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Pyrolysis and gasification of biomass in solar and simulated solar environments. The pioneering works of Michael J. ANTAL in the period 1976-1989. Jacques Lédé Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00887 • Publication Date (Web): 15 Jun 2016 Downloaded from http://pubs.acs.org on June 19, 2016

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PYROLYSIS AND GASIFICATION OF BIOMASS IN SOLAR AND SIMULATED SOLAR ENVIRONMENTS. THE PIONEERING WORKS OF Michael J. ANTAL IN THE PERIOD 1976-1989.

By Dr Jacques Lédé Laboratoire Réactions et Génie des Procédés, CNRS-Université de Lorraine 1, rue Grandville – BP 20451 – 54001 NANCY Cedex (France) Tél. +33 (0)3.83.17.52.40 – Fax : +33 (0)3.83.32.29.75 e-mail : [email protected]

ABSTRACT The present paper summarizes the main works that M.J. ANTAL performed between 1976 and 1989 in the field of biomass conversion under the influence of concentrated radiation. Both solar furnaces and solar simulators have been used. The experiments and modelling (pyrolysis and gasification) have been made under a great number of conditions (types of biomasses, reactors, …). The numerous results include new insights in the best knowledge of radiant reactors and of fundamental general aspects of biomass gasification and fast pyrolysis. Two other types of reactions carried out in solar simulators are also described in this review. More than 30 years later, many of these pioneering works have unfortunately been forgotten. It is concluded that authors working in these topics would greatly benefit by examining the related works published by Michael J. ANTAL before starting new research programs.

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I. Introduction I.1 General considerations In the context of the development of high performance solar furnaces around the middle of the XX century 1,2, numerous works have been published on the use of concentrated solar energy for driving chemical reactions such as flash pyrolysis 3. The related papers first described feasibility experiments and also a great number of theoretical considerations

4-8

. Much of

them have been presented during the Solar Thermal Test Facilities Users Association (STTFUA) meetings 9-14. Many types of possible reaction systems have been considered with specific interest focused on the preparation of solar fuels for several economical and energy reasons and as a means of solar energy storage. It was also expected that concentrated radiation could provide high temperature reactions and heating rates (flash pyrolysis). Such severe conditions could – a priori - favour new chemical mechanisms in comparison to those observed in the usual milder experimental conditions. The thermochemical transformation of carbonaceous materials

15-18

had been previously

suggested with first works related to coal 19-25. Rapidly, the authors included biomass (and its main components) in their works. For biomass, the reactions were identified as both pyrolysis and gasification. If pyrolysis clearly denotes a reaction carried out under inert atmosphere, the gasification concept was sometimes more confusing. Actually, some authors spoke of gasification for reactions made under O2, air, steam or CO2 atmosphere (with the objective of preparing H2, syngas, …), while others spoke of gasification for reactions made under flash pyrolysis which could favour high fractions of gases (CO, CO2, light hydrocarbons such as ethylene) and (condensable) vapours.

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In order to mimic solar conditions, several experiments have been carried out with solar furnace simulators (image furnaces) associating high power lamps with specific mirrors

26

,

and also with lasers 27-31. Obviously, these laboratory devices brought the advantages of more reproducible and controllable conditions.

I.2. Michael J. ANTAL in the context of solar driven biomass upgrading Michael J. ANTAL (MJA) was one of the first to work on biomass reactions (and related reactors) under concentrated radiation. His main works were performed roughly between 1976 and 1989. During this 13 years period, MJA worked with several types of solar furnaces and solar simulators to transform different types of biomass, celluloses and lignins. His main objectives were: to bring experimental evidences of differences between products obtained under flash and slow pyrolysis, to search new reactor concepts adapted to the specific nature of a concentrated radiation and to bring new insights in kinetic mechanisms. It is instructive to remember the 12 anticipated main advantages (Table 1) of solar-fired biomass pyrolysis already summarized by MJA in a paper of 1983 32 :

Table 1

I.3. Aim of the present paper It is to describe MJA’s numerous papers and results published between 1976 and 1989 in the field of chemical reactions performed under the influence of concentrated radiant energy. They will be summarized and chronologically commented on in order to observe the logical evolution of MJA’s ideas and results.

