solar thermochemical conversion of biomass

Solar Energy Vol. 65, No. 1, pp. 3–13, 19991998 Elsevier Science Ltd

Pergamon PII: S0038 – 092X( 98 )00109 – 1 All rights reserved. Printed in Great Britain0038-092X/99/$ – see front matter

SOLAR THERMOCHEMICAL CONVERSION OF BIOMASS

´ ´JACQUES LEDELaboratoire des Sciences du Genie Chimique (CNRS-ENSIC), 1, rue Grandville-BP 451, F-54001 Nancy

Cedex, France

Revised version accepted 19 August 1998

Abstract—The purpose of this paper is first to briefly describe the usual routes of biomass thermochemicalconversion and then to discuss the possibility of using concentrated solar energy to provide the necessary heatfor the processes. Gasification, fast and slow pyrolysis are more particularly described. They can be carried outfor the preparation of a vast range of possible products that can be used as energy carriers and/or as a sourceof chemical commodities. The gasification processes are intended for the preparation of gas mixtures (CO, H ,2

etc.) for chemical synthesis, heat or electricity generation. The fast pyrolysis formerly carried out for gasproduction (CO, H , light hydrocarbons, etc.) is now mainly studied with the objective to produce liquids2

(bio-oils). Slow pyrolysis is in use for a long time for the preparation of solids (charcoal). The nature andquality of the products depend mainly on the experimental conditions of the process (temperature, heatingrates, residence times, etc.). The possibility of a solar entry in the gasification and pyrolysis processes is thendiscussed. The technical and scientific benefits, as well as the difficulties, are underlined, showing the necessityto design new types of specific reactors. From a fundamental point of view the advantages are also underlinedof using a concentrated radiation as a laboratory tool for studying the very fast primary steps of biomassthermal decomposition as well as the possible existence of intermediate short life time species that are still notwell known. 1998 Elsevier Science Ltd. All rights reserved

1. INTRODUCTION been often suggested in the last 20 years even ifonly few projects have been effectively carried

Biomass has been in use for a very long time byout. The advantages and problems are underlined.

mankind to satisfy its energy requirements. How-The third part of the paper is devoted to some

ever it was almost abandoned after the emergenceaspects of the current basic research in the domain

of the use of coal and oil in the 19th and 20thof biomass thermal decomposition where it is

centuries. After the energy crisis of the 1970–shown that concentrated radiation can be

1980s and now because of the increasing prob-favourably used to study the first fast elementary

lems in the environment, biomass is, with solarsteps of reactions. Actually these aspects are still

energy, one of the most often considered sourcesvery much unknown and even the source of

of renewable energy. The present paper is dividedcontroversies. Obtaining new basic data in that

into three parts.field would be of primary importance for improv-

In the first part, the main sources and propertiesing the efficiencies of usual pyrolysis and gasifi-

of biomass are listed, followed by a brief descrip-cation processes, but also for a better design of

tion of the main usual processes of biomassfuture solar thermochemical biomass conversion

thermochemical conversion. The purpose is not toreactors.

make an exhaustive review of the great number ofpapers (several hundreds) published in that field,but to remind about the basic differences between 2. MAIN SOURCES, PROPERTIES ANDgasification, fast and slow pyrolysis. This is often ADVANTAGES OF BIOMASSa source of confusion in the scientific com-

2.1. The three main existing resources ofmunities that are not directly involved in biomass´ ´biomass (Deglise and Lede, 1982 )thermal conversion studies.

In the second part, the possibilities of a solar (1) Forest-derived biomass (wood) includes theentry in these processes are discussed. Solar residues (bark; sawdust; etc.) of the directenergy is responsible for the formation of biomass exploitation of the forests but also of theby the natural mechanisms of photosynthesis and wood industry. Wood is characterized by ahence, the formation of biomass represents a relatively high yield of lignin and a moisturenatural route of solar energy storage. The associa- content lower than 50%.tion of concentrated solar energy and biomass has (2) Agricultural biomass derived from cultiva-

3

´ ´4 J. Lede

tions developed for food or industry purposes represents about 2 /3 (2230 MTEP) of the annual(wool; straw; etc.). The level of lignin is potential of renewable energies in the world thatrelatively low ( , 20%). can be mobilized at the present time (Dessus,

(3) Aquatic biomass includes seaweeds character- 1994). It brings a clear answer to problems ofized by a very high moisture constant. environment (less emission of CO , SO , NO ,2 2 x

