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Fast Pyrolysis of Biomass
Pyrolysis of biomass is an important process option either as a pretreatment for gasification (i. e., the first stage of two-stage gasification) or as an independent process such as fast pyrolysis. The former is usually aimed at producing biosyngas, whereas the latter is intended to produce a liquid fuel product. Typical biomass pyrolysis takes place actively at around 500°C and produces a liquid product via fast cooling (shorter than two seconds) of volatile pyrolytic products. The liquid product produced is called bio-oil or pyrolysis oil. Bio-oil is considered greenhouse gas neutral, because it only puts back into the atmosphere what was initially removed by the plant during its lifetime [50]. Bio-oil is nearly sulfur-free.
As can be seen from the prevailing pyrolysis temperature of ~500°C, pyrolysis of biomass as a process treatment is quite similar to oil shale pyrolysis [51] and coal pyrolysis [14], wherein hydrocarbon species are devolatilized and thermally cracked. Bio-oil production via biomass pyrolysis is typically carried out via flash-pyrolysis or fast pyrolysis. The biomass fast pyrolysis process [23, 52, 53] is a thermochemical conversion process that converts biomass feedstock into gaseous, solid, and liquid products via heating of biomass in the absence of oxygen or air. The principal product of typical fast pyrolysis of biomass is a liquid product of bio-oil. As the name of the process implies, the process is intended to take place very fast in a matter of a couple of seconds or shorter; that is, т > 2 s. As such, heat and mass transfer conditions in the reactor become crucially important in both design and operation of fast pyrolysis of biomass. A variety of reactor designs has been proposed and tested on pilot scales, and they include traditional fluidized bed reactors, circulating fluidized bed reactors, rotating cone reactors, vacuum reactors, ablative tubes, and more. Table 5.11 shows a partial list of operational pilot-scale biomass pyrolysis units.
A typical biomass fast pyrolysis process involves several stages of operation including biomass feed drying, comminuting, fast pyrolysis, separation
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TABLE 5.11 Pilot-scale Fast Pyrolysis of Biomass
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of char, and liquid recovery. Fast pyrolysis of biomass has several distinct merits as an alternative fuel process technology and they are as follows:
• The principal product of biomass fast pyrolysis is bio-oil, a liquid product. As such, storage and transportation of the product are easy.
• The process takes place very quickly in a matter of 0.5-2 seconds and as such the reactor residence time is very short.
• The process technology is very widely and universally applicable to a variety of biomass feedstock.
• The process chemistry is simple and straightforward.
• The process equipment needed is relatively simple and not complex.
• Due to the high reactor throughput and simple chemistry, the capital cost is not high.
• Small-scale process feasibility has been demonstrated. However, process economics for a small-scale power generation (<5 MWe) based on fast pyrolysis of biomass is substantially less favorable than that of a larger scale (>10 MWe) system.
However, the drawbacks of fast pyrolysis of biomass include the following:
• The moisture level of the feed biomass needs to be controlled below 10% or even lower. Otherwise, the feed water and reaction-produced water will end up in the final liquid oil product.
• The biomass feedstock needs to go through size reduction and preconditioning. The feed material has to be in particulate form in order to minimize the heat and mass transfer resistance; preferably ~2 mm for bubbling bed and ~6 mm for circulating fluidized bed (CFB).
• The quality of bio-oil product is generally of poor quality. Bio-oil has a high oxygen content which makes the oil more corrosive and unstable, in addition to possessing a lower heating value. Bio-oil also contains metallic compounds and nitrogen species, which can foul and deactivate most of the fuel upgrading catalysts. The upgrading process requires a large amount of hydrogen and becomes costly.
• The process requires a very fast heating rate, which is costly in both operation and capital investment.
• The overall energy efficiency of the process is not high, by itself.
• Large-scale process operation may be subjected to significant logistical burdens of feedstock collection, storage, and pretreatment.
The first biocrude in the United States was produced at a 30 kg/hr scale [64] at operating conditions of approximately 500°C and a residence time of one second, that is, at a typical fast pyrolysis condition. Since then, Canadian researchers have converted woody biomass into fuel via pyrolysis in a 200 kg/hr pilot plant [65]. The fuel oil substitute was produced (on 1,000 ton/day dry basis) at approximately $3.4/GJ, based on the 1990 fixed price (1990 U. S. dollars). At the time, the cost for light fuel oil was $4.0-4.6/GJ [66], thus indicating that the pyrolyzed biomass fuel was a more economical alternative. However, the skepticism of high transportation costs of the biomass to the pyrolyzer outweighing potential profits limited R&D funding for the following years. This was when the petroleum-based liquid fuel price was much lower than that in the twenty-first century. To circumvent this biomass transportation and logistical problem, the Energy Resources Company (ERCO) in Massachusetts developed a mobile pyrolysis prototype for the U. S. EPA.