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The aim is not to make a comprehensive review of biomass solar reactions and reactors at the date of the present paper. However, in the discussion, the results and opinions of MJA will be briefly compared to others more recently published (after roughly 1990) by other authors. The objectives will be to show that MJA’s pioneering works (including advantages and drawbacks of biomass conversion under concentrated radiation) should not be ignored, but conversely to be a source of inspiration.

II. The pioneering works of MJA in the period 1976-1989 II.1. Summary of the main dates of MJA scientific career - 1970-1973: Ph.D. at Harvard Graduate School of Arts and Sciences. - 1973-1975 : Los Alamos Scientific University. Thermonuclear Weapons Physics Group, transport in a thermonuclear plasma - 1975-1982 : Princeton University. Lecturer (1975-1976) and subsequently Assistant Professor. Introduction of new courses on energy alternatives and solar thermal engineering. - 1982-2015 : University of Hawaii. Full Professor of Mechanical Engineering and distinguished Professor of Renewable Energy Resources in the Hawaii Natural Energy Institute (HNEI) of the University of Hawaii. Introduction of 6 new courses on chemical thermodynamics, kinetics and reactor design, exergy analysis, laboratory methods in fuel conversion and solar thermal engineering. - As shown in this paper, the published works of MJA on solar driven biomass upgrading were performed at Princeton University and afterwards at the University of Hawaii.

II.2. Preliminary works

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In 1976, MJA was granted a patent 33 on a method for producing gaseous synthetic fuels from solid wastes. It described a steam gasifier fluidized bed reactor heated by sunlight provided by a solar furnace. In the same period, several papers were presented by MJA in various meetings. They identified concentrated solar radiation as an attractive means for rapidly heating biomass and thereby achieving flash pyrolysis

9,10,34-42

. They included conceptual concerns, first

experimental works performed at Princeton, and economic projections. The first experimentation with biomass gasification was made in summer 1979 at the focus of Odeillo (France) 1 MW (Thermal) solar furnace

43,44

. The various tests included the use of a

vertical drop reactor where the particles were fed at the top through a vibrating system under inert or steam atmosphere. The reactor was settled at the focus of the horizontal beam furnace. These first successful feasibility experiments showed the formation of high gas fractions with noticeable hydrocarbons (CH4, C2H4, …) contents. However, MJA and his coworkers encountered difficulties in maintaining controlled conditions, resulting from the variability of solar conditions and also from carbon deposits on the quartz walls of the reactor.

II.3. First quantitative results obtained in solar furnace simulators The first results obtained at Princeton University were presented in October 1980 at Copper Mountain (Co, USA), an important specialists workshop entirely devoted to fast pyrolysis of biomass 45,46. In order to simulate the experiments at Odeillo, the same reactor (a transparent quartz tube) was used, with biomass particles being injected at the top. All the reaction products were collected. The solar simulators included a high power lamp connected to two mirrors. Two configurations were tested.

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- The first one provided a horizontal beam system (as in Odeillo). It included a 650 W and later a 2000 W tungsten halogen bulb connected to two horizontal elliptical mirrors. Steam was injected in co- and also in counter current. - In the second one a vertical axis system employed two elliptical mirrors, with light provided by a 5 kW tungsten halogen bulb. A flat front-surface mirror was used to redirect the horizontal light beam issued from the first elliptical mirror to a vertical orientation towards the second vertical light beam elliptical mirror. The reactor was set in the axis of this second elliptical mirror. Finally, a cylindrical mirror was added concentric to the reactor. In the first configuration, about only 50 % of the cellulose particles were pyrolyzed. In the second one, steam, CO2 or N2 could be used in counter current. A few experiments were also made under vacuum. Particles included lignin, cellulose and wood. In this new configuration, it was also impossible to reach complete conversion (maximum 50 %). The explanation was that all particles did not cross the focus of the furnace and probably had different residence times. However, several important conclusions were drawn on the basis of the pyrolyzed fraction of particles : - High yields of liquids could be obtained, mainly for experiments made under vacuum (up to 70 %). The explanation was that no gas phase reactions occured. - In the other cases, permanent gases fractions were favored. They contained high yields of CO, CH4 and C2H4. It was concluded that liquids (so called sirups) were favored by low gas phase temperature and short residence times. These observations showed evidence of the ability of the radiant flash pyrolysis reactor to decouple the gas from the solid reactions and so to provide an unusually flexible system for the selective production of either sirups or gases.

