Ind. Eng. Chem. Process Des. Dev. 1984,23, 355-363
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Biomass Pyrolysis with an Entrained Flow Reactor Mark S. Bohn' Solar Energy Research Institute, Golden, Colorado 8040 1
Charles 8. Benham RenteCh, InC., Golden, Colorado 8040 1
A tubular entrained flow reactor has been used to study the effect of process variables on biomass pyrolysis. I n this type of reactor, finely ground biomass particles are entrained by carrier gas and transported through a reactor tube which is heated to about 900 O C . Biomass particles pyrolyze as a result of heat transfer from the reactor wall yielding a gas composed primarily of carbon monoxide, carbon dioxide, hydrogen, methane, and unsaturated hydrocarbons. In this experimental program three dependent variables, percent conversion to gas, gas composition, and process heat, have been measured as a function of several process control variables. These process variables include reactor temperature, carrier gas to biomass flow ratio, reactor residence time, biomass particle size, and reactor Reynolds number. The data allow one to design and predict the performance of large-scale reactors and also elucidate heat transfer mechanisms in biomass pyrolysis.
Introduction Several techniques have been proposed recently for the thermal conversion of solid organic materials to more valuable or more convenient solid, liquid, or gaseous forms. These organic materials, although readily available and abundant, have limited uses in terms of fuels or chemical feedstocks. Generally speaking, the conversion schemes use heat and various combinations of steam, hydrogen, or catalysts to convert the feedstock to various amounts of char, pyrolytic oils, hydrocarbon gases, hydrogen, and carbon oxides, with ash being a byproduct of most biomass feedstocks. Each technique employs a different reactor configuration which is determined by the feedstock heating method and the method used to transport the feedstock through the reactor. The method used to heat the feedstock is the primary determinant of the reactor product or products; reactor operating conditions determine product distribution. Thus, reactor design and operation are dictated by the desired product of the entire conversion scheme. Each of the aforementioned products has some degree of usefulness depending on the application, but each represents a higher value product than the feedstock in that application. Char production depends on temperature, rate of heating, atmosphere, and-to some extent-pressure (Shafizedeh, 1975, 1968). I t is generally associated with slow heating of cellulosic materials (Lincoln, 1965; Lewellen et al., 1976). Char produced from wheat straw in this study is composed of about 50% carbon, 3% hydrogen, 1% nitrogen, 0.5% sulfur, 32% ash, and 10% oxygen by mass. The same composition has been obtained for pyrolysis of the organic component of municipal solid waste at 500 OC and 1 atm pressure (Finney and Garrett, 1973). The product has value as solid fuel with a higher heating value of about 25 kJ/g (ash free basis) (Shafizedeh, 19751, as a substitute for carbon black (Finney and Garrett, 1973),or as an activated filtering medium. Pyrolytic oils are produced at temperatures above 250 "C and consist of oxygenated organic compounds suitable as liquid fuel in some applications (Office of Technology Assessment, 1980). Production of the oils can be increased by higher reactor pressure, use of catalysts, or by condensation of the oils before they react further (cracking) to produce char or gases. The oil has been compared by Finney and Garrett (1973) to number 6 fuel oil, although 0196-4305/84/1123-0355$01.50/0
it is acidic, has about half the heating value, and is much more viscous than the fuel oil. At significantly higher temperatures, perhaps 600 "C or higher, further degradation and decomposition of the vapors produces increasing amounts of hydrocarbon gases, hydrogen, and carbon monoxide. Very rapid heating of the feedstock favors this production of gases and minimizes char and pyrolytic oil production. Thus, applications requiring gas as the primary product tend to utilize reactors which employ heat-transfer mechanisms yielding high feedstock heating rates. Feedstock may be heated by heat that is either generated within the reactor or produced outside and transferred to the feedstock. For internal heat generation, air or oxygen is introduced into the reactor, and partial combustion of the feedstock or of char, oils, or pyrolysis gases provides the necessary heat of reaction. For externally generated heat, heat of combustion from these fuels may also be used, but utilization of other heat sources and fuels is possible. In addition, the product stream is not contaminated by nitrogen from the air or carbon dioxide produced by partial feedstock combustion. Reactors which utilize internally generated heat are known as air or oxygen gasifiers and exist in a variety of configurations, depending on how the feedstock is transported through the reactor and how the air or oxygen is pumped through the reactor (Reed, 1980). Some configurations operate in a batch mode: the reactor is loaded with feedstock, and air or oxygen is blown through the reactor. Continuous gasifiers typically have a provision for feeding biomass in the top of a vessel, with air or oxygen being blown through the vessel either top-to-bottom (downdraft) or vice versa (updraft). In a fluidized-bed gasifier, the air or oxygen enters the reactor bottom and fluidizes a bed, greatly increasing heat and mass transfer. Reactors which employ externally generated heat are of one of two configurations. In the first, heat transfer to the feedstock is accomplished by bringing the feedstock into contact with hot char or another solid material which has been heated in a separate vessel. This can be carried out in a dual fluidized bed (Kuester, 1980) or in a dilute-phase, entrained-flow reactor (Mallan, 1974). In the second configuration employing externally generated heat, heat transfer to the feedstock is accomplished by entraining the feedstock in an inert carrier gas and propelling the 0 1984 American Chemical Society