ERCO’s unit was designed to accept biomass with 10% moisture content at a rate of 100 tons/day. At this rate, the system had a minimal net energy efficiency of 70% and produced gaseous, liquid, and char end products. The process, which was initially started using an outside fuel source, became completely self-sufficient shortly after its startup. This was achieved by implementing a cogeneration system to convert the pyrolysis gas into the electricity required for operation. A small fraction of the pyrolysis gas is also
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FIGURE 5.3
A schematic and material balance for ERCO’s Mobile Pyrolysis Unit.
used to dry the entering feedstock to the required 10% moisture. A simplified version of ERCO’s mobile unit is shown in Figure 5.3 [67].
The end-products are pyrolysis oil and pyrolytic char, both of which are more economical to transport than the original biomass feedstock. The average heating values for the pyrolysis oil and char are 10,000 BTU/lb and 12,000 BTU/lb, respectively [67]. The pyrolysis gas, which has a nominal heating value of 150 BTU/scf, is not considered an end product because it is directly used in the cogeneration system. If classified based on the coal syngas criterion, typical pyrolysis gas of biomass would be classified as a low-BTU gas whose usual criterion for the heating value is less than 300 BTU/scf [5]. The mobility, self-sufficiency, and profitability of the system lifted some of the hesitancy of funding research on the pyrolysis of biomass. In addition, ERCO’s success led to additional investigation of "dual," or cogeneration systems, which produce both useful heat and electric power, that is, combined heat and power (CHP).
In a fluidized bed fast pyrolysis reactor, fine particles (2-3 mm size) of biomass are introduced into a bed fluidized by a gas, which is usually a recirculated product gas [68]. High heat and mass transfer rates result in rapid heating of biomass particles via convective heat. Char attrition and bio-oil contamination may take place and char carbon could appear in the bio-oil product. Heat can be supplied externally to the bed or by a hot fluidizing gas. In most designs, reactor heat is usually provided by heat exchanger tubes through which hot gas from char combustion flows. The reactor effluent
leaves from the top of the reactor as noncondensable gas, char, bio-oil vapor, and aerosol. The fluid bed may contain, in addition to biomass particles, fluidizing media such as hot sand particles for enhanced heat transfer. Aerosol is generally defined as a suspension of solid and liquid particles in a gas. The term aerosol includes both the particles and the suspending gas and the particle size may range from 0.002 to larger than 100 pm [49]. There is some dilution of the products due to the use of fluidizing gas, which makes it more difficult to condense and then separate the bio-oil vapor from the gas exiting the condensers. The general operational principle is quite similar to that of the traditional gas-solid fluidized bed reactor [69]. The fluidized bed fast pyrolysis process has been pilot tested by several companies including Dynamotive [54], Wellman [56], and Agri-Therm [70], as listed in Table 5.11. Special versions of fluidized bed reactors popularly used in biomass fast pyrolysis are bubbling fluidized bed (BFB) and circulating fluidized bed (CFB) reactors. A principal difference between the two types is that the former has a freeboard space above the fluid bed (as shown in Figure 5.4), whereas the latter achieves a full entrainment of the particle-fluid mixture in the reactor.
Bubbling fluidized bed reactors have long been used in chemical and petroleum processing. An earlier version of bubbling fluidized sand bed reactor was utilized by the WFPP (Waterloo Fast Pyrolysis Process) [71, 72]. Larger units based on the BFB are a 200 kg/hr system by Union Fenosa (Spain) and a
400 kg/hr system by DynaMotive (Canada). Both systems were based on the WFPP developed at the University of Waterloo (Canada) and designed by its spin-off company Resource Transforms International (RTI) in Canada [55]. Bubbling fluidized beds (BFBs) occur when the incoming carrier gas velocity is sufficiently above the minimum fluidization velocity to cause the formation of bubblelike structures within the particulate bed. In such a condition, the bed appears more or less bubbling.
