The gas is of low to medium heating value, 5-15 MJ/Nm3, depending on the method of pyrolysis. Even after efficient vapour condensation and liquid collection, it still has a relatively high oil content and either needs to be burned hot such as for drying feedstock, or a tertiary clean-up stage may be needed according to how it is used. Physical gas cleaning is difficult due to the complex physical and chemical characteristics of the organics. If water scrubbing is used, this gives a substantial wastewater disposal problem. The most effective utilisation method for the gas is as the fluidising medium if a fluid bed is used and use in-plant for some of the process energy requirement, although its specific energy content is rather low. The gas from high temperature processes can contain a significant proportion of non­equilibrium products such as olefins. These are of potential interest to the petrochemical industry when high temperature flash pyrolysis is employed (9). The yields of any individual or group of constituents is, however, relatively low. Ethylene yields have not exceeded 15% for example which is considered too low to be of economic interest (10). Catalytic conversion of such intermediates is possible by any established petroleum or petrochemical process, although this is not known to have been investigated.

The solid product from pyrolysis is char with any ash or inerts present in the feed.

Carbonisation is pyrolysis operated at low reaction rates and low temperatures to maximise charcoal production. This is established technology in both industrialised and developing countries. High pressure pyrolysis gives higher solid yields, but low pressure (vacuum) pyrolysis behaves analogously to flash pyrolysis in terms of liquids as the primary products are rapidly removed and do not contribute to char formation. In flash pyrolysis the char can be exported or used in — plant for process energy demands.

It is necessary to distinguish between temperature of reaction and reactor temperature. The latter is much higher due to the need for a temperature gradient to effect heat transfer. For fast pyrolysis the lower limit on wood decomposition is approximately 435 *C for obtaining acceptable liquid yields of at least 50% with low reaction times.

The effect of temperature is well understood in terms of total product yield with a maximum at typically 500-520’C for most forms of woody biomass. Other crops may have maxima at different temperatures. The effect of temperature is less well understood in terms of product fuel quality. Work by the University of Waterloo has demonstrated the effects of ash, DP, heating rate and reactor temperature on chemical yields (9). As progress is made in defining bio-oil quality better in fuel terms, the secondary gas/vapour phase history may be more important. At prolonged residence times (> 1 s), the lignin derived fraction may be further depolymerised to produce a more homogeneous liquids. This is also influenced by the reactor configuration. Work done by McKinley (1) has demonstrated that liquid produced in an ablative pyrolysis reactor has a much lower molecular weight average due to depolymerisation and cracking of the liquids on the metal surface prior to vaporisation.

The pyrolysis liquid was black and viscous and contained a significant proportion of charcoal of up to 15% by weight of the oil. This caused a "lumpy" texture in early samples. Atypical pyrolysis liquid analysis and a gas analysis is given in Table 5.3. The char was a coarse powder with particle sizes ranging from 40-50 microns to several mm, similar to the size and shape characteristics to the feed.

7.1 INTRODUCTION

One of the main activities in this contract was to support and help develop a biomass information system (BIS) that would establish the basic structure of a comprehensive database of biomass related activities for access and analysis by EC officials and interested researchers. The contract was let to Hendyplan.

7.2 BIOMASS INFORMATION SYSTEM

7.2.1 Introduction

A database was designed that would allow the collation and interpretation of state of the art information in each of the areas covered by the bio-energy R&D activities. Unfortunately each area had such diverse information requirements and availability that fusion was proved impossible and the rest of this section is devoted to describing the thermal conversion contribution.

The database was initially envisaged as taking the form of three separate files linked by a common set of keywords as follows:

• Database: Containing basic contacting information;

• Technology: Providing information on the basic technologies and

who is involved with their development; and

• Products: Providing information on the individual products that

may be derived and how they can be used.

A flow diagram of the anticipated and planned interactions is given in Figure 7.1 which is described in more detail below.

Having identified the necessary interactions required for a successful and self- evolving system the database methodology was then simplified to aid a successful outcome to the project and the resultant database was formulated and is summarised in sections 7.2.2 to 7.2.4 below as a preliminary outline of the database worksheet.

Subsequent discussions and requests for the database to include assessments and comparisons of each EC contract included led to the final version shown below in Table 7.1, preceding the 20 sets of specimen data supplied.

Although the final data was sent in January 1994, no further contact has been possible with Hendyplan and requests for reports and working versions of the final database for testing and appraisal have not been fruitful. A critical review of the final database would have provided valuable feedback to Hendyplan and the EC to help formulating future plans for such an activity.