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feedstock through an externally heated reactor tube. This entrained-flow tubular reactor is most familiar in the petrochemical industry where it is used for cracking gaseous or liquid feedstocks, e.g., ethylene production from ethane or naphtha (Schutt and Zdonik, 1956). In the area of processing solid feedstocks, the entrained-flow tubular reactor has been used to dry and carbonize coal (Parry, 1953), to retort oil shale (Sohns, 1959), and to dry and carbonize wood (Boley and Landers, 1969). Patents for processing solid carbonaceous material into gas in a tubular reador date as early as 1925 (Murrie, 1925). More recently, a transparent wall reactor has been proposed for biomass pyrolysis with direct solar radiation heat input (Antal, 1981). In applications requiring maximum conversion of feedstock to gas, the tubular reactor has several advantages over other conversion technologies. The tubular reactor subjects the feedstocks to very rapid heating rates, of the order of lo4 OC/s, thereby minimizing char and tar production and maximizing gas production, especially the valuable unsaturated hydrocarbons (Finney and Garrett, 1973). Because heat is generated external to the reactor tube, pyrolysis gases are not contaminated by products of combustion or nitrogen, yielding a gas with a higher heating value of about 17 MJ/m3 and with maximum quantities of hydrogen, carbon monoxide, and the unsaturated hydrocarbon species. External heating of the reactor tube also allows use of a wide variety of heating methods, including combustion of feedstock, process offgas, natural gas, or fuel oil, or use of solar energy. The tubular reactor is operated in a continuous mode, has a high throughput capability, and can be configured in a compact furnace. Results presented in this paper show that particle heating is by radiation from the tube wall; therefore, the fluid mechanics of particle transport and particle heating are not coupled. This feature leads to an easily controlled reactor and simplifies reactor scaling to larger capacities. Finally, the technology for heating gaseous or liquid feedstocks by transporting them through a heated tubular reactor is well established and should be applicable to solid feedstock heating. Disadvantages of the tubular reactor are primarily related to tube life and cost of the high alloy tubes. Vessel reactors can be refractory lined, thereby minimizing exposure of metal components to high temperature. In the tubular reactor, however, the metal reactor tube is exposed on its outside surface to the heat source and on its inside surface to high-temperature steam and hydrocarbon gases. Materials limitations will necessitate scheduled tube replacement-standard procedure in the petrochemical industry. Previous work Research to date on tubular reactor pyrolysis of organic feedstocks has provided valuable insight in several areas. In the experiments described by Boley and Landers (1969), a vertical reactor of 20 cm i.d. and 490 cm length was used to produce charcoal from waste wood materials. Heat was supplied either external to the reactor tube by hot gases or internally by partial combustion. Feedstock was transported upward through the reactor tube by a carrier gas (not identified); products including charcoal, ash, tars, gas, and water were separated for analysis. Reactor outlet temperature was nominally 550 OC. Data for partial combustion indicate about 27% mass conversion to charcoal (76% carbon, 3% hydrogen, 0.1% nitrogen, 0.2% sulfur, 2% ash, 18% oxygen by mass), 28% mass conversion to nitrogen-free gas (10% C02,22% CO, 2.9% Ha, 2% CHI, 61% Nzby volume), 33% mass conversion to water, and
12% mass conversion to nonaqueous condensates. Data for external heating were not reported. Mallan and Finney (1972) discuss a pyrolysis reactor which was designed to maximize liquid yield. Details of the proprietary reactor were not given, but information in a subsequent patent (Mallan, 1974) implies that the reactor was a tubular entrained-flow reactor in which the primary heat-transfer mechanism was contact between the feedstock and hot char introduced into the reactor. A t 500 OC and 1 atm pressure, the 2 kg/h reactor yielded 40% mass conversion to oil, 20% mass conversion to char, 27% mass conversion to gas, and 13% mass conversion to water. The value of the oil produced in the process is discussed by Finney and Garrett (1973). A paper more concerned with gas production from organic materials (Rensfelt, 1978) discusses pyrolysis of bark, straw, peat, wood, waste, and coal in a laboratory-scale reactor. A straight, vertical, tubular quartz reactor of 2 cm i.d. and 100 cm length was heated with electrical resistance heaters. Feedstock was dropped through the reactor, and carrier gas (steam, nitrogen, hydrogen) was introduced at the reactor top or bottom to control particle residence time. Mass fractions of gas, tar, char, and gas composition were reported for pyrolysis of solid waste as a function of reactor gas-phase temperature