A variety of different designs has also been introduced. The particulate or granular bed in the BFB may be composed of biomass particles only without any inert media and all the process heat can be supplied by hot fluidizing gas. Alternately, the granular bed can contain a hot heat-transfer solid medium such as indirectly heated sand, which enhances heat transfer efficiency and generally allows for a larger throughput for the process. An earlier version of the DynaMotive system used natural gas to heat their pilot-scale reactor, but most of modern designs use the exothermic heat of char combustion to supply the necessary heat to the pyrolysis reactor. The BFB is, in principle, self-cleaning as the by-product char is carried out of the reactor by the product gases and oil vapors [55]. For this process feature, the density of char will have to be less than that of fluidizing media so that the by-product char will literally "float" on top of the bed. The allowable particle size range for this type of reactor is quite narrow and should be carefully managed. Furthermore, the bio-oil produced has a carbon contamination possibility, inasmuch as the oil vapor has to pass through the char-rich layer on its way out of the bed [55]. The gas flow rate for this reactor will have to be determined based on the dimensions of fluidizing media and the desired residence time of gas and oil vapor in the freeboard section of the reactor, which is above the bed. The residence time is generally between 0.5 and 2.0 seconds. The BFB is currently most popularly used for both fast pyrolysis and gasification processes of biomass. In order to achieve a short residence time for volatiles, a shallow bed depth, a high gas flow rate, or usually both are utilized [57, 73]. A high gas-to-biomass feed ratio is adopted for necessary fluidization and a short residence time, which in turn results in product dilution and lowers the thermal efficiency of the process.
Circulating fluid bed reactors and derivative types of reactor design are frequently utilized for fast pyrolysis of biomass. The basic concept of CFB reactors involves efficient and rapid heat transfer in a convective mode and short residence times for both biomass particles and product vapors. Fine biomass particles are introduced into a circulating fluidized bed of hot sand. Hot sand and biomass particles move together with the transport gas which is usually a recirculated product gas. In reactor engineering, this transport gas is usually referred to as a fluidizing gas. In practice, the residence times for solid biomass particles are not uniform and only a little longer than the volatiles. Therefore, solid recycling of partially reacted feed would become necessary or very fine particle size of biomass would have to be used. Most CFB reactors are dilute phase units and their heat transfer rates are high,
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Gas, Oil Vapors, and Aerosol
FIGURE 5.5 Circulating fluidized bed reactor for fast pyrolysis of biomass. |
but not as high as particularly desired, because the mode of heat transfer is gas-solid convective heat transfer [57, 74]. If a twin-bed reactor system is used, that is, the first for fast pyrolysis and the second as a char combustor to reheat the circulating solids, as shown in Figure 5.5, there is a strong possibility for ash carry-over to the pyrolysis reactor and ash buildup in circulating solids [57].
The ash attrition and char carry-over problem could also be high and if not controlled properly, some level of contamination of bio-oil products is also possible. One of the main advantages of the CFB is the possibility to achieve a short and controllable residence time for char [75]. The alkaline compounds in biomass ash are known to possess a catalytic effect for cracking organic molecules contained in volatile vapors, thereby potentially lowering the volatile bio-oil yield. Red Arrow and VTT processes are based on circulating fluid bed reactors.
A rotating cone fast pyrolysis system (Figure 5.6) operates based on the idea of intensive mixing between biomass particles and hot sand particles, thereby providing good heat and mass transfer. This type of reactor requires very fine to fine biomass particle size. This process does not
Aerosols and
Hot Sand Oil VaPors
Rotating
FIGURE 5.6
A schematic of rotating cone reactor for fast pyrolysis of biomass.
require any carrier gas for its operation and therefore the reactor size can be made compact. The BTG’s fast pyrolysis process developed by Biomass Technology Group (the Netherlands) is based on a modified rotating cone reactor [60].
In this reactor, efficient heat transfer between the hot sand particles and biomass particles is accomplished, and a good portion of the process heat is retained in the hot sand particles. A wide variety of different biomass feedstock can be processed in the pyrolysis process which is operated at 450- 600°C. Before entering the reactor, the biomass feedstock must be reduced in size to a particulate form finer than 6 mm, and its moisture content to below 10 wt%. Sufficient excess heat is normally available from the pyrolysis plant to dry the feed biomass from 40-50 wt% moisture to below 10 wt%. A schematic of rotating cone reactor is shown in Figure 5.6.
From the BTG process, up to 75 wt% pyrolysis oil and 25 wt% char and gas are produced as primary products [60]. Because no "inert" carrier gas is used in this process, no additional gas heating is required and the pyrolysis products are undiluted vapor. This undiluted vapor flow allows the downstream equipment to be of a minimum size. In a condenser, the oil vapor product is rapidly cooled yielding the oil product and some permanent (noncondensable) gases. In only a few seconds of process treatment, the biomass is transformed into pyrolysis oil. Biomass char and hot sand used in the reactor are recycled to a combustor, where char is combusted to reheat the sand. After reheating the sand by char combustion heat, the sand is recirculated back
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to the reactor. The permanent gases (or noncondensable gas) can be utilized in a gas engine to generate electricity or simply flared off. In principle, no external utilities are required for operation of the process, that is, energy — wise self-sufficient. A schematic of a BTG process for fast flash of biomass is shown in Figure 5.7.