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II.4. Experiments carried out at the focus of the DOE Avanced Components Test Facility at Georgia Institute of Technology (Atlanta, Georgia) 32 The facility included a 550 octogonally shaped mirrors field that focused sunlight into a focal zone situated 21.1 m above the center of the field. The maximum power was of the order of 325 kW. A vertical quartz reactor was fed by biomass particles (cellulose, corn cob, hardwood) at its top (screw feeder). Steam flowed upwards. Experiences under CO2 failed because of pyrolytic carbon deposition on the quartz walls. Char was recovered at the bottom. Gases and vapours were trapped beyond the top and later analyzed. As in the previous experiments in solar simulators, only 50 % pyrolysis conversion was obtained, because the residence time of the falling solid particles was insufficient. The authors pointed out the importance of optimizing gas flow within the reactor for securing longer solid residence times in future reactors. The quartz reactor was also affected by devitrification, evidencing an important problem that should be solved in future solar driven chemical reactors. As in previous experiments at Princeton, the reaction could selectively produce either high yields of gases or of liquid sirups, a valuable feature of such reactors. The gases contained high fractions of hydrocarbons, such as C2H4. The recovered sirups were further analyzed at Princeton 47. The pyrolysis process appeared to result in the selective formation of levoglucosan from cellulose and of similar monomeric fragments from wood. These primary products were subjected to recycling inside the reactor and underwent further pyrolysis due to the countercurrent flow of gases and solids. The results suggested designing future co-current flow conditions in order to produce larger yields of relatively few monomeric fragments. In their 1983 paper, MJA et al.

32

described 7 potential high value uses for sirups produced

from ligno cellulosic biomass in a solar fired flash pyrolysis reactor. They are summarized in table 2.

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Table 2

In this same paper

32

, the solid phase pyrolysis chemistry was also discussed with special

interest in cellulose. Two kinetic models were reported. The first one supposed a single rate equation relying on a simple Arrhenius constant 48. The second one was a model relying on three kinetic constants and which introduced the notion of active cellulose

49

. Both models

were used 32 for the numerical simulation of the reactor’s performance, including heat transfer phenomena. The results agreed with experiments and confirmed the existence of two temperatures within the reactor for gas and particles.

II.5. Experiments with a spouted bed reactor 50 This new paper relied on the search for favourable conditions for preparing high yields of sirups from biomass (mainly cellulose). The authors first stated that levoglucosan and its furanose isomer would be primary cellulose pyrolysis products. The authors reported literature results showing that trace amounts of ash could reduce sirups yields, thus requiring pretreatments. Such expensive operations could be prevented by the use of radiant flash pyrolysis because of the quench effects (two temperatures reactors) that they previously discovered. For that purpose and encouraged by their previous results, the authors recommended the use of a new type of reactor : the spouted bed, which offered several potential advantages in comparison with a usual fluidized bed, for example, the ability to keep the reactor char free in continuous operation, while the quenching conditions of primary products would be potentially excellent.

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The reactor was a cylinder made from 22 mm i.d. quartz tubing. It had a topered conical bottom (cone angle of 20°). At the base of the cone was a 1 mm capillary tube whose axis was colinear with that of the reactor. Steam entered the reactor to spout the bed through this tube. The reactor was fed from the top with a controlled falling flow of particles. The evolving vapours were entrained out of the reactor before condensation. The particles repeatedly passed through the same region (spouted bed). This mixed zone was adjusted at the focus of an arc image furnace. Conversely to the furnaces previously studied, this new one was made of two parabolic mirrors having perpendicular axis through the intermediate use of a flat mirror. The source of light was a 5 kW short arc bulb producing a spectrum similar to that of the sun. The results showed the formation of up to 63 % sirups for cellulose and 30 % for Kraft paper. The large proportions of levoglucosan compared favourably with those obtained by other authors

51

in vacuum conditions. However, the slightly lower yields in the work of Hopkins,

De Jenga and Antal

50

with cellulose were explained by the fact that cellulose is white and

may not absorb sufficient energy for rapid heating, leading to highest fractions of char. The presence of C2H2 in the gases suggested that very high temperatures were probably reached within the reactor.

