Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines—Part 1 : emulsion production

Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines—Part 1 : emulsion production

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  Available online at Biomass and Bioenergy 25 (2003) 85–99 Development of emulsions from biomass pyrolysis liquid anddiesel and their use in engines—Part 1: emulsion production D. Chiaramonti a ; ∗ , M. Bonini a , E. Fratini a , G. Tondi  b , K. Gartner  c , A.V. Bridgwater  d ,H.P. Grimm e , I. Soldaini f  , A. Webster  g , P. Baglioni a ; ∗ a Department of Chemistry and CSGI (Consorzio Sistemi a Grande Interfase), University of Florence,via della Lastruccia 3, I-50019 Florence, Italy  b Energetics Department “S. Stecco”, Faculty of Mechanical Engineering, University of Florence,Via S. Marta 3, I-50139 Florence, Italy c Institute f ur Elektrische Energietechnik (IEE), Universit at Gh-Kassel, Energietechnik, Fb 16,Rationelle Energiewandlung, D-34109 Kassel, Germany d Aston University, Bio-Energy Research Group, Aston Triangle, Birmingham B4 7ET, UK  e WIP, Sylvensteinstr. 2, D81369 M  unchen, Germany f  Pasquali Macchine Agricole, Via Nuova 30, I-50041 Calenzano, Italy g ORMROD Diesels, Unit 4, Peel Industrial Estate, West Pimbo, Skelmersdale, Lancashire WN8 9PT, UK  Received 18 March 2002; received in revised form 28 October 2002; accepted 28 October 2002 Abstract The current method of utilising biomass derived fast liquid (bio-crude oil or bio-oil) in a diesel engine requires three fuelsand a complex start-up and shut down procedure. For more rapid and successful commercialisation of this renewable liquidfuel, a more convenient and cheaper method of utilisation is needed that provides a single fuel that is stable and readilyignites in a compression engine. This paper describes the production of emulsions from biomass fast pyrolysis liquid anddiesel fuel for utilisation in diesel engines. The objective is to allow unmodied diesel engines to run on fast pyrolysis liquidderived from biomass without the cost and complexity of a dual fuel system. The immediate application is in stationaryengines for power generation, but there are longer term opportunities for use as a transport fuel. This paper describes the production of the emulsions that have been tested in dierent diesel engines (tests in engines is reported in a separate paper). ?  2002 Elsevier Science Ltd. All rights reserved. Keywords:  Biomass pyrolysis; Biomass power production; Emulsion; Diesel engine; Bio-crude oil 1. Introduction Pyrolysis is the thermochemical process thatconverts biomass into liquid, charcoal and non- ∗ Corresponding authors. Tel.: +39-055-457-3033; fax: +39-055-457-3032. E-mail addresses:  chiaramonti@apple.csgi.uni.it(D. Chiaramonti), piero.baglioni@uni.it (P. Baglioni). condensable gases, acetic acid, acetone and methanol by heating the biomass to about 750 K in absence of air. The process can be adjusted to favor charcoal, pyrolytic oil, gas or methanol production with a fuelto feed eciency up to 95%. Pyrolysis can be usedfor the production of bio-oil by various processes (e.g.ash pyrolysis) [1 – 5]. Bio-crude oil (BCO) (pyrolysis oil) use for power and heat production represents a main goal in the 0961-9534/03/$-see front matter   ?  2002 Elsevier Science Ltd. All rights reserved.PII: S0961-9534(02)00183-6  86  D. Chiaramonti et al./Biomass and Bioenergy 25 (2003) 85–99  biomass sector. The concept of using biomass-basedfuels, specically vegetable oils as diesel fuelalternatives, is particularly interesting and it is notnew. Rudolf Diesel himself demonstrated that hisengine could run on vegetable oil fuels. Since then anumber of vegetable oils have been tested as dieselfuel alternatives [6 – 10]. These fuels are non-toxic, renewable sources of energy, which do not contributeto the net global CO 2  buildup.A large number of studies on biomass derived oilsas biofuels have been performed [10 – 13]. However, direct use of pyrolysis oils (BCOs) requires signicantadaptations of technology to fuel characteristics, andit has not yet been fully proven at demonstration scale[8,10,14]. BCOs have already been used in modied dieselengines, in which a dedicated fuel feeding system has been designed, constructed and installed in parallel tothe diesel feeding line: in addition, the operation of the engine becomes more sophisticated (use of clean-ing fuels, as methanol, in a particular sequence dur-ing start-up and shut-down), and a pilot diesel fuelinjection is necessary. This approach requires a sig-nicant adaptation of the engine