In all cases, the wastes of biomass are produced etc.) and respects the natural cycle of carbon.by industries, but specific energy plantations for However, biomass has several drawbacks. Itunique energetical purposes can be also consid- has often a high moisture content (20 to 50%ered. depending on its origin and time elapsed since its

harvesting). Drying needs an external source of2.2. Chemical properties of biomass (Deglise energy (3000 to 5000 kJ /kg of water removed

´ ´ ´and Lede, 1982; Dumon and Gelus, 1982; which may represent up to 25% of the heat ofDiebold and Bridgwater, 1997 ) combustion of dry wood). The other main dis-

The elementary analysis of wood leads to an advantages are: biomass is much dispersed on theaverage mean formula which is valid for a wide earth surface; its density is relatively low (400–

3variety of sources: CH O . 600 kg/m ); the available sizes are very different1.44 0.66

Three main parts are often considered: the according to the type of wastes (from a tree trunkextracts present in variable quantities: 4–15% to a sawdust particle). The results are high costs(resins; terpens; pigments; tannins; etc.); the con- of transport and of grinding (the price increasesstituents of the cells (cellulose; lignin; hemicellul- sharply as the desired final size decreases), andose); the ashes (mineral matter: mainly oxides) relatively large volumes of storage. As will berepresenting about 1% and playing catalytic roles seen later, the ability for biomass to give rise toin the conversion processes. different gaseous, solid or liquid products by

Cellulose is the main constituent. It is a linear thermal conversion is an important problem be-polymer with a degree of polymerization of up to cause the products of any conversion route have10 000 size-carbon anhydroglucose sugar units. to undergo efficient separations and purifications.The cellulose fibers are held together in a matrix The dispersion of biomass is of course aof lignin and hemicellulose. Lignin is a complex consequence of the dispersion of the energythree dimensional polymer of phenyl-propane provided by the sun on the earth surface. Theunits containing a large number of aromatic rings simultaneous use of solar energy and biomass tobound to each other by furane rings and ether perform solar thermal conversion processes needsbonds. Hemicellulose is a polymer of lower to solve the two problems of concentrations ofdegree of polymerization (100 to 200) formed solar energy and biomass wastes in specificfrom pentosane monomers such as xylose. The locations where both resources are available.fractions of cellulose, lignin and hemicellulosedepend on the type of biomass.

3. DIFFERENT ROUTES OF BIOMASSThe overall range for the oxygen content of

THERMOCHEMICAL CONVERSIONbiomass is 40 to 45 wt.% on a moisture and ashfree basis. The consequence is a much lower Apart from combustion (that transforms direct-heating value compared to other hydrocarbon ly biomass into energy) there are several possiblefuels. The energy content on a volume basis is routes of biomass upgrading producing differentstill even less favourable (Diebold and Bridgwa- kinds of compounds that can be used as chemicalter, 1997). commodities or for energy production. The aim of

the present paper is related to thermochemical2.3. Advantages and drawbacks of biomass for conversion, and processes like hydrolysis, fermen-

´ ´an energetical use (Lede, 1995 ) tation, etc. will not be described.Compared to coal for example, its decomposi- The three main thermal processes are: gasifica-

tion temperature is lower. It contains relatively tion, pyrolysis and direct liquefaction. This last´ ´less ash fractions and has a lower sulfur content possible route (Deglise and Lede, 1982; Boocock

(an advantage in the case of catalytic upgrading of et al., 1988; Bouvier et al., 1988) correspondingits thermal decomposition products). The mean to a thermal and catalyzed reaction of biomass inbiomass global composition is relatively constant a solvent and under pressure is less often studiedwhatever its origin with a molar H/C ratio close and will not be considered in this paper. More-to 1.44. It is of course a renewable resource, the over, it has never been suggested in associationwastes of which are immediately available. It with solar energy.

Solar thermochemical conversion of biomass 5

´ ´3.1. Gasification (Deglise and Lede, 1982; H /CO ratios. In any case, the selectivities can be2

Kaltschmitt and Bridgwater, 1997; Bridgwater controlled by conducting the process with specificand Boocock, 1997 ) catalysts.