and residence time. Particle sizes between 0.4 and 0.5 mm and residence times of less than 0.5 s produced less than 10% mass conversion to gas, while increasing the residence time to 0.75 s increased gas yield to 53% for 700 "C gas-phase temperature and 73% for 800 "C. A residence time of approximately 0.6 s produced no less than 15% char and no more than 53% gas for gas-phase temperatures less than 100 OC. In terms of composition of the gas produced by pyrolysis of solid waste, a maximum ethylene concentration was found for 800 OC gas-phase temperature; residence time was not given. Gas composition was 43% CO, 18% CHI, 16% Hz, 11% C02, 10% C2H4, and 2% C2Hz/C2H6,volume percent. Ethylene production fell to about 2% at 500 "C or 1000 OC gas-phase temperature. Similar trends were reported by Bohn and Benham (1980) for municipal solid waste and wheat straw pyrolyzed in a horizontal quartz tubular reactor heated by a tube furnace. Steam was used as a carrier gas to entrain the feedstock and transport it through the reactor. Particle size was about 0.25 mm and residence times of less than 0.05 s were used. Ethylene concentrations peaked at about 9% but required higher temperatures than for the longer residence time used by Rensfelt (1978). Wheat straw yielded no more than 5% ethylene by volume. The difference in maximum ethylene yield between municipal solid waste and wheat straw was attributed to the presence of plastics in the former. Variation in gas composition with residence time and reactor temperature was explored in a careful study described by Antal(1979). Three tubular furnaces connected in series allowed the independent study of gas-phase reactions among the volatile substances produced in the early stages of pyrolysis. The first furnace produced superheated steam, the second furnace pyrolyzed a small sample of cellulose in the presence of the steam, and the third furnace cracked the volatile matter produced in the second furnace. The pyrolysis furnace was operated at 500 "C producing heating rates of 50 to 200 OC/min. This is a relatively slow heating rate but was fixed during the experimental program-only temperature and residence time in the third furnace were varied. Composition of the gas produced by the third furnace was comparable to that reported in earlier studies of cellulosic materials. Results
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were presented in terms of the grams of each gas species produced per gram of feedstock input. This type of data presentation includes mass conversion of feedstock to volatile matter in the pyrolysis furnace, but since conditions there were not varied, one may assume the rate of mass conversion of feedstock to volatile matter was fixed. Thus, the data may be treated by using volumetric gas composition. The results show increasing ethylene yield with temperature (up to 750 OC) and a maximum ethylene yield at a given residence time depending on temperature. Propylene production is favored at lower temperatures and shorter residence times. Carbon dioxide production varied weakly with temperature and residence time, and it was concluded that the primary mechanism for C02production occurs during the initial pyrolysis of the solid material. High solid heat rates were seen as one way to minimize C02 production. Methane, carbon monoxide, and hydrogen production increase with increasing gas-phase temperature and gas residence time. The importance of gas-phase reactions was therefore demonstrated and probably explains variations in gas composition with reactor temperature (Rensfelt, 1978; Bohn and Benham, 1980). Antal(1979) also explains that transport reactors, which have high throughputs, low-pressure operation, high heating rates, and short residence times, are ideally suited for biomass gasification. In a study of the conversion of solid waste to gasoline, Diebold (1979) describes results from a tubular pyrolysis reactor heated with a gas flame. This work represents the first attempt to quantify results from a practical, entrained-flow, tubular reactor for biomass gasification. The coiled reactor was 1.9 cm in i.d. and 2.1 or 6.1 m long, and it utilized carbon dioxide as the entraining/transport medium. Residence times were 0.042 to 0.195 s, particle sizes were 0.25 mm, and gas-phase temperature at the reactor outlet varied from 700 to 840 "C. One major conclusion was that dilute reactor conditions (less feedstock per total volume of gas in the reactor) produced more gas per unit feedstock. It was also noted that molar gas composition did not vary significantly and that dilution therefore favors production of a gas of essentially constant composition. From this discussion, it is clear that several variables, including temperatures, residence time, dilution, and particle size affect operation of a tubular entrained-flow biomass pyrolysis reactor. From previous work, however, it would be difficult to get a clear picture of the influence of these variables or other variables which may be important. A systematic study of a practical entrained-flow tubular reactor which produces a clear picture of the interrelationship of the important variables is needed. The purpose of the work described herein is to meet that need. This paper first describes the variables of importance in a tubular entrained-flow reactor. Next, the experimental apparatus and methods designed to explore those variables are described. Results of the experimental program are then described. A discussion then attempts to put the results both in the context of previous work and of entrained-flow tubular reactor design.
