Pyrolysis Promising Route

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  • Bioresource Technology 42 (1992) 219-231

    Pyrolysis, a Promising Route for Biomass Utilization G. Maschio

    Dipartimento di Chimica Industriale, Universith di Messina, Salita Sperone 31 CP 29, 1-98166 S'Agata di Messina, Italy

    C. Koufopanos* & A. Lucchesi

    Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universit~ di Pisa, Pisa, Italy

    (Received 30 July 1991; revised version received 15 January 1992; accepted 4 February 1992)

    Abstract

    The pyrolysis of biomass is a thermal treatment which results in the production of char, liquid and gaseous products.

    In this laboratory the pyrolysis process has been studied experimentally using apparatus of different scales. In particular, the influence of the main process parameters on the yields and characteristics of the products has been investigated. On the basis of these results the differences between conven- tional and fast pyrolysis can be discussed.

    The most attractive product of conventional pyrolysis is charcoal, as the handling and use of bio-oil presents some problems due to its charac- teristics. The pyrolysis gas is a medium BTU gas and can be easily burnt. Fast pyrolysis minimizes charcoal production. This process gives as the main product a high yieM of a medium BTU gas rich in hydrogen and carbon monoxide.

    The feasibility of the process on an industrial scale is discussed.

    Key words: Biomass, pyrolysis, thermochemical conversion, wood, ligno-cellulosic components.

    INTRODUCTION

    Amongst the different processes which have been proposed for the energetic utilization of biomass, pyrolysis remains one of the most promising. The

    *Present address: Hellenic Cement Research Centre. 15, K. Pateli, Gr-14123 Likovrisi, Greece.

    Bioresource Technology 0960-8524/92/S05.00 1992 Great Britain

    term 'pyrolysis', is defined as the thermal treat- ment of biomass, in the absence of oxygen, which results in the production of solid (charcoal), liquid (tar and an aqueous solution of organics) and gaseous products. Pyrolysis is interesting, not only as an independent process leading to the produc- tion of energetically-dense products, but also as an intermediate step in a gasification or combus- tion process.

    A large number of research projects in the field of thermochemical conversion of biomass and particularly on biomass pyrolysis have been carried out (Knight, 1979; Sorer & Zaborsky, 1981; Bridgwater & Beenackers, 1985; Bridg- water & Van Swaaij, 1987; Bridgwater, 1988; Beenackers et al., 1989; Bridgwater & Bridge, 1991; Diebold, 1991 ). The results of this research have proved the feasibility of this technology. Many results regarding the identification of the wide spectrum of substances produced and their physico-chemical characterization are now avail- able. The problems associated with the realization of the process and the utilization of the products have been made evident. Bridgwater (1988) in a recent review analyzes the state of the art of dif- ferent pyrolysis technologies. Different interesting approaches to the efficient solution of the scale- up problems have appeared (Bridgwater & Bridge, 1991; Diebold, 1991 ).

    During the past 15 years many different pyroly- sis processes have been researched and developed in USA (Knight et al., 1986; Diebold & Power, 1988; Kovac et al., 1987); Diebold ( 1991 ) reviews the development of pyrolysis reactor concepts in the USA. However, the complexity of the process

    219 Elsevier Science Publishers Ltd, England. Printed in

  • 220 G. Maschio, C. Koufopanos, A. Lucchesi

    requires a variety of solutions suitable for the particular needs of each application.

    It is reported (Bridgwater & Van Swaaij, 1987; Beenackers et al., 1989) that, considering the oil prices of 1988, the cost of an energy unit pro- duced via pyrolysis is double that derived from fuel oil. However, the trend of oil prices indicates that significant variations may occur even in a short period. A significant increase in the price of oil is a likely scenario. As a consequence, the development of pyrolysis and other thermo- chemical processing of biomass can play an important role in the programming of short-term strategies.

    In the past few years (Elliot, 1985; Bridgewater & Bridge, 1991) much effort has been focused on the optimization of the operating conditions in order to obtain the most favorable yields of products and to improve their quality. In this study particular attention has been paid to improving the characteristics of the bio-oil. By changing the operating conditions during pyroly- sis we can modify the actual course of reactions and, thus, modify the final product distribution. In particular the kinetics of the process are influ- enced by the values of the main process para- meters: temperature, solid residence time, compo- sition of feedstock, particle size and heating rate. It has been shown (Koufopanos et al., 1989, 1991), that the heating conditions strongly affect the progress of the process. High heating rates (above 1000 K/s), which are employed in flash pyrolysis, minimize the yields of solid pyrolysis products and maximize those of liquid products (Scott & P iskorz, 1982; Antal, 1983 ).