At present, biomass gasification is commercial-Gasification is designed to produce non-con-ly practised primarily to produce a fuel gas afterdensable gases (Diebold and Bridgwater, 1997). Itmore or less extensive cleaning with the advan-is carried out under reactive atmosphere in atages of a gaseous fuel compared to solidgasifier. Most of the time, the main steps of thebiomass. The biggest plants process up to severalprocess occur in the same reactor. They includetons /hour of feedstock. The gases can be useddrying of the feedstock; primary steps oflocally or in distribution networks depending onpyrolysis; secondary reactions of gasification.Verythe conditions (process heat in industry, drying,schematically, after the drying step, the fasturban heating, cooking, etc.). Electricity genera-processes of pyrolysis give rise to gases, tars andtion is another very important domain of potentialchar that undergo further reactions of reductionapplication of biomass gasification. Co-gasifica-and cracking giving gases (CO, H , CO , etc.). In2 2

tion of biomass and coal and/or plastic wastesorder to provide the heat for these elementaryalso attracts much interest. Other possible applica-reactions, variable amounts of O , air, etc. are2

added to the gasifier leading to the partial com- tions are also H and/or syngas (CO 1 H )2 2

bustion of some of the pyrolysis species. It preparation. The mixture of CO and H has the2

amounts to saying that globally, a fraction of the advantage of being relatively poor in sulfur butbiomass feedstock is only used to provide the heat the residual fractions of hydrocarbons must befor the process. This fraction which is not up- eliminated (for example by catalytic steam re-graded in valuable gases can be as high as 30%. forming). It can be used for methanol synthesis or

The reactors are very often inspired by coal the operation of fuel cells. However, it must begasifiers as for example the packed bed systems pointed out that methanol synthesis needs a H/Cfor which many models have been proposed and ratio of 4 while in biomass it is only of about 1.5,where the different zones of reactions are clearly and 2.12 in an ideal and complete steam gasifica-distinguished. They can operate in co-current or tion process. It would be therefore necessary tocounter current conditions. More recently, new enrich the mixture with H in specific hybrid2

types of reactors have been developed: fluidized, systems that would include in parallel a methanecirculating fluidized, spouted, etc. bed reactors (biogas or natural gas) reforming unit, or a(Kaltschmitt and Bridgwater, 1997; Bridgwater hydrogen source produced by electrolysis or fromand Boocock, 1997). They need the use of a solar process. Hydrogasification is another routefluidizing particles that can be neutral (sand) or in which H is added under high pressure to the2

have catalytic roles. The composition and heating gasifier in order to prepare high yields ofvalue of the gaseous products depends on the methane.nature of the feeding gas. The gasifiers can

3.2. Pyrolysis (Kaltschmitt and Bridgwater,operate under atmospheric, but also under high1997; Bridgwater and Boocock, 1997 )pressure in order to improve the overall process

efficiency (Alden et al., 1997). The temperatures Pyrolysis is a thermal decomposition processcan be much higher than 10008C. The gas prod- producing a mixture of products under threeucts contain residual fractions of condensible tars, phases. The different types of pyrolysis processesaerosols and particulate ashes. Their removal is a differ in the type of product that is searchedmajor barrier to the use of these gases in gas (solids: charcoal; liquids: oils; gases). The resultsturbines, fuel cells, etc. A frequent way of purifi- are mainly dependent on the operating conditions:cation is based on the catalytic cracking of the temperatures; residence times; heating rates. Thecontaminants (Bridgwater and Boocock, 1997). processes need a heat input. Reactor designs

Air gasification is the most simple and well include the addition of small amounts of air to aknown type of process. The products contain high fraction of the recycled products such as char oryields of N (up to 60%) leading to a high gases to provide the process heat by partial2

dilution of the gases with low calorific values combustion. External fuel can also be considered.3(4–6000 kJ /Nm ). Oxygen gasification leads to Pyrolysis can be performed above or near atmos-

gas mixtures richer in CO and H with much pheric pressure but also under vacuum (Roy et al.,23higher calorific values (about 12 000 kJ /Nm ) but 1985).

with the cost of an O unit. Steam gasification 3.2.1. Slow pyrolysis. It is sometimes called2

with variable amounts of O or air lead to higher carbonization and is in operation for several2

´ ´6 J. Lede

centuries. The aim is to enhance the production of applications for heat and power generation. Otherpotential markets for the oils include their use ascharcoal by slow heating (up to several dayschemical commodities or as petroleum fuelsreaction times). In traditional units the charcoalsubstitutes in the long term. However the crudefractions hardly reach yields of about 30%. How-bio oils need further upgrading in order to im-ever recently, Antal et al. (1996) claim to produceprove their stability, heating value, ash content,up to 42–62% charcoal yields in less than 2 hetc. (Diebold and Bridgwater, 1997).depending on the moisture content of the feed and

The most usual reactors for pyrolysis arethe pressure of the reactor. Such a result wouldfluidized, circulating fluidized, entrained, etc. bedsreduce deforestation and pollution induced by oldwith pilot plants processing up to several hundredprocesses still working in several developingkg/h. They are designed for oil productioncountries. Charcoal can have several possible(Kaltschmitt and Bridgwater, 1997; Bridgwateruses: active carbon; metallurgy; electrodes; waterand Boocock, 1997). A specific design uses atreatment; barbecues; etc. Besides charcoal, all therotating cone (Janse et al., 1997). The pyrolysisprocesses produce non-negligible fractions ofcan be also carried out under vacuum (Roy et al.,tarry materials and gases for which markets must1985). Special interest is devoted to ablativebe found.