II.6. Numerical simulation of solar fired flash pyrolysis reactors 52 The model made in the case of cellulose relied on the solving of competing phenomena of heat and mass transfer and chemical reactions inside a continuous two phase (gas and particles) reactor placed inside a concentrated radiation energy environment. Several simplification assumptions were made. Among them, the chemical kinetics was supposed to obey a one-step reaction even if the authors underlined that the three- step mechanism 49 was usually favoured.

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The model was applied to two reactors previously described: the countercurrent flow reactor test in the Georgia Tech facility 32 and the spouted bed reactor 50. The results of the model were presented under the form of particle temperature and weight fraction profiles along the reactor and for a great number of experimental conditions. The authors also defined a magnification factor as the heat of combustion of the fuel products divided by the input of radiant energy from the light concentrator. The main conclusions were: - The difference between the solid and gas phases temperatures were often as large as several K during pyrolysis with gas phase lower than 773K. This decoupling phenomenon confirmed that such reactors offered ideal conditions for quenching the primary pyrolysis products and hence sirups production. - The magnification factors exceed 5. They could be theoretically increased to 10 or more by reducing various heat losses and/or operating under lower pressure operations. This factor for the spouted bed was less sensitive to most of the experimental factors. These high values appeared to auger well for the economics of solar fired reactors. - The predicted residence time required to achieve complete pyrolysis was strongly dependent upon the chosen form of chemical rate law. More experimental work was needed to identify the accurate rate law.

II.7. Radiant flash pyrolysis of biomass using a Xenon flash tube 53 In order to study in more detail the primary mechanisms of biomass pyrolysis, a series of experiments were performed in a Xenon flash tube. Small masses (a few tens of mg) of biomass were placed in a tubular pyrex reactor cell inserted into the core of an helical xenon flash tube. The experiments were made under vacuum. The flux densities exceeded 8 kW cm-2

10


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during 1 ms flash. The products were recovered, weighed and analyzed (except the sirups). Many different types of biomass were tested. Almost any biomass material could be pyrolyzed except pure cellulose because of its high reflectivity. Hence, a limiting factor appeared to be the optical properties of the material. Conversely to other experiments described above and carried out in steady state and at atmospheric pressure, the sirups yields were low, in this case, while the gas yield was comparatively high. The most abundant gas product was CO and also C2H2 and H2, with CO/CO2 mass ratios ranging between 8 and 20 in contrast with low temperature pyrolysis studies where the ratio was generally less than 1. Different types of primary and secondary chars were observed after the flash. These surprising findings were related to the low sirups yields in spite of the fact that experimental conditions (rapid heating and vacuum) would have favoured their yields at the expense of char and gases. The existence of secondary reactions could not account for these observations. As an explanation, the authors posited the existence of a new high temperature solid phase pyrolysis pathway involving the catastrophic fragmentation of the polymer structure of each of the materials studied with formation of CO and H2. Such a mechanism which had already been speculated upon by other authors

49

for cellulose would therefore be

valid for all types of biomass materials.

II.8. Design and operation of a high power arc image furnace at the University of Hawaii 54 Fabrication and operation of a bench scale solar simulator were implemented in the mideighties. This furnace was capable of providing an intense light beam with a total power of up to 2 kW (thermal) and a peak flux density of 107 Wm-2 (10 000 suns), similar to the conditions in Odeillo’s solar furnace 2. At that time, it was the most intense downward facing light beam available in the USA.

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The light source was a vertical 30 kW water cooled short arc high pressure xenon lamp (light spectrum similar to that of the sun). The optical system included two 1.52 m diameter parabolic dish reflectors (0.648 m focal lengths), with the size of the focus about 0.012 m in diameter. The xenon lamp was placed at the focus of the first horizontal axis parabolic mirror. A system of 24 flat and cooled mirrors was used to reflect light upward to the second mirror downward facing the parabolic system. A great number of tests and calibrations were made in order to define the performances of the furnace. They include, for example, the overall efficiency as well as radiant flux densities near the focus in greatly varied conditions. They were reported with sufficient detail to enable other authors to design and operate similar systems in order to simulate solar thermal research. II.9. Fabrication of an original fast-thermogravimetric analyzer (Fast-TGA) 55 Conventional thermogravimetric (TGA) systems are not suited for studying radiative decomposition of highly reactive compounds at high temperatures and heating rates. Actually, errors due to the dynamic response systems can be significant. In order to overcome these problems, an original thermogravimetric analyzer (Fast-TGA) using the 30 kW image furnace previously described

