat transfer by convection to the volatile products while diffusing out through the particle, Shrinkage of the biomass particle as it undergoes pyrolysis, etc.
Mechanism and products of biomass pyrolysis
Mobile pyrolysis unit
Mobile pyrolysis unit
Mobile pyrolysis system (MPS) Mobile pyrolysis takes the pyrolysis plant directly into the agricultural and forestry operation, reducing transportation costs and converting low value stalks, leaves, straws, bagasse, chips, sawdust and branches, into higher value and high-energy density bio-oil and bio-char. Mobile pyrolysis unit engineered to address four major issues: Mobility – bring the processor to the biomass to reduce collection, handling and shipping expenses. Simplicity – operator-friendly, easy-to-learn, easy-to-use, mitigate operational labour costs. Adaptability – designed for multiple market uses and feedstocks. Affordability – one of the least expensive technologies on the market.
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