Biomass Gasification With Circle Draft

Ò

Process

Gian Claudio Faussone
Inser Cso Appio Claudio 229/5, Turin, Italy; [e-mail] (for correspondence)

Published online 21 March 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11608

Gasification seems to be a promising technology for small

scale power generation, but while coal gasification has a
long history and many facilities operate the process success-
fully, biomass hasn’t the same success. Biomass gasifiers are
usually based on ‘‘down draft’’ or ‘‘up draft’’ configuration
and are not able to guarantee good quality syngas. Besides,
they require very dry and selected feedstock, and the gas’
cleaning process is usually very difficult and complex. Fluid-
ized-bed gasifiers are also very promising but they require
feedstock’s preparation in terms of size and moisture and
they are quite complex to operate in comparison to ‘‘down
draft’’ or ‘‘up draft’’ configurations. In order to overcome
these limitations INSER has developed a new gasification
system, air blown and not pressurized: the ‘‘Circle Draft

Ò

.’’

Ó 2012 American Institute of Chemical Engineers Environ Prog, 31:
216–218, 2012

Keywords: biofuels, alternative energy

INTRODUCTION

This technology is able to provide the same good quality

syngas already experienced in coal gasification (e.g., Tys-
senKrupp Uhde Prenflo

Ò

) but using any kind of biomass and

carbonaceous feedstock. Lab analysis on the syngas’ compo-
sition states that almost 75% of the produced syngas is com-
bustible (H

2

, CO, and CH

4

) with very little CO

2

and N

2

even

using air as oxidant. The lower heating value (LHV) is about
11,500 kJ/nm

3

well above the 5000 kJ/nm

3

value considered

as reference for biomass air gasification syngas.

The potential high percentage of combustible syngas

achievable by this process is mainly due to the fact that the
syngas is a mixture of pyrolysis syngas and gasification syn-
gas, the latter obtained with air and steam, so limited quan-
tity of inert N

2

is introduced into the vessel.

Hence, the produced syngas can be used not only for

power generation using reciprocating engines or gas tur-
bines, but also for the synthesis of second generation bio-
fuels and bio-methane through Fischer–Tropsch reactors or
similar well experienced technologies.

Besides, the gasifier performs the cleaning of the syngas

as an intrinsic step of the gasification process itself, thanks to
the particular circulation of the syngas through the inner
chamber of the reactor. High content of moisture in the feed-
stock (20–30%) can also be accepted. This syngas’ circula-
tion, which gives the process the name of ‘‘Circle Draft

Ò

,’’

makes easier the subsequent treatment of the syngas without
the necessity of wet scrubbing or washing.

DISCUSSION

Usually, biomass gasification is performed in gasifiers

based on ‘‘down draft’’ design: like the ‘‘Imbert’’ one, design

widely used during WWII in Europe for automotive and
static applications; or ‘‘up draft’’ air blown, directly derived
from large coal gasifiers initially used for the production of
‘‘lighting gas.’’

People with experience in biomass gasification know very

well the limits of those designs: very sensitive to feedstock’s
size, moisture and composition, poor quality of produced
syngas, tars removals from syngas, just to quote some of the
most common.

A partial solution was attained working extensively on the

feedstock: regular sizing, shaping, strict moisture control, dry-
ing; combining hard effort for syngas cleaning using wet
scrubbing, fabric filtering, electrostatic separation of tars, etc.
But the overall complexity of the system makes it less attrac-
tive and more fragile.

Other solutions borrow the experience on coal gasifica-

tion transforming the biomass into charcoal first and then
using coal gasifiers technology, accepting a large loss of effi-
ciency.

Fluidized-bed gasifiers are also used in biomass gasifica-

tion. However, if on one side they allow a continuous syngas
production, on the other side they also require strict feed-
stock’s preparation both in terms of size and moisture con-
tent. In some cases, they can use only very standardized
sized feedstock like pellets or they even require biomass
grinding to suit the feed line. The intrinsic complexity of the
process limits fluidized bed gasifiers operation in small scale
applications.

To those problems and limits must be add the lack of

experience and engagement of engines and turbines pro-
viders who, despite the high demand worldwide for inno-
vative syngas fueled power generators, are reluctant to
leave the well-known ‘‘natural gas’’ field, with very few
exceptions.

‘‘Circle Draft

Ò

’’ process is quite different from a standard

biomass gasification system. It combines the pyrolysis and
gasification in a single vessel, with a circulation of the syngas
which enables a self-cleaning of the syngas itself. The results
are very interesting: feedstock does not have to be regular in
shape and size, high content of moisture can be accepted,
and the syngas may even have a higher heating value com-
pared to standard biomass gasifiers.

As the syngas’ composition can be adjusted throttling the

amount of oxidant, it can be used as fuel for power genera-
tion or as raw gas for chemical synthesis.

The pilot plant in Cherasco, Italy, was designed to per-

form a series of tests on different feedstock and to collect a
significant experience on this type of gasification, as well as
testing if and what type of extra equipment would be neces-
sary for commercial scale application. It is composed by one
gasifier with a screw feeder, two heat exchangers water
cooled for syngas cooling, one test engine (otto cycle), one
torch, and one steam generator. The upper chamber of the
gasifiers is equipped with electric heaters. The gasifier can

Ó 2012 American Institute of Chemical Engineers

216

July 2012

Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep

produce from 50 to 150 nm

3

per hour of syngas depending

on the working condition.

CIRCLE DRAFT

Ò

PROCESS DESCRIPTION (PCT PATENT PENDING)

The vessel is composed by two chambers.
In the upper one, the feedstock is pyrolyzed and con-

verted into charcoal (yellow in Figure 1).

Then the charcoal produced in the pyrolysis chamber

descends the vessel (gray in Figure 1), it is gasified in the
lower chamber with oxidant (generally air) and a variable
amount of steam or water.

The feeder must provide seal against the entrance of air

in the pyrolysis zone and it must provide seal against leakage
of syngas. Different solutions can be used to address this
problem: lock hoppers with rotating valves, dome valves or
cone valves are all solutions commercially available and
widely used in different applications.

The upper chamber is equipped with electric heaters in

order to raise quickly the temperature (if required) and to
ease the process with very humid feedstock. Although elec-
tric heaters are not the most efficient way to raise tempera-
ture, they allow precise control over it. Besides, as they oper-
ate only when the temperature in the upper chambers drops
below a set point, they are switched off most of the time,
making a good technical solution to add energy to the pyrol-
ysis chamber only when the amount of heat carried by the
flux of the gasification syngas is not sufficient.

In the pyrolysis chamber where the temperature can reach

500

8C, and no air is present, pyrolysis syngas is produced.

In the gasification chamber where charcoal and tar are

gasified at temperature of about 900–1000

8C, gasification syn-

gas is produced.

Both syngases mix together in the pyrolysis chamber and

are extracted from the side of the vessel, forced to pass
through a ‘‘bed of charcoal’’ (gray in Figure 2), which acts as
a ‘‘built-in-filter’’ trapping the tar which is gasified in the gasi-

fication chamber. This ‘‘bed of charcoal’’ is continuously
renewed during the operation as fresh feedstock is converted
into charcoal in the pyrolysis chamber.

The syngas exits the gasifier at about 250

8C as result of

heat losses and has to be cooled in a heat exchanger (not
shown). With better insulation the temperature of the syngas
might be probably higher.

Ash and slag are extracted from the bottom of the vessel.

FEEDSTOCKS

Different feedstocks have been successfully gasified like

woodchips at different moisture, straw