54

was fabricated, calibrated and tested. A small sample mass (few mg)

of powder material was placed inside a small metallic holder shielded by a small platinum lid. A very thin thermocouple was placed inside the sample holder. This assembly, placed inside a fused silica tube and fed upwardly by a N2 flow, was suspended to an electro balance. This vertical system was placed along the optical axis of the second parabolic system. Several original calibrations of temperature and weight measurements in conditions of high heating rates (greater than 2 K s-1) were carried out to define dynamic system parameters and then to determine mathematically the effects of the system’s dynamic response on the Fast-TGA results.

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II.10. Pyrolysis experiments performed in the Fast-TGA Analyser They included cellulose pyroloysis and decomposition of ZnSO4 (thermochemical cycles). - Cellulose pyrolysis kinetics 55 The decomposition of Avicel Cellulose was successfully modeled on the basis of a simple, single step nearly first order reaction. The activation energy was close to 100.5 kJ mol-1 in good agreement with rate constants reported from other low temperature studies. The authors noted that if such a law was suitable for correlating their results, it did not incorporate the implications of all known kinetic pathways available in the literature. Under these conditions, the primary char constituted a substantial portion of residues and accounted for more than 8 % of the weight of the original dry cellulose. These fractions appeared higher than those expected in these conditions of high heating rate. An explanation was the presence of the platinum lid which hindered easy escape of primary vapours from the sample boat into the inert environment. - ZnSO4 decomposition kinetics 56 Thermochemical water-splitting cycles for the production of H2 have been the object of numerous studies. One of the reaction steps of these cycles involves a high temperature reaction which can be carried out within a concentrating solar furnace. One of these cycles relies on the decomposition of ZnSO4. At the end of the 80ies, many kinetic studies performed in low heating rates conditions were available in the literature, but none in conditions which mimic those existing within a solar fired thermochemical reactor. A small ZnSO4 mass (few mg) was placed inside the sample holder previously described 55. The reaction temperatures were in excess of 1400 K. The results showed that the solid’s heating rate was sufficient to affect the α-to-β phase transformation of ZnSO4 prior to the onset of decomposition. The β phase then decomposed through an oxysulfate intermediate.

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Apparent activation energy between 210 and 250 kJ mol-1 and an apparent reaction order between 0 and 0.3 were obtained for the β-phase ZnSO4. The results proved to be different from those published in conventional TGA analyzers (lower temperatures and heating rates 57

). They suggested a change in reaction mechanisms in conditions of a concentrated solar

environment.

II.11. Photocatalytic formation of free radicals and their effects on hydrocarbon pyrolysis chemistry in a new image furnace 58 High energy photons present in concentrated sunlight can be used to photolytically dissociate certain vapour-phase compounds known to be sources of free radicals. These free radicals can subsequently initiate or influence pyrolysis reactions involving hydrocarbons. The experiments were conducted in a new – horizontal axis – image furnace type, which included an ellipsoidal mirror and a 1 kW xenon lamp placed at its first focus. Reactions were studied at its second focus where the end part of a tubular flow reactor was placed. Acetone was chosen as a source of free radicals. In the first part of the experiments, acetone was injected in the reactor through a mixture with steam at 150°C. In the second part, it was introduced with a hydrocarbon reactant (n butane) at 350°C. The results showed that acetone readily photo dissociated in a 1000 sun environment into methyl radicals. These radicals sensitized the pyrolysis chemistry of n butane at 350°C (conversion to butane, hexane, propene and other hydrocarbons) .Without photosensitization, no pyrolysis was observed. Numerical simulations showed that major photosensitization effects could be observed at higher temperatures between 400 and 500°C.