3.2.2. Fast ( flash) pyrolysis. It is a concept pyrolysis derived from former basic experimentsraised | 20 years ago. It requires rapid heating developed in France and in which biomass isconditions in such a way that relatively high heated by more or less close contact with a

´ ´ ´ ´temperatures of reaction are reached. These con- moving hot surface (Lede et al., 1985; Lede and´ ´ ´ ´ditions are obtained with specific reactors where Villermaux, 1987; Lede et al., 1988; Lede, 1996).

2temperature, transfer efficiencies and residence Heat transfer coefficients of up to 40 000 W/m K´ ´times must be well adjusted in order to control the (Lede et al., 1985) can be reached in these

gas /oil ratios and to minimize the formation of conditions with, at the same time, very good masscharcoal (charcoal is the result of recondensations transfer efficiencies (rapid elimination of theand/or secondary vapour–solid interactions). The primary liquid products).reaction temperature of biomass is usually around

´ ´ ´ ´5008C (Lede et al., 1985; Lede and Villermaux,4. POSSIBILITY OF A SOLAR ENERGY

´ ´ ´ ´1987; Lede et al., 1988; Lede, 1994, 1996;ENTRY IN THE GASIFICATION AND

Narayan and Antal, 1996) and residence times ofPYROLYSIS PROCESSES

solid and gas phases lower than a few seconds.Initially, flash pyrolysis was carried out in The biomass thermochemical conversion pro-

conditions that favoured gas production, i.e. fast cesses are globally endothermic and the heatheating of biomass followed by relatively long required can be supplied by concentrated solarresidence times and high temperatures allowing energy in such a way that the energy evolvedefficient cracking of the primary products. High from the products ideally represents the sum offraction of CO (35–55%) and H (20–30%) can energy stored during the photosynthesis and the2

then be obtained with noticeable fractions (up to thermal processes. The association of a concen-20% weight fraction) of hydrocarbons (CH , trated radiation with biomass has often been4

´ ´C H , C H , C H , etc.) increasing with tempera- suggested in the last 20 years (Antal, 1979; Lede2 6 2 4 2 2

ture. The heating values of the gases may reach et al., 1980; Lincoln, 1980; Taylor et al., 1980;3 ´ ´18 000 kJ /Nm with gasification yields of up to Antal et al., 1981, 1983; Lede et al., 1983;

´ ´95% (SERI, 1980; Lede et al., 1986). Hofmann and Antal, 1984; Hopkins and Antal,Nowadays flash pyrolysis is mainly considered 1984; Hopkins et al., 1984; Tabatabaie-Raissi et

for bio oils production (Kaltschmitt and Bridgwa- al., 1989; Gronli, 1996; Lanzetta et al., 1997).ter, 1997; Bridgwater and Boocock, 1997). Exten- However, relatively few projects have been ex-sive cracking is prevented and vapour products perimentally carried out. The work, usually per-are recovered after condensation. The oils are formed at laboratory scale, has been done mainlycomplex mixtures of a large number of species for gasification with less efforts for pyrolysis.derived from more or less complete depolymeri-

4.1. Gasificationzation of the biomass components. The com-position can be changed by the use of catalysts or It has been already mentioned in a previousafter several possible biomass pretreatments ac- section that in usual gasifiers, heat is supplied incording to reactor conditions. Fractions of oils situ by several elementary exothermic steps. Thehigher than 70% can be reached with possible gasifiers are fed by air or O and large amounts of2


the biomass feedstock (sometimes more than walls of the pyrolysis reactor or through the30%) are not upgraded into valuable products. intermediate of a heat carrier acting to the wallsMoreover the gases are much diluted by N in air or directly inside the reactor (for example sand in2

gasifiers. These drawbacks clearly explain why fluidized bed reactors). Such a heat supply can besolar energy has been suggested to provide the used immediately or after a more or less longheat input with the following main advantages: period of storage.economy of external fuel and of biomass that is The very high heat flux densities that can becompletely upgraded; high calorific value of the reached with concentrated solar energy can be