and, consequently,capital and O&M costs per kW installed are higher than standard diesel engines [15,16]. This fact repre- sents a considerable barrier to the use of BCO in dieselengines.Measurements of the combustion performance of  biofuel oils, blends with diesel fuels and emulsionswith water have been reported [12]. The development and use of BCO/diesel oil emul-sions represent a relatively short-term approach tothe exploitation of the signicant biomass resources potentially available by pyrolysis. The upgrading of the fuel itself by means of the production of stableemulsions allows the use of low-cost large-scale mass production diesel engines with only minor modica-tions, thus reducing signicantly the investment costin comparison to dual fuel engines. The fuel pro-duction process aims therefore at producing low-costand stable emulsions, compatible with the existingdiesel technology (only minor modications to theengine) [17]. This approach to fuel upgrading is investigated byvarious research groups. In particular, CANMET hasdeveloped a method for microemulsion productionfrom pyrolysis oil and diesel oil: stable emulsionshave been produced in the range of 5–40% have been produced and tested.In the present work, background knowledge onemulsication processes has been used to understandthe forces that play a fundamental role in the pro-duction of pyrolysis oil/diesel oil emulsions and toformulate stable systems [18]. Initially, a large num-  ber of tests at laboratory level have been realized toselect the most appropriate combination of additives.Once dened the surfactants mixture to be used,the emulsication process has been adjusted and re-alized at demonstration level in a semi-automatic plant [17]. 2. Theory  2.1. Introduction to the production of emulsions Though emulsions are ubiquitous in everyday life,the study of these systems has been often carried onin empirical ways [19]. Dispersions of two or more not miscible uids are produced for many applica-tions. Examples are abundant either between natu-ral products (such as milk) and industrial products[20,21]. Systems obtained from homogenization of  two or more not miscible uids are indicated as emul-sions, miniemulsions or microemulsions (or swollenmicelles) depending on the size of the particlesdispersed in the continuous phase [22,23]. These systems are produced using one or several additives(surfactants and co-surfactants) that are able to lower the surface energy of the interface of the produceddroplets [24]. Emulsions are obtained from a dispersion of twonon-miscible uids. The dispersion produces a con-tinuous phase and a nely dispersed droplets phase.Most of the properties of the emulsion systems (sta- bility, viscosity, etc.) depend on the droplets sizeand size distribution that usually cover a rather wideinterval, i.e. from 300 to 400 nm to about 10  m.A commonly used classication of emulsions is based on the polarity of the dispersant phase comparedto that of the dispersed one. In almost all the appli-cations, water is one of the uids, while the other ischaracterized by a lower dielectric constant. In generalthis second uid is indicated as oil. Therefore emul-sions are generally indicated as dispersions of water   D. Chiaramonti et al./Biomass and Bioenergy 25 (2003) 85–99  87Fig. 1. Most important mechanisms of emulsion breaking: (a) stable emulsion; (b) creaming; (c) Ostwald ripening; (d) occulation;(e) coalescence; (f) breaking. droplets in oil (water-in-oil emulsions, W/O) or oildroplets in water (oil-in-water emulsions, O/W).  2.2. Stability of emulsions The formation and stability of droplets depends ontwo competitive factors:(i) the migration of the surfactant at the dropletsinterface (stabilizing process),(ii) the droplets coalescence (destabilization process).The phase characterized by the highest coalescencerate will become the continuous phase.Emulsions formation and stability are also aected by the sequence and the methodology used to mixtogether the emulsion components. For example, wecan either dissolve the emulsier in the oil and in thewater phase or dissolve the emulsier in one uid andadd it to the other one, etc. Moreover, the mean dropletdiameter depends on the intensity and on the amountof the introduced energy for the particular preparationtechnique.Stability of emulsions can be described by theDerajaguin, Landau, Vervey and Overbeek (DLVO)theory and its implementations that take into accountdierent kind of interactions, as Van der Waals in-teractions, electrostatic interactions, steric, hydration,etc. [18,25 – 28]. The main mechanisms of emulsion