II.12. Further topics

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During this 1976-1989 period, MJA also performed and published a lot of works on biomass (and its main components such as cellulose) thermal degradation besides the solar context 5970

. These important works continue to be authoritative at the present time and should

absolutely not be ignored from any one working in these areas. At the end of this period (roughly 1988-1989), MJA abandoned his activities in the field of radiant heating and initiated new research in the field of supercritical water as a medium which supports and enhances aqueous phase chemistry ordinarily observed at much lower temperatures

71,72,73

.

MJA’s activities in the field of biomass continued without respite until 2015 74.

III – Discussion and conclusions The previous sections of this paper have summarized the main research performed by Michael J. ANTAL between 1976 and 1989 in the field of biomass thermal degradation under the influence of concentrated radiant energy. It has been recalled that more than 35 years ago, MJA had already described all the potential scientific and economic advantages of solar pyrolysis and gasification of biomass. Even at that time, MJA mentioned the idea of biomass central refinery (see points 10 and 11 in table 1 32). Numerous approaches have been explored. The sources of radiant heating included two solar facilities (Odeillo, France and Atlanta, USA) as well as many solar simulators (at Princeton and University of Hawaii). Experiments have been carried out with many diverse varieties of biomasses and components in conditions of pyrolysis and also of gasification. The experimental conditions included transient and steady state regimes with several types of reactors: drop tubes (co- and countercurrent), continuous spouted bed and an original FastTGA system. Most of these systems have been mathematically modelled. Fundamentally MJA and his coworkers clearly emphasized the advantages of gas/solid solar reactors which offer the existence of two operating temperatures.

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- The solid particles alone absorb the radiation while the gas phase is mainly transparent and hence colder. Such a property brings the possibility of quenching gaseous primary vapours. This has been experimentally evidenced through the measurement of noticeable fractions of so-called sirups. From the beginning, MJA anticipated the potential interests of these sirups. Actually, they can be roughly compared to the bio oils which today give rise to top interest in the literature. In other hydrodynamic conditions, the formation of permanent gases (including CO, CH4, C2H4) was favoured. It should be remarked that the existence of two different temperatures in gas-solid reactors evidences the fact that the simple notion of reactor temperature does not make sense. These two specific temperatures should always be distinguished even if the measurement of the temperature of a fast reacting solid is a very difficult task. - In one of his papers, MJA and his coworkers described a new possible mechanism of high temperature primary decomposition occurring inside the solid phase referred to as the catastrophic fragmentation phenomenon of polymers, which would be valid for any type of biomass. Such a mechanism should be taken into consideration in the context of the formation of cellulose primary pyrolysis compounds (sometimes called active cellulose) which continues to give rise to numerous debates 75. - More generally, MJA’s works introduced discussion of several cellulose pyrolysis mechanisms with comparison with low pyrolysis behaviour. These studies are still matters of topical interest. However, MJA and his coworkers met several experimental difficulties and reported some unexpected observations. - In certain conditions, higher char fractions than those in more conventional slow pyrolysis conditions were formed. Several explanations were given. Among them, a bad hydrodynamic control of solids and gases inside the reactor (for example, particles crossing the focal zone

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several times after intermediate coolings). Also, in the case of Fast-TGA, the difficulty of the vapours to rapidly escape from the sample boat with the possibility of secondary crackings. - In several cases, it was difficult to reach high conversion yields, often lower than 50 %. This problem was attributed to the high reflectivity of several types of biomasses, such as cellulose. A non-optimized hydrodynamic behaviour of the solids could be also another reason, considering that several particles could by-pass the focal zone or have different residence times, principally if there exists a particle size distribution. - Recurrent problems resulted from the deposition of different products (carbonaceous materials, tars, …) on the initially transparent walls of the reactor after a certain time of the experiment,

resulting in possible devitrification of the walls and mainly a progressive

decrease of available radiant flux inside the reactor, with also the possibility of cracks inside the walls. Hence it was difficult to reach steady state conditions. Steam was injected for cleaning, but in conditions where pyrolysis conditions were no longer reached. - It is basically difficult to accurately measure the true temperature at which a solid undergoes a fast primary decomposition inside a highly concentrated radiation. Also, the measured values may be those of char which is secondarily formed (external and not internal temperature). In parallel to these works and until now, many works continue to be published under the forms of reviews of the radiant transformation of biomass76-78 and also of numerous experiments, modellings, and economic considerations. However, most of the difficulties met by MJA and his coworkers have not been satisfactorily solved since 1989 in the case of reactors relying on the direct absorption of the radiation by the solid fuel. They include for example : problems of windows transparency ; high cellulose reflectivity ; non optimal selectivities resulting from complex hydrodynamics in combination with the existence of flux densities gradients in the focal zone. Of course, significant improvements have been made in