´ ´products: less CO and no N ; no need of an favourably considered for flash pyrolysis (Lede et2 2

external unit of O ; clean process; etc.). These are al., 1980) with a priori two main advantages. First2

some of the reasons why most of the projects of of all, the conditions of flash pyrolysis (fastsolar upgrading of biomass are related to gasifica- heating rates and high temperatures) can betion processes. Most of them rely on the use of reached. Secondly, the available flux is concen-small particles processed in fluidized bed type trated inside the volume of the relatively smallreactors (Murray and Fletcher, 1994) or in free focal zone and the primary products are hencefalling systems with direct illumination by con- liberated in a rather cold environment in such acentrated radiation. However other systems using way that secondary cracking reactions are mini-massive biomass can be considered. For example, mized. In order to take advantage of the qualitiesthe cross section of a cylindrical rod of wood can of a concentrated solar energy (clean; high fluxbe settled at the focus of a concentrator and densities) it is advantageous to heat the biomasscontinuously adjusted as the wood is consumed by direct absorption of the radiation. Several

´ ´(Lede et al., 1983). The reaction must take place attempts have been reported in the literature butinside a reactor fed by steam and having a often with disappointing results: formation oftransparent window. Note that, with massive relatively high fractions of charcoal and of lesswood, the reactor can also be directly fed by fractions of vapours than expected; difficulties ofliquid water arriving on the reacting surface. The reactions with cellulose; etc. These problems canreaction is reported to occur according to two be explained by four main reasons.very distinct stages: a rapid step of pyrolysisgiving rise to a charry material, further slowly (1) At the first moments of the exposition to thegasified by steam at high temperature. The re- radiation, the feedstock undergoes a thermalaction is slow because of transfer limitations and flash corresponding to the required conditionsof the slow rate of the gas–solid gasification for flash pyrolysis. The reaction occurs rapid-reactions. Other similar experiments have been ly at high temperature with the formation ofdone with packed beds of several types of car- intermediate and unstable liquid products

´ ´bonaceous materials adjusted at the focal zone (Lede et al., 1997). However, because of the(Taylor et al., 1980). The windows have shown relatively poor mass transfer efficiencies,good behaviour, because of local gasifications by these products form a screen to the incomingsteam of the tar and char depositions. The mole radiation with two consequences: the internalfractions of H and CO can reach, respectively, layers of virgin biomass receive less and less2

50% and 40% with fractions of light hydrocar- high heat flux densities and wind up beingbons. In these processes of solar gasification as heated only by conduction; the primary liquidwell as in all the biomass thermochemical conver- products exposed to the radiation go onsion processes, the products contain more or less reacting with the formation of secondaryimportant fractions of aerosols that must be species. The results are that the flashremoved before the end use of the gaseous pyrolysis conditions are less and less satisfiedproducts. and that a slow pyrolysis behaviour takes

place with the formation of increasing frac-4.2. Pyrolysis tions of charcoal. Of course, these secondary

In usual pyrolysis processes, the necessary heat effects are more important for a highcan be provided by the combustion of one of the granulometry feedstock, as for example withbyproducts (for example, gas and/or char frac- massive pieces of wood (Gronli, 1996). Theytions in pyrolysis plants designed for the pro- are less important with a highly concentratedduction of bio oils). The use of external fuels is radiation favouring the formation of vapoursalso possible (fossil fuels, part of biomass, etc.). instead of char. These behaviours represent aThe heat produced can be transferred either to the noticeable difference with ablative pyrolysis

´ ´8 J. Lede

where a mechanical device removes the pri- reactors have also other applications in the overallfield of solar chemistry.mary liquids as they are formed and where

steady state conditions of flash pyrolysis are4.3. Solar thermochemical conversion ofrapidly and easily reached.biomass: problems in reactor design(2) The optical properties of some of the com-

The main difficulties to overcome are con-ponents of biomass are not compatible withnected to the very complicated reaction itselfan efficient absorption of the radiation. Theyproducing solids (charcoal), liquids (tars, oils) andare highly reflecting and semi-absorbing ma-gases, and to the problems induced by the use ofterials in such a way that high fractions of theconcentrated solar energy.incoming radiant flux is not used for heating