The Variables Operation of a tubular entrained-flow reactor involves a number of independent variables related to the reactor, the feedstock, and the carrier gas used to transport the feedstock through the reactor. Output of the reactor involves several dependent variables related to the product of the reactor, in this case pyrolysis gas. In this section we determine what the various independent and dependent variables are, and in the context of desired reactor
product determine the important parameter groups made up from two or more variables. Variables related to the reactor include reactor length 1, reactor tube inside diameter d,, and reactor tube wall temperature T,. Reactor wall temperature has been used here rather than gas-phase temperature, because it is the variable one would control in an actual reactor. Gas-phase temperature is determined by T, and several other variables related to heat transfer and chemical kinetics. Variables related to the feedstock include particle diameter dp, particle density p,, particle specific heat C,, particle thermal conductivity k,, activation energy E , preexponential factor A , and feedstock flow rate mb. Particle diameter must be defined here, because feedstock particles are not spherical. We define dp as the screen size through which one-half of a feedstock sample will pass when subjected to a screen classification. The activation energy and preexponential factor are for the reaction or reactions involved in the particle pyrolysis and determine the rate at which the solid particle is converted to gas. Variables related to the carrier gas include carrier density p c , carrier constant pressure specific heat C,, carrier thermal conductivity k,, carrier viscosity p c , and carrier flow rate mc. The dependent variables one chooses are somewhat arbitrary, but the choice depends on the intended use of the reactor and expected product or products from the reactor. We have identified the tubular entrained-flow reactor as the optimum reactor configuration for maximizing gas yield; therefore it is logical to identify one of the dependent variables as pyrolysis gas mass flow rate mF. Having produced the pyrolysis gas, the next question is what the gas composition is. We identify gas volumetric composition by xl,the mole fraction of each gas species, or for brevity we may use pyrolysis gas density p p . Finally, energy input to the reactor required for the process Q is the remaining dependent variable of importance. From a practical standpoint, one must know how'much energy to supply to the pyrolysis furnace. Rather than combining m, and xzto give a dependent variable describing the mass of a given gas component produced per unit mass of feedstock, we have chosen to separate the two variables-the value of this approach will be seen in the Results section. In simplified terms, our choice of dependent variables tells how much gas is produced, what the composition of the gas is, and how much energy is required to produce the gas. Having identified the variables, the next step is to determine groups of variables. This step eliminates the need of experimentally testing the response of all the dependent variables to every independent variable. The method used here to construct the parameter groups is dimensional analysis. This method produces all the dimensionless groups for the list of dimensional variables produced from a given problem. New dimensionless groups may be produced from the original list by multiplying powers of groups; in this way a list of dimensionless groups can be constructed which is most closely related to the physics of the problem under study. This reconstruction results in a new, complete list of physically sensible dimensionless groups but does allow some subjectivity to enter into the resulting list. The reconstructep list of dimensionless groups is shown in Table I. The first group is the ratio of carrier gas mass flow rate to feedstock flow rate. The second and third groups are self-explanatory. Group four is the Reynolds number (based on reactor tube diameter and carrier mass flow), which influences convective heat transfer from the
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Table I. Dimensionless Groups
Table 111. Dependant Variables variable ~OOm,/mb
percent gasification
t 14%
Qlm,
process heat, J/g molar gas composition
+16%
xi
Table 11. Modified Parameter Groups parameter
description
measurement accuracy
steam-to-biomass ratio Reynolds number residence time reactor wall temperature particle size
t12% t 7% f 7% t 10 "C t 10%
measurement accuracy
description
Flare
Air
fl
10.03
Hopper
water Quench Electrically Heated Reactor Tube
rew Feeder --Steam
Flex Joint
mslmb
4m,/(npSdt) 7, = l/U, T, d , Id,