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the fields of modellings, simulations, controls, Computational Fluid Dynamics, pyrolysis chemistry, analysis and materials behaviours. Also the socio-economical context has changed. Several types of contacting systems have been and continue to be tested : direct heating through a window (as in most of MJA’s works), but also indirect heating through an intermediate absorbing surface, and heating via a fluid (molten salt, supercritical fluid), which bring partial solutions to the difficulties connected to direct light absorption by the solid fuel. The various types of reactors include: fixed beds, fluidized beds, cyclones, vortex types with transparent windows, etc. Their optimizations are often difficult in the context of the specific nature of solar energy. In each case, the objectives remain quite similar to those of MJA : fast pyrolysis for producing bio oils or gasification for producing syngas and/or H2. Unfortunately many authors who initiate new research programs with similar objectives seem to ignore most of the previous works of MJA and his coworkers performed more than 30 years ago. It is evident that the achievements of MJA and his coworkers and the difficulties they had to overcome are still topical questions and could be useful informations for anyone starting a new research program in these topics. It is the same for solar simulators which continue to be built on quite the same principles as those already optimized by MJA and his coworkers

79-85

. If such image furnaces are

considered for simulating solar conditions, MJA has also shown that they are original and excellent laboratory tools for studying the fundamental mechanisms of biomass pyrolysis in well- controlled conditions. It should be remarked that in such devices (solar furnaces and image furnaces), the pyrolysing materials are submitted to imposed flux densities. Conversely, in more traditional pyrolysis laboratory tools (TGA, …), the imposed parameters are temperature and heating rate. So there are basic complementarities between these two types of systems. Also, let us remember that the actual temperatures and heating rates at which a solid undergoes an endothermal primary

18


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reaction may be quite smaller than those which could be expected mainly for high external temperatures, heating rates and flux densities 86. This so-called thermal lag phenomenon was clearly demonstrated by Narayan and Antal in 1996 87. In other words, expecting that using high severity conditions of radiant energy would lead to related high temperatures and heating rates of the solid during its first moments of pyrolysis (primary decomposition) appears to be a utopian dream. Actually, the high measured temperatures are often those of char formed by secondary processes. Such phenomena could explain several failures and disappointments including those experienced by MJA and his coworkers in their first experiments. Their paper published later

87

could help to bring a possible first explanation.

ACKNOWLEGMENTS The author warmly thanks Ann ANTAL who sent him documents related to Michael’s career and also for having agreed to re-read this manuscript. Many thanks also for all the unforgettable moments that we spent together with Michael in France and during numerous meetings. The author thanks the two reviewers for their constructive comments.

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(14) Antal, M.J. Flash pyrolysis of biomass using concentrated solar radiation. Solar Thermal Test Facilities Users Association (STTFUA), Proceedings Annual

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(42) Moreira, J.R.; Antal, M.J. Gasification of Biomass as a Source of Synfuels for Developing Countries. 2nd Miami International Conference on Alternative Energy Sources 1979, Miami. (43) Antal, M.J.; Rodot, M.; Royère, C; Vialaron, A. Solar Flash Pyrolysis of Biomass. ISED Silver Jubilee 1979 International Congress, 1979, Atlanta. (44) Antal, M.J.; Rodot, M.; Royère, C; Vialaron, A. Biomass Gasification at the Focus of the Odeillo (France) 1MWth Solar Furnace. American Chemical Society Symposium on Thermal Conversion of Solid Wastes and Biomass 1980, vol. 130, Washington D.C. (45) Antal, M.J.; Hofmann, L.; Moreira, J.R. Bench Scale radiant flash pyrolysis of biomass in : Proceedings of Specialist’s Workshop on Fast Pyrolysis of Biomass. Copper Mountain (Co, USA), October 19-22 1980, 175-182. (46) Antal, M.J.; Hofmann, L.; Moreira, J.R.; Brown, C.I.; Steenblick, R. Design and Operation of a Solar fired Biomass Pyrolysis Reactor, Institute of Gas Technology’s Conference on Energy from Biomass and Waste, Florida, January 1981. (47) De Jenga, C.; Antal, M.J.; Jones, M. J. Yields and composition of sirups resulting from the flash pyrolysis of cellulosic materials using radiant energy. Appl. Polym. Sci. 1982, 27, 4313-4322. (48) Antal, M.J.; Friedman, H.L.; Rogers, F.E. Kinetics of cellulose pyrolysis in nitrogen and steam. Comb. Sci. Technology 1980, 21, 141-152. (49) Bradbury, A.G.W.; Sakai, Y.; Shafizadeh, F. A kinetic model for pyrolysis of cellulose. J. Appl. Polym. Sci. 1979, 23, 3271-3280.