Masterfile, directory, keywords

Description, performance, cost

Characteristics, costs

Characteristics, values

Calculation of capital costs & product costs

Evaluation of costs and performance

Figure 7.1 Components of projected database and interactions

7.2.2 “DATABASE” section of BIS

The initial proposed composition is summarised below:

Abbreviation, full name

Main person by surname and initials

Including fields for town/city, post code, country.

Define, maximum 10 words Define as keywords from list supplied:

Pyrolysis, Gasification, Combustion, Liquefaction, Upgrading, Power generation, Chemicals, Catalysts, Other, to be defined — for cross referencing to TECHNOLOGIES and PRODUCTS.

Feed rate kg/h

Define all by name including pseudonyms and alternatives. Can be established to link to a FEEDSTOCK file.

Define any special requirements such as maximum

particle size or maximum moisture content

Define major product produced or required — for cross

referencing to TECHNOLOGY and PRODUCTS

Define any special or peculiar properties

Define

Define

Names in full and abbreviations where relevant Names in full and abbreviations where relevant Names in full and abbreviations where relevant

Work has been recently carried out on fast pyrolysis of Euphorbia in a 6 g/h externally heated fluid bed microreactor. A maximum total liquid yield of 44 wt% was obtained at 500*C (29).

3.5.2.8 _ University of Twente (Netherlands)

An ablative pyrolysis reactor was developed based on particles sliding over a heated rotating cone mixed with heated sand. Total liquid yields of up to 50 wt% were obtained at capacity of 7 kg/h (30). Work is continuing on development of the system to recycle the sand and char in associated fluid beds and a standpipe to provide a continuous system (31). A 50 kg/h unit has been sold to the University of Beijing in China and is due for commissioning early in 1995 (31).

3.5.2.9 Other

TGA studies are being carried out at the University of Zaragoza, Spain (32) and the Hungarian Academy of Sciences (18); free fall or drop tube experiments at RIT, Sweden (33); modelling studies at the University of Naples, Itafy (34); bio-oil analysis studies at the University of Corsica, France (35) and UCL, Belgium (36); wire mesh pyrolysis at Imperial College, London, UK (37); waste wood fast pyrolysis at the Institute of Wood Chemistry, Hamburg, Germany; and waste pyrolysis at low temperature at the University of Cardiff, UK (38).

The Stenau process is based on the work carried out at the University of Tubingen. There is little information about technical aspects of the process other than a rotary kiln is used to slow pyrolyse refuse/MSW at fairly low temperatures to effect partial liquefaction at a capacity of 1 t/day refuse (66, 67). The performance is summarised in Table 5.13. A flowsheet is given in Figure 5.12 (68). The liquid

yield is about 31% weight on dry MSW feed with 71 m3 off gas per tonne feed, some of which is used for process heat, and a solid residue of 56% wt on feed.

Table 5.13 Summary of the Stenau Process.

Feedstock throughput (d. a.f.) 1 tonne/day

Liquid product yield 0.309 tonne bio-oil/tonne feed

Gas product yield 71 m3/tonne feed

Solid residue yield 0.56 tonne/tonne feed

The kinetics of wood degradation and its respective components have been and still are obtained by measuring the rate of weight loss of the sample as a function of time and temperature. The most common technique for this investigation is thermogravimetric analysis (TGA). TGA involves continuous weighing and recording of data obtained from a sample, heated at either constant temperature or a fixed heating rate, enclosed in a furnace (e. g. 1,2, 3, 4, 5, 6, 9, 13, 15, 18, 20, 40, 41, 42, 43, 44, 45, 46, 55]. These experiments are normally carried out under vacuum or in a nitrogen atmosphere at both low temperatures and heating rates. Some workers have used steam as the gaseous environment in their experimental system [45, 46, 56]. As there is no provision for the collection of volatile pyrolysis products, the TGA data are normally used to derive overall kinetic expressions.

The global thermal degradation process can be described by a simplistic reaction scheme as shown in equation M.2.

biomass —> char + volatiles M.2

and the rate of the above reaction is then described in the form of a first order Arrhenius type rate law as shown in equation M.3.

dW = — A exp (-E ) (W — Щ) M.3 dt RT .

where: W: residual weight fraction,

Wf: final weight,

A: pre-exponential (frequency) factor,

E: activation energy,

R: Universal gas constant.

T: reactor temperature

If the sample is heated at a constant rate M, then dW = — A M.4

dt M exp (- E) (W — Wf) where: M = dT

RT dt

Sometimes the weight term is replaced by a density term. It is assumed here that no shrinkage occurs during char formation. Table 2.2 shows some selected kinetic parameters for overall reaction rate expressions.