4.3.1. Problems connected to the reaction. Thethe feedstock. This is for example the casechemical characteristics of the biomass thermoch-with very small particles of pure celluloseemical decomposition reactions are not well(Hopkins and Antal, 1984; Boutin et al.,known (see next section). The primary steps of1998a,b).pyrolysis are very fast and produce unknown and(3) In moving bed reactors, the particles ofshort life time intermediate species. Moreover, thebiomass may cross several times the focalfinal products obtained result from a great numberzone and hence undergo a succession of flashof secondary reactions of crackings and recon-heatings and coolings. These conditions aredensations involving several phases. Some of thealso a source of secondary reactions andelementary chemical steps are very fast and arehence of charcoal.hence in competition with other physical and rate(4) In several moving bed reactors, some par-

´ ´controlling factors (Lede, 1997) taking place inticles directly heated by the radiation causethe reactor (heat and mass transfer processes,screening effects for others that receive muchhydrodynamics, etc.). The data published in thelower flux densities.literature are the source of many controversies.They are often valid for the reactor where theyThe best conditions of flash pyrolysis (i.e.have been measured and cannot be extended tomaximum fractions of primary liquids and ofother designs. If these problems are valid for anyvolatile compounds) can hence be ideally reachedbiomass thermochemical conversion reactor, theyby two possible means. A single short flash ofare greatly enhanced with a solar energy input.light is made on the feedstock followed immedi-

4.3.2. Problems connected to the use of con-ately by a fast quench (see next section). In acentrated solar energy.second possibility, a mechanical device eliminates

the primary products from the irradiated surface,as soon as they are formed. This can be performed (1) The constraints related to solar energy areby combining the advantages of concentrated first of all due to the problems connected tosolar energy and ablation conditions: solar energy the use of transparent windows that mustheats up the biomass particles and also the walls operate for a long time with chemical systemsof a reactor against which the pieces of biomass involving several phases. A fraction of theslide with high velocities (for example by cen- liquids (tar, oil) and dusts (charcoal, ashes)trifugal forces effects). The frictions between the are produced under the form of aerosols.two solids remove immediately the primary They may deposit or condense on the win-pyrolysis liquids. This is possible with vortex type dow. The consequences are that the available

´ ´reactors such as the cyclone reactor (Lede, 1979; flux decreases and that the window is heated´ ´Lede et al., 1986). First experiments of feasibility up and can break. Possible local gasification

have shown the ability of such a device to operate processes can favourably occur locally whenwith solar energy even if in these preliminary the reactor is fed by steam, keeping thetests, the concentrated radiation did not penetrate window clean. Other solutions rely on theinside the vessel. The results show that the search of specific hydrodynamics: use offractions of condensable matter and of gases can secondary inert flows (but creating unfavour-be easily changed by the simple adjustment of the able dead zones or by passes in the reactor);amount of incoming solar flux inside the solar design of new types of reactors opened to thecavity and/or of the residence times of the gases atmosphere; etc. In any case, these modi-(the measured gasification yields vary from a few fications create new flow behaviours that may% to 95% with a maximum of 5% of charcoal) change the yield and selectivity of the whole

´ ´(Lede et al., 1986). Notice that such vortex type process. An interesting answer to this prob-


lem has been suggested (Epstein, 1995) in the must be solved. A second cause of transientcase of coal particles transported by liquid regime can be observed for biomass particleswater and absorbing directly the concentrated crossing several times the focal zone andradiation. No window is needed. In that case, undergoing a succession of transient regimesone can expect a competition between the of heatings and coolings, inducing importantvery fast quenching of the primary products consequences on the selectivity of the re-

´ ínside the cold water bath, and the gasifica- action (Lede, 1996).tion reaction with the very thin layer of steam (4) Finally, the modelling of the process (a basicsurrounding the particle. Operations relying step for scaling up) is complicated by theon indirect heating through an intermediate introduction of solar energy inside the reac-wall can of course be advantageous but with tor. To the usual mass, heat and momentumpartial loss of the high quality of solar energy balances (Villermaux et al., 1986) are(cleanness, concentration). This is the case for superimposed radiative balances. Usually dif-

´ éxample with the cyclone reactor (Lede et al., ficult to establish, they imply the walls of the1986) previously mentioned, or with devices reactor, the reacting particles and the possibleusing intermediate baths, melted by the con- presence of catalysts. They must take intocentrated solar energy (Epstein, 1995). Modi- account the absorption of the solar radiation,fied fluidized bed reactors (annular or two the losses by re-radiation and the exchangesdimensional) could be also favourably con- between and inside the hot species. Thesidered if the solution of indirect heating is difficulties are still enhanced by the fact that

´ ćhosen (Lede et al., 1996). Standard rotary many parameters and data are not available inkilns opened to the atmosphere are probably the literature: kinetics and thermodynamics ofinconsistent with biomass thermal conversion. the very complicated chemical pathways;