The vacuum pyrolysis process operates under low pressure (vacuum or atmospheric) and has principal process merits in its processability of larger biomass particles as well as its short residence time for volatiles. These biomass particles are taken out using a vacuum pump from the reactor regardless of the particles’ residence time.
Due to the lack of convective gas flow inside the reactor, however, the heat and mass transfer rates are slower than those for the fluidized bed reactors, hence requiring a longer residence time for biomass particles in the vacuum reactor. A longer biomass residence time in turn makes the reactor and equipment size inevitably larger. Biomass in a vacuum reactor moves downward by gravity and rotating scrapers through the multiple hearth pyrolyzer with the temperature increasing from about 200°C to 400°C, as shown in Figure 5.8. Pyrovac’s pyrocycling™ process is based on vacuum pyrolysis technology [61]. According to Pyrovac, their process technology has the following features:
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FIGURE 5.8 A schematic of vacuum pyrolysis reactor for biomass fast flash. |
• There is no need to pulverize or grind the feed biomass materials. Particles of up to 20 mm can be fed without any difficulty.
• The process system can be operated under torrefaction mode (<300°C) or pyrolysis mode (>450°C). The first mode generates biochar as a principal product, whereas the latter mode produces bio-oil.
• The process can be operated under vacuum or atmospheric conditions to enhance the production of either bio-oil or biochar. The process operation as well as its product portfolio is more versatile compared to competing process technologies.
• The process adopts a moving and stirred bed reactor.
• Molten salt heat carrier at 575°C is in indirect contact with biomass feedstock, thus aiding in efficient heat transfer.
• There are two heating plates with internal raking systems.
• The process uses two condensing towers. The first tower mainly collects heavy bio-oil and contains little water and acids. The second tower mainly recovers the aqueous acidic phase.
The ablative pyrolysis process is based on the heat transfer taking place when a biomass particle slides over a hot surface, as shown in Figure 5.9. High pressure applied to biomass particles on a hot reactor wall or surface to provide good contact is achieved by centrifugal or mechanical motion [68].
Bio-oil Collection
FIGURE 5.9
A schematic of ablative pyrolysis reactor for fast pyrolysis of biomass.
This type of reactor does not require a small particle size and can handle large particles without difficulty. The reactor does not require any carrier gas or sweep gas. The reactors of cyclonic type have difficulty in achieving sufficiently long residence times for the biomass particles that are required to allow a high degree of conversion.
Therefore, it is usually necessary to recycle partially reacted solids back to the reactor, as is the case with a circulating fluidized bed reactor. A high degree of char attrition also takes place and tends to contaminate the product bio-oil with a high level of carry-over carbon [57]. Some variations of the ablative pyrolysis process include the cone type and plate type for hot surfaces. These processes are mechanically more complex and difficult to scale up, because the moving parts are subjected to the high temperatures required for pyrolysis. Furthermore, the loss of thermal energy from the ablative process in general could be high, inasmuch as the hot surface needs to be at a substantially higher temperature than the desired pyrolysis temperature.
Fast pyrolysis of biomass can be achieved in an auger-type reactor or auger reactor, in which an auger or an advancing screw assembly drives the biomass and hot sand through the reactor barrel. The operational principle is very similar to that of a polymer twin-screw extruder, as shown in Figure 5.10. This type of reactor achieves a very good mixing of materials
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FIGURE 5.10 An auger type reactor for biomass fast pyrolysis. |
in the reactor, thereby enhancing the heat and mass transfer efficiency. In an auger-type reactor for biomass fast flash, the usual mechanism of heat transfer is via direct heat transfer between hot sand and biomass particles.
At the outlet of the reactor, solids including hot sands, char, and ash are separately recovered from the oil vapors and aerosol. Hot sand is reheated using char combustion heat and recirculated back to the reactor, and the oil vapor and aerosol are sent to the condenser for bio-oil recovery. The process does not require a carrier gas, but the biomass particle size is preferentially small for smooth operation. The design of auger reactors has received extensive benefits from the industrial practice and design experience of the polymer and twin-screw extruder industries [76]. Therefore, this type of process is relatively straightforward to design and fabricate and is deemed to be suitable for small-scale production.