    Depending on the operating conditions, the pyrolysis processes can be divided into three sub- classes: Conventional Pyrolysis, Fast Pyrolysis and Flash Pyrolysis.

    The range of the values of the main operating parameters are summarized in Table 1.

    The evolution of fast- and flash-pyrolysis tech- nologies must be attributed to the fact that the utilization of liquid fuels is very attractive (Antal, 1983; Scott et al., 1985; Radlein et al., 1987).

    Some interesting results concerning the improve- ment of bio-oil characteristics by using fast- or flash-pyrolysis followed by a secondary upgrading process have been reported in the literature (Knight et al., 1986; Bridgwater & Bridge, 1991). The characteristics of the bio-oil produced and the economics of the process suggest further research developments in this field.

    In our laboratory, the pyrolysis process has been systematically studied using different experi- mental apparatus of laboratory-, bench-, and large (pilot)-scale (Lucchesi & Maschio, 1987; Koufo- panos et al., 1989, 1991). Some significant results are presented here. The large amounts of experi- mental data, regarding the yields and the charac- terization of the pyrolysis products as well as pyrolysis reactor design and performance, offer a basis for the assessment of the process and pro- pose the most attractive paths to follow. As these data concern both conventional and fast pyrolysis, the differences between these two versions of pyrolysis can be discussed, their boundaries can be explored and possible interpretations of their behavior can be provided. This work helps to fill the gap existing between the research and the application of biomass conversion technologies.

    METHODS

    Experimental apparatus

    Conventional pyrolysis The conventional pyrolysis process was studied experimentally in apparatus of different scale and type. A first series of experimental runs was carried out in order to investigate the influence of temperature, composition and biomass particle size on the rate of pyrolysis. Pulverized biomass particles (d< 0"5 mm) were pyrolyzed in a thermo- balance (Mettler TA 3000).

    TG runs were carried out on samples up to 150 mg in a temperature range from 200 to 900C (+ 0.5C), using heating rates ranging from 5 to 80C/rain. In order to analyze the effect of parti-

    Table 1. Range of the main operating parameters for pyrolysis processes

    C. pyrolysis Fast pyrolysis Flash pyrolysis

    Operating temperature (C) 300-700 600-1000 800-1000" Heating rate (C/s) 0.1-1 10-200 >I 1000 Solid residence time (s) 600-6 000 0.5-5 < 0.5 Particle size (mm) 5-50 < 1 Dust

    "Up to 2 000C with solar furnaces.

  • Pyrolysis of biomass 221

    Fig. 1. change determination. 1, sample; 2, metallic rod; 3, furnace; 4, helical coil; 5, flow meter; 6, quenching; 7, gas exit; 8, balance.

    :i 1 r I

    carrier ',,, / N gas _..._,.._1.,,

    Experimental apparatus for isothermal mass- ] 41 ! I char L_ 1

    cle size a reactor specially designed for isothermal

    gas

    7 ~'

    cooling oil water

    Fig. 2. Flow diagram of the bench-scale pyrolysis reactor. 1, Biomass feeder; 2, moving bed reactor; 3, electric oven; 4, char collector; 5, hot cyclone; 6, heat exchanger; 7, entrain- ment separator.

    mass-change determination (Koufopanos et al., 1989, 1991 ) was used.

    The experimental runs were performed in a device designed for this purpose (Fig. 1). A tubu- lar reactor (38 mm inside diameter) was inserted into an electrically-heated tubular furnace. The sample material was placed in a stainless-steel wire mesh basket hung on a metallic rod con- nected to a balance in order to determine the weight loss of the sample.

    Isothermal mass-change determination was carried out on samples of different size (from sawdust of 0.3-0"5 mm up to cylinders of diam- eter 20 mm and length 100 mm) in a temperature range from 200 to 700C ( + 1 C).

    In order to analyze the overall performances of the process (yields of pyrolysis products and global kinetics rate) a series of experimental runs was carried out in a semi-batch bench-scale reac- tor (Lucchesi & Maschio, 1986).

    The apparatus, shown schematically in Fig. 2, consisted of a moving-bed reactor (inner diameter id= 100 ram, height h--500 ram) placed in an electrically heated oven. The biomass was fed to the reactor by a screw feeder and deposited on a rotating grate, where it met a countercurrent gas stream introduced at the