(2) As long as the reactions involve several optical properties of all the involved speciesphases, it is not possible to associate any kind (emissivity, absorptivity, reflectivity);of reactor with any kind of solar concen- physicochemical constants. These problemstrating facility. For example, solar furnaces are much more complicated when these prop-with horizontal axis create dissymmetries if erties rapidly change as the reaction proceeds.associated with vertical axis reactors (fluid- Of course the optical characteristics of char-ized beds, falling beds, spouted beds, coal, tar, etc. are quite different from those ofcyclones, etc.). All the biomass particles are biomass! Because of the uncertainties on thenot similarly irradiated and hence do not chemical characteristics found in the litera-undergo reactions with the same rates and ture, specific laboratory devices must beselectivities. In the case of parabolic dish imagined for measuring accurate data inconcentrators, following the sun, the position similar conditions as those imposed by solarof the reactor changes at any time. It is hence energy (i.e. high temperatures and high heat-unsuitable if gravity plays a role in the flow ing rates).behaviour of the reactor. In that case and alsowith solar towers, additional difficulties may

5. CONCENTRATED SOLAR ENERGY: Aappear in the conveying of biomass from the

TOOL FOR STUDYING THE FUNDAMENTALground to the reaction zone.

ASPECT OF BIOMASS PYROLYSIS(3) Another main difficulty induced by solar

energy is that the reacting system must have Several hundred papers have been published inthe ability to operate in transient conditions. the field of biomass thermal conversion aiming atA first cause is due to the intermittency of the a better understanding of the elementary processessolar radiation at the short (clouds) and daily of decomposition. The purpose of many workstime scales, causing problems of stability and (mainly performed with cellulose) is to derivecontrol of the gasifier or of the pyrolyser. If kinetic data of the elementary first steps ofthe reaction is very slow (ex. carbonization), reaction and to imagine different possible reactionfast variations of the available flux have only pathways. In spite of the very abundant literature,few consequences. In the same way, very fast there exists many basic uncertainties and con-pyrolysis occurring in fractions of seconds troversies related to these results and also to thecan be also compatible with sudden variations possible existence of intermediate short life timeof the illumination. However, in that case, species (Diebold, 1994; Varhegyi et al., 1994;

´ ´difficult problems of feeding of the particles Lede et al., 1997). The important discrepancies

´ ´10 J. Lede

observed in the published results are the conse- This is particularly important for small particles.quences of the fact that the primary reactions are These observations explain some of the failuresvery fast (a few milliseconds) and that the transfer reported in the literature in the solar flashand hydrodynamic processes are rate controlling. pyrolysis of cellulose.Very often, the published kinetic data are in fact These basic results are of primary importancerelated to these physical steps and not to pure for designing solar biomass gasification orchemical processes. Moreover, many of these pyrolysis reactors.fundamental works are performed by thermo-gravimetric analysis (TGA) (Antal and Varhegyi,

6. CONCLUSIONS1995), an efficient method that is however incom-patible with the very fast reactions involved and Biomass and its wastes can be upgraded ac-that is unable to detect the presence of species cording to a large number of possible routes offormed without mass loss. The mass transfer gasification and pyrolysis processes. Many typesefficiencies in TGA are also very poor and of end compounds can be obtained: gases (syngas;secondary reactions are likely to occur. H ; light hydrocarbons); liquids (bio oils); solids2

As previously mentioned, flash pyrolysis needs (charcoal). Their potential fields of use are alsoheat sources providing high heat flux densities (up very large and diversified (electricity; fuel for

6 7 2to 10 –10 W/m ) making it possible for biomass transportation; chemical commodities). The pro-to react at relatively high temperatures (around cesses always need an external heat input. Solar5008C). These high fluxes can be reached in very energy is a more and more suggested possibilityclean conditions with a concentrated radiation. If offering the advantages to upgrade the totality ofa sample of biomass is irradiated during short biomass feedstock and to design 100% renewabletimes (less than 1 s), secondary reactions are plants with no use of fossil fuels. A gasificationprevented and the observation and analysis of the process would not require an expensive O plant2

surface of the feedstock after the flash reaction nor the use of air and would produce high qualityenables to elucidate the nature of the primary fuel gases much less diluted in N . The emissions2

processes of pyrolysis. It is also possible, under would be reduced and the natural cycle of carboncontrolled conditions of flux to measure some of respected. This approach would bring a clearthe optical properties (reflectivity and absorptiv- answer to social pressures related to the presentity) of the biomass components. These two in- and increasing environment problems. The energyvestigations are carried out by LSGC-CNRS- recovered in the products is the sum of energiesNancy Laboratory in collaboration with IMP- stored during the photosynthesis processes andCNRS-Odeillo Laboratory in the framework of a during the thermal reaction (double storage ofFrench program supported by Agrice (Ademe). solar energy).