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(59) Antal, M.J. Biomass Pyrolysis : a Review of the Literature. Part I : Carbohydrate Pyrolysis. In Boer, K.W. and Duffie, J.A. (Ed.). Advances in Solar Energy, American Solar Energy Society, New York 1982, 61-111. (60) Antal, M.J. Biomass Pyrolysis : a Review of the Literature. Part 2 : Lignocellulose, Pyrolysis.. In Boer, K.W. and Duffie, J.A. (Ed.). Advances in Solar Energy, American Solar Energy Society, New York 1985, 175-255. (61) Varhegyi, G.; Antal, M.J.; Szekely, T.; Till, F.; Jakab, E. Simultaneous thermogravimetric mass spectrometric studies of the thermal decomposition of biopolymers. 1. Avicel cellulose in the presence and absence of catalysts. Energy and Fuels 1988, 2, 267272. (62) Varhegyi, G.; Antal, M.J., Szekely, T.; Hill, F.; Jakab, E.; Szabo, P. Simultaneous thermogravimetric mass spectrometric studies of the thermal decomposition of biopolymers. 2. Sugar cane bagasse in the presence and absence of catalysts. Energy and Fuels 1988, 2, 273-277. (63) Antal, M.J. Mathematical modelling of biomass pyrolysis phenomena. Fuel 1985, 64, 1483-1486. (64) Antal, M.J.; Mok, W.S.L.; Roy, J.C.; Tabatabaie-Raissi, A. Pyrolytic sources of hydrocarbons from biomass. J. Anal. Appl. Pyrolysis 1985, 8, 291-303. (65) Mok, W.S.L.;

Antal, M.J. Effects of pressure on biomass pyrolysis. 1. Cellulose

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1

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2

Storage of solar heat under the form of a fluid fuel overcoming the intermittent nature of solar energy. Magnification of the energy of biomass feedstock in gasifiers.

3

Possibility of sizing solar furnaces to a modest scale without economic penalty.

4 5

No requirement of an oxygen plant to a gasifier. Production of low CO2 and N2 contents in the gaseous fuel. Modest role of the solar furnace cost in the overall economics of the system.

6

Production of valuable products such as ethylene in fast pyrolysis conditions.

7

Reduction of the capital costs per unit fuel produced because biomass is rapidly heated in fast pyrolysis conditions. Good adaptation of the reactor to partly cloudy day situations because the system has negligible thermal mass and low biomass residence time in the reactor. Production of high yields of liquid fuels (sirups) because of the existence of two temperatures inside the reactor. Easy storage of the sirups with the possibility to serve as a buffer between intermittency of solar energy and the continuous operation of a central refinery. Possibility for the sirups to serve as an interface between small scales of biomass conversion reactors and the large scale of a central refinery. Higher potential uses of the sirups than petroleum.

8 9 10 11 12

Table 1 : Advantages of a solar driven biomass thermal upgrading reactor 32.

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1 Polyurethanes from polydiethylene glycol levoglucosan 2 Pharmaceutical products 3 Ethanol by levoglucosan fermentation 4 Levulinic acid from levoglucosan 5 Glucose by levoglucosan hydrolysis 6 Surfactants from levoglucosan and anhydrous sugars 7 Source of olefins by pyrolysis of the sirups

Table 2 : Potential uses of sirups produced by a solar driven biomass fast pyrolysis reactor 32.

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