tube wall to the carrier gas. Groups five through seven are ratios of time scales for the problem. One such time scale is the time period in which a feedstock particle is in the reactor, exposed to the wall temperature T,. During this period, the particle temperatures must increase to the temperature at which the kinetics predicts an appreciable reaction rate (pyrolysis begins), and the particle must then react to form the pyrolysis gas. Particle heat-up is due to convection from the tube wall to the carrier/particle stream, or by radiation from the tube wall to the particles. If residence time is long enough, the particle will completely gasify. Shorter residence times will result in less than complete gasification of the particle, or if insufficient time is allowed for the particle to reach the reaction temperature, no pyrolysis gas will be produced. These three groups relate particle residence time to the reaction time required by the kinetics for complete gasification and the time required to heat the particle to the reaction temperature by convection or radiation. The dimensionless groups may be further simplified, because a number of the independent variables do not lend themselves easily to experimental modification. These include the feedstock properties-p,, C,, E, A , k,-and the carrier properties C, and k,. In this experimental program one feedstock and one carrier gas were used (wheat straw and steam). For a majority or organic materials, the feedstock properties listed above will probably not change appreciably. Steam is the most practical carrier gas for reasons explained in the next section. Reactivity of the steam with pyrolysis gases will differ from that of other carrier gases which may produce different gas compositions. This difference may require experiments with each carrier gas. Groups five and six reduce to the residence time, and group seven then reduces to wall temperature T,. Table I1 lists the modified groups. The most convenient grouping of the dependent variables is listed in Table 111. In this experimental program Reynolds number and residence time (T,,) are based on carrier gas conditions at the reactor inlet, an easily determined condition. Since actual particle residence time depends on local carrier gas conditions through the reactor as well as local pyrolysis gas production rate, ro cannot be used directly in a particle pyrolysis model. It is, however, the correct parameter to use in a correlation. Experiment Design The experimental apparatus shown in Figure 1 was designed to allow one to adjust the independent groups listed in Table I1 and to measure the resulting influence
Gas Sample
Mass Balanc Wet Test Meter
Gas Cleanup
Figure 1. Entrained flow pyrolysis apparatus.
on the three dependent groups listed in Table 111. Reactor tubes used had two configurations. Straight tubes with d, = 1.2 or 1.4 cm and 1 = 60 or 120 cm were used for low Reynolds number runs (Re < 4000), and a helical tube with d, = 1.2 cm and 1 = 645 cm was used for high Reynolds number runs. Straight tubes were heated electrically by nichrome wire coils in four quartz tubes (5 mm i.d., 7 mm 0.d.) held to the outside periphery of the reactor by woven glass tape at 90' spacing, parallel to the reactor axis. The 60-cm reactor was heated in three independent sections, 15, 15, and 30 cm in length. The 120-cm reactor was heated in five independent sections, 15, 15,30, 30, and 30 cm in length. Shorter sections were used at the inlet so that the prescribed reactor wall temperature profile could be maintained at that area of highest heat load. Chromel-Alumel thermocouples held in contact with the reactor outside wall sensed wall temperature every 15 cm of wall length. Based on observed variations in the recorded tube wall temperature and expected thermocouple errors, the reported temperatures are accurate to h10 "C. A computer read these thermocouples and adjusted each heater to maintain constant wall temperature along the reactor length and monitored electrical power delivered to each heater. The coiled reactor was heated by a natural gas burner in a kiln, and wall temperature was measured every 100 cm of reactor length. Coiled reactor wall temperature was controlled manually by adjusting the burner gas supply. The screw feeder shown in Figure 1was used to meter feedstock into the steam flow, which was then transported through the heated reactor tube. The screw feeder, sealed feedstock hopper, and reactor rested on a digital mass balance and were isolated from the downstream components by a flex joint. A flexible steam line provided adequate isolation from the boiler/superheater. The computer read the mass balance output approximately every 10 s and plotted the reading on a strip chart recorder. In this way, loss of mass was recorded as a series of data points falling near a straight line on the strip chart and the slope of the best straight line through the data was proportional to the feedstock flow rate m b This method has proven to be much more reliable than other methods, such as calibration of the screw feeder. Accuracy of this method is primarily determined by sensitivity to system upsets, especially at low feedstock flow rates,