The first experiments have been performed with A great number of works are reported in thecellulose with an image furnace (5 kW xenon last 20 years but mainly at the laboratory andlamp associated to two elliptical mirrors) and a 2 fundamental levels, with special interest for solarkW solar furnace. They show that cellulose steam gasification for power generation, H and2

pyrolysis produces primarily intermediate species methanol preparations. However, in that case, athat are liquid at reaction temperature but solid at parallel additional source of (solar) H must be2

room temperature. They are 100% water soluble considered for methanol synthesis (Kesselring,and are made of a relatively small number of 1995). A review of the different technologies forproducts. They result from the depolymerisation solar gasification of carbonaceous materialsof the cellulose polymer (Boutin et al., 1998a,b). (biomass, coal, natural gas, residual oil, oil-shale,The presence of such species is of primary etc.) is given by Epstein et al. (1994). Even if, atimportance because they can induce agglomera- the present time, few and probably not enoughtions (and hence pluggings) in the reactor. Very basic data are available, it could be highly rec-local experiments relying on the use of small ommended to support a few projects of pilotoptical fibers have also proved that pure cellulose plants where the technico-economic feasibility ofis a highly reflecting material (more than 80% solar biomass gasification plants including areflectivity) and that the remaining radiation is superheated steam generator would be tested. Aonly progressively absorbed in the solid according provisional cost analysis of a solar process ofto an exponential law (Boutin et al., 1998a,b). thermochemical conversion of biomass can beThese last results show that only a small fraction hardly made in the present state of knowledge.of the incoming radiation is used for the reaction. There are many possible situations and the results


depend to a large extent on the chosen route rapidity of the reactions (flash pyrolysis) are well(pyrolysis; gasification; etc.), of specific local compatible with the intermittent nature of solarconditions including the type of resources of energy (ability of the reactor to operate in tran-biomass, of the end use of the products (gases, sient conditions). The concentrated radiationliquids, solids for heat or electricity generation or could also be used for cracking the contaminantspreparation of chemicals, etc.). They depend also formed in any pyrolysis and gasification processon environmental factors that are, a priori, dif- and hence play the role of the usual catalytic stepsficult to estimate at the present time. Solar usually carried out for cleaning the gaseousbiomass thermal conversion is of course only products. Even if it has not been extensivelyconceivable in areas where both sun and biomass mentioned in this paper, the direct liquefaction ofare available. Potentialities exist in very sunny biomass could probably be rethought in associa-locations where high quantities of biomass are tion with a concentrated solar energy entry. Theproduced, as for example around the mediterra- solar thermal conversion of carbonaceous materi-nean basin but also in Australia, Brazil, India, etc. als that was formerly suggested for coal and forThis is for example the case for sunny greenhouse biomass, could be also extended to the co-pro-intensive agriculture areas where large quantities cessing of biomass and coal, and/or plastic wastesof biomass wastes are produced and that, in (NREL, 1995). Of course, all the progress madeaddition, pose serious problems of environment in these fields can induce spin offs in the domain

´ ´(south of Spain). Finally solar biomass upgrading of solar chemistry (Lede and Pharabod, 1997) andcould also create technological leaderships aiming more generally in the design of multiphase andat exporting new technologies. high temperature reactors. Finally it has been

In addition to solar gasification, there are also pointed out the very interesting possibilities ofmany other potentialities in the field of solar using, as a laboratory tool, the specific qualities ofpyrolysis (leading to different types of possible a concentrated radiation for making progress inproducts), and also of two-stage processes as- the fundamental knowledge of biomass thermoch-sociating a first step of fast pyrolysis followed by emical degradation.a second step of slow gasification, offering a These unusual aspects of biomass pyrolysis andprobable higher flexibility of operation. All these gasification open new fascinating fields of re-potential solar processes must be considered in search and applications. They bring also theassociation with more extensive research efforts. opportunity of new fruitful collaborations betweenThey include search of new efficient types of two different scientific communities involvedreactors such as vortex types (cyclones); imping- separately in the domains of ‘solar chemistry’ anding jets; etc. for small particles. Reactors able to of ‘biomass thermochemical conversion’.process big pieces of biomass must not be ignoredwith the advantage to prevent the high cost of

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