UNIVERSITY OF LAVAL, CANADA

5.17.1 Summary

The objective of this project was to investigate the potential of low pressure pyrolysis of biomass to produce high yields of condensable vapours and selectively condense fractions from different sections of the reactor to examine the potential for fractionation and recovery of chemicals. The reactor is a 30 kg/h multiple hearth furnace operated under vacuum. A range of feedstocks have been tested of which tyre pyrolysis was subsequently successfully scaled up to 250 kg/h. A notable feature of the vacuum type pyrolysis plant was handling whole tyres. Recent work has concentrated on operating the system for waste disposal. A system is believed to have been sold in Switzerland in 1994.

5.17.2 Description

A 30 kg/h vacuum pilot plant multiple hearth reaction system with liquid

condensation and collection was designed, constructed and tested (74). Figure

5.15 shows the arrangement of equipment. The optimum temperature range was found to be between 350-400°C and a yield of 60 wt % (on a dry ash free wood basis) of pyrolytic oil was obtained at an average heating rate of 10°C/min and at a total system pressure between 0.3 and 2.3 mm Hg (40-307 Pa). The feedstock used was aspen poplar. There is an extensive literature of which some key references are quoted (75, 76, 77, 78, 79). See also (3) for a more comprehensive overview and extensive referencing. Wood chips with a particle size from 1/4" to 1/2" Tyler mesh (6 mm to 12.7 mm) are fed via a hopper on the top of the reactor, which is hermetically sealed. This is equipped with a variable rate feeding device that feeds the chips into the preheated reactor at a constant feed rate of between 0.8 to 4 kg/h.

The reactor is a multiple hearth furnace 2 m high and 0.7 m diameter, with six hearths. Electric heating elements are used to heat the reactor. The temperatures of the heating plates increase from top to bottom of the reactor. A typical temperature profile is 200°C to 400°C. At steady state conditions, the absolute system pressure of the system is less than 80 mm Hg (10.7 kPa). The organic vapours and gaseous products are removed from the reactor by a mechanical vacuum pump via six outlet manifolds which correspond to the six heating plates. The char falls to the bottom of the reactor where it is collected in a metallic jar on a load cell. The process unit is connected to a central microprocessor which simultaneously gathers data and controls about 75 operating parameters.

image39

Figure 5.15 University of Laval Pilot Plant Flow Diagram

The clean-up system is a series of shell and tube heat exchangers (primary condensing unit) and a train of receivers (secondary condensing unit). Each outlet manifold is connected to a heat exchanger where the vapours are condensed and recovered as organic liquid in individual receivers. Cool to warm tap water is used as the cooling medium on the shell side. The vapours from the heat exchangers are then collected in the secondary condensing unit where the aqueous phase is primarily recovered. The first receiver is immersed in a bath of water-ethylene glycol mixture. The next two are immersed in baths of dry ice-acetone while the final receiver is filled with glass wool at room temperature. The non-condensable gases are pumped into a 500 litre vacuum vessel.

The low pressure removes the primary products quickly and avoids secondary reactions. Fractionation provides some separation of liquids evolved at different temperatures in a continuously operating system, but this has not proved as effective as had been hoped.

Recent testing of this unit at throughputs of 30 kg/h showed that the primary condensing unit composed of six shell and tube heat exchangers were inefficient due to clogging problems. The six individual exchangers were substituted by a single spray type condensing unit, similar to the secondary condensing unit. The new system proved to work very satisfactorily.

A major design consideration is the large volume of equipment due to operation at low pressure and the high cost of maintaining vacuum operation. Scale-up will have to consider the optimum operating pressure and the problems of heat transfer in the multiple hearth furnace.

5.17.3 Products

Some results are shown in Table 5.15. The highest yields of bio-oil are obtained at the lowest pressure and the higher temperature conditions. The optimum temperature range for maximum oil yield from wood was found to be between 425- 450°C at the bottom of the reactor.

One potential advantage of using a multiple-hearth reactor configuration is the capacity to fractionate the pyrolysis products by use of multiple outlets at different levels (see Table 5.16). The separation of the aqueous and the oil phases is important at the industrial level because the recovery of chemicals during distillation of large amount of water is less economical.

A typical analysis of the oif is shown in Table 5.17. This oil is highly oxygenated and consists of phenols, sugars and both aliphatic and aromatic hydrocarbons. The gases are mainly CO and CO2.

Table 5.15 Product Yields For Low Pressure Pyrolysis at Laval University

Temperature, °С

425

363

465

450

Pressure, mm Hg

12

18

80

12

Feedstock, kg

5.98

5.99

3.39

15.43

Yields (% wt wood organic basis)

Oil

46.4

41.6

39.7

50.9

Water

18.2

14.9

21.6

16.5

Char

24.2

33.0

24.7

21.3

Total Gas

11.2

10.5

14.0

11.3

Gas composition (vol %, dry basis)

CO

59.2

60.4

60.0

60.7

C02

33.6

34.9

31.4

31.6

CH4

2.4

0.9

3.3

2.7

H2

0.9

0.1

0.7

Others

3.9

3.7

4.6

5.0

Table 5.16

Separation of Condensation

Water

and

Pyrolytic

Oil

During

Run no

Reactor

Temp., °С

Primary

Secondary

Pressure

Hearth VI

Cooling

Condensing

Condensing

(mm Hg)

Water

Unit

Unit

Oil

Water

Oil

Water

(%)

(%)

(%)

(%)

C019

80

465

11-28

52.2

19.2

7.4

21.2

C023

12

450

50-55

32.2

1.5

36.7

29.6

C024

30

450

30-35

39.8

1.6

27.2

31.4

C025

10

450

15-20

47.8

3.4

27.2

21.6

Percentages are based on total condensates

Table 5.17 Elemental Analysis of Bio-oil from Laval University (80, 81)

Elemental Composition, wt%

Carbon

49.9

Hydrogen

7.0

Oxygen

43.0

Nitrogen

H/C ratio

1.68

O/C ratio

0.65

Ash

Moisture

18.4

Density, g/cm3 @ 55°C

1.23

Viscosity, cp

Heating Value, MJ/kg

21.1