The Arrhenius kinetic parameters, the activation energy (E) and the pre-exponential (frequency) factor (A) are derived by obtaining best fit curves through the experimental data and solving the Arrhenius rate law, using a least squares method. A comparison of the different kinetic parameter estimation methods is made by Vovelle et al. which highlights the variability in the values obtained (57). Most of these values have been obtained from weight loss data.

Kinetic parameters estimated by the researchers given in Table 2.2 show a wide variability in the values, even with similar feedstocks. Some of the variability in the parameters may be accounted for by the neglect of temperature variation of the sample during heat up and the use of the steady state temperature as the overall reaction temperature. Some of the kinetic modelling has also be performed with large biomass samples where the effects of mass and heat transfer cannot be neglected. This is evidenced by Salazar who gives different value for the pre­exponential factor and the activation energy for two differing cylinders of eucalyptus (68). Other researchers have used two or more consecutive steps of zero and first order reactions to describe the pyrolytic degradation of the biomass (13, 58, 59, 60, 61,62,63,64).

Bilbao et al. have taken into account the influence of heating rate on the kinetic parameters and have therefore studied a range of heating rates from 1.25 C°/min to 80C7min and the effects on the reaction order (2, 3, 4, 5, 6). Other discrepancies arise from too simplistic modelling and the presence of impurities or ash which may influence the decomposition kinetics. Varhegyi et al. have investigated the effect of NaCI, FeS04 and ZnCl2 on the pyrolysis of Avicel cellulose under different

conditions with four different modelling approaches (55). To predict the product distribution, stepwise models should be applied.

Europe has only relatively recently become involved in direct production of liquid fuels from biomass. Up to 1989, a conventional (slow) pyrolysis demonstration plant of 500 kg/h was operated by Alten in Italy for liquid and char production with approximately 25% yield of each (4). The product was derived in relatively low yields from an undefined reaction system. It is likely that the product was at least partially secondary liquids due to the relatively slow reaction, probable high reaction temperature and long vapour residence time, Bio-Alternative in Switzerland operated a fixed bed carbonisation pilot plant fed with wood, waste and MSW for charcoal production with liquids as a low yielding by-product (5). Liquids were recovered in a direct contact scroll cooler with selective condensation of oil at around 120°C which gave low yields of secondary pyrolysis products that were also more tarry.

More recently, a 200 kg/h flash pyrolysis pilot plant based on the University of Waterloo (Canada) process has been constructed in Spain by Union Fenosa which started up in mid 1993 (6). Egemin in Belgium have built and operated a 200 kg/h entrained downflow pilot plant to their own design which started up in July 1991 and operated until late 1992 (7). ENEL are purchasing a 15 t/d Ensyn (Canada) RTPIll pilot plant to produce bio-oils for testing. All these processes and other exploratory studies sponsored by the EEC JOULE (8) and AIR programmes are included in Table 4.1 and described below.

Most biomass contains natural salts that will influence the decomposition products. However, since the salts cannot readily be removed without affecting the organic substrate, any catalytic effects become part of the simple thermal degradation process. Much of the early work on fundamentals such as Shafizadeh (11) recognised the effect but did not take advantage of the phenomena. A particular example is pyrolysis of sewage sludge, initially investigated at the University of Tubingen (12) and later applied to development of a commercial plant to recover oil (13, 14, 15). Bayer claimed that the inorganic constituents of sewage acted as a "natural catalyst" (12) and this work was extended to recovery of fatty acids (16). The University of Waterloo has clearly demonstrated the effects of natural catalysts on chemicals production, and details are given in Table 6.3 and beyond.

6.3.1 Chemicals production

The extraction and recovery of chemicals from biomass pyrolysis liquids is rapidly growing in interest as the natural catalysts in most biomass forms are enhanced or removed to emphasise production of specific chemicals or families of chemicals. In addition, specific chemicals are recovered by physical and/or chemical processing and may be subjected to catalytic processing to improve the product quality or yield or derive higher value chemicals. Since the primary formation of organics is significantly influenced by the presence or absence of natural or added catalysts, it is not practicable to differentiate between catalytic and non-catalytic processes, so all chemical extraction and recovery work is included. Secondary upgrading of the recovered compounds or fractions is also included here, but catalytic conversion of the primary vapours and whole product oil is-discussed separately below.

Table 6.3 lists the organisations currently and recently involved in chemicals extraction/recovery and derivation, while Table 6.4 lists the chemicals and where they are being investigated.