breaking areshown in Fig. 1.The destabilization of an emulsion goes throughseveral consecutive and parallel steps before the nalstage of separated layers is reached. At rsts, dropletsmove due to diusion (or stirring), and if the repul-sion potential is too weak, they become aggregated toeach other: in practice, occulation has taken place.The single droplets are now replaced by twins (or multiples) separated by a thin lm. The thickness of the thin lm is reduced due to the Van der Waals at-traction, and when a critical value of its dimension isreached the lm bursts and the two droplets unite toa single droplet. Coalescence has occurred. In parallelwith these phenomena, the droplets rise through themedium (creaming) or sink to the bottom (sedimen-tation) due to dierences in density of the dispersedand continuous phase. The nal result is a highly  88  D. Chiaramonti et al./Biomass and Bioenergy 25 (2003) 85–99 concentrated emulsion at the top or bottom of the con-tainer, and the increased number of droplets per vol-ume increases the occulation rate in a most decisivemanner.Theocculationandcoalescenceprocessleadto larger and larger droplets until, nally, a phase sep-aration has occurred.Since emulsions are not thermodynamically stable,it is obvious that the control of the stability of theemulsions is limited to the kinetic control of the sepa-ration of the constituents (oil and water). After sometime, emulsions destabilize as a result of the agingeect, and the total area of the system decreases. Themost common method to verify the stability of emul-sions is simply by testing the formation of separate phases at room temperature, by heating the samplesor by centrifuging. More rened methods consist inthe analysis of the particle size or of the viscosity asa function of time [18]. Coalescence is by denition slow in stable emul-sions and measurement of the ageing rate is accel-erated by use of centrifugation at between 1000 and25 ; 000 g.In addition, in the case of pyrolysis oils, one of thesignicantproblemsintheirutilizationistheirclaimedinstability, which manifests as changes in viscosity. Ameasure of stability or instability is a valuable indi-cator of one of the key properties of pyrolysis liquidsand a viscosity index is proposed to provide such ameasure. The viscosity index is dened in the equa-tion below and provides a measure of viscosity changeover a given time period: VI   = (  t  2 −  t  1 )  t  1 ; where  VI   is the viscosity index,   t  1  the viscosity attime  t  1  (i.e. before storage) and   t  2  the viscosity attime  t  2 (i.e. after 24 h storage at 80 ◦ C).The lower the number, the more stable the liquid.A perfectly stable liquid has a viscosity index of 0.00.This approach has also been applied to evaluate thestability properties of the emulsions.  2.3. BCO =  diesel emulsions Asreportedabove,theprocessforobtainingastableemulsion may be very dicult and time consuming:in the case of BCO and diesel (Fig. 2), approximately Fig. 2. BCO/diesel oil mixture (left) and emulsion (right). one hundred dierent surfactants, and a consistentnumber of mixtures of them, have been tested. Thereason for that mainly relies on the composition of theBCO itself. In fact, BCO is made of a mixture of sev-eral organic products: from short chain products, asaldehyde or carboxylic acid, to polymers.The interactions that are responsible for the stabilityof an emulsion are Van der Waals, electrostatic andsteric interactions. Clearly, if the phases are single products it is rather simple to evaluate the kind of molecular interaction and to understand the class of surfactants that should be used to obtain the emulsion.If instead the molecular composition of one phase iscomplex, it becomes very dicult to gure out theadditive to be used for the emulsication process. Thisis exactly the case of the BCO/diesel oil system.The methodology to be used is the following: (1) aneective solvent for the BCO has to be identied. Thisallows a surfactant to be used even if it is not solublein the BCO: it is sucient that the surfactant is solublein the same solvent, (ii) the additive for the emulsionformation is tested. As the additive class showing the better results in terms of stability is determined, asmany surfactants as possible belonging to the additiveclassmustbetested,(iii)asmallnumberofsurfactantscan therefore be chosen, and the emulsion propertiesoptimized. A mixture of dierent additives can also be adopted.As the surfactants to be used for emulsion prepara-tion are nally dened, the process for the productionof the nal emulsion has to be optimized.  D. Chiaramonti et al./Biomass and Bioenergy 25 (2003) 85–99  89 Three kinds of emulsions can be prepared in respectto the BCO/diesel oil ratio: •  water-in-oil emulsions (W/O) are obtained if up to ∼ 45% w/w BCO is added to the diesel oil phase, •  oil-in-water emulsions (O/W) are obtained if up to ∼ 45% w/w of diesel oil is added in BCO, •  bicontinuous emulsions are obtained when the per-centage by weight of the two phases is close to 50%.In the rst case, BCO’s droplets dispersed in dieseloil (continuous medium) form the emulsion. In thesecond case Diesel oil’s droplets dispersed in BCO(continuous medium) form the emulsion. Descriptionof the third case is more complex: in fact, from a the-oretical point of view, both oil and water phase arecontinuous (no droplets) and form a so-called bicon-tinuous emulsion.The choice of the emulsifying agent, which changesthe interfacial properties (namely interaction potential between droplets) of the system avoiding (or delay-ing) the emulsion’s breaking, is a procedure driven byseveral empirical rules. These rules are built on the primary condition for an emulsier to be ecient in atwo phase, two-liquid emulsion. It has to be localizedat the oil/water interface to a maximal extent. Thiscriterion is exemplied in the hydrophilic lipophilic balance (HLB) number.It is important to emphasize that the HLB number for a certain molecule denotes the balance between itshydrophilic/lipophilic properties only; the number per se does not give any information about the stabilizingecacy of the emulsier. The HLB number meansthat the emulsier is optimal in a water-oil system inwhich the properties of the oil match the surfactant.Hence, each water-oil combination is characterized by an HLB number. The denition of this number israther complicated and depends on the family of thesurfactant. A more detailed treatment of this subjectis given in Ref. [18]. More practically, emulsiers characterized by HLBnumber: •  from 4 to ∼ 8, stabilize W/O emulsions, •  from 8 to ∼ 10, stabilize bicontinuous emulsions, •  from 10 to ∼ 18, stabilize O/W emulsions.The phase inversion temperature (PIT) or HLB tem- perature concept relates the emulsier selection tothe temperature at which an emulsion stabilized by anon-ionic emulsier of the polyethylene glycol typechanged from oil-in-water to water-in-oil with risingtemperature.A large number (approximately one hundred) of surfactants were tested, including both of commer-cial blends and chemically pure composition. Amongthem, cationic, anionic, zwitterionic and non-ionic sta- bilizers. Moreover, for each of these classes of addi-tives, polymeric and non-polymeric surfactants werealso tested. Additives with sulfur or nitrogen in their composition have been neglected, in order to avoid anincrease in the NO  x  and SO  x  gaseous emissions dur-ing the emulsion combustion. A list of the most sig-nicant tested surfactants is given in Table 1.The water-in-oil emulsions have been prepared by adding the surfactant to diesel oil and thereafter adding the bio-oil to the resulting mixture. Bicon-tinuous emulsions have been formulated by addingthe surfactant to diesel oil, and to BCO then mixingtogether the resulting mixtures. Oil-in-water emul-sions have been formulated by adding to the bio-oilthe surfactant and thereafter adding to the resultingmixture the diesel oil during emulsication.The emulsication process has been carried out byusing a homogenization unit. The temperature dur-ing mixing has been preferably maintained between60–65 ◦ C and the emulsication process has been con-tinued until a homogeneous single phase has been ob-tained. During the emulsier choice phase, emulsionshave been prepared by using a discontinuous processon a little amount of mixture (from 20 to 500 g). Onceidentiedtheemulsier,acontinuousprocesshasbeenemployed to produce a greater amount of material (upto 10 kg). In this conguration the tank was a heatingcase to have the desired temperature in the continuoushomogenization unit.It is important to underline that emulsions can be prepared also at room temperature: however, in thiscase the emulsions stability decreases ( ∼  20 days). Nevertheless, the destabilization of the emulsions, prepared at both high and room temperature, is anon-irreversible process: mixing (by a stirrer) issucient to obtain again a homogenous system.The phase diagram of the system BCO \ diesel oil \ emulsier has been investigated in order to identifythe best w/w ratio in terms of homogeneity, stability,engine performance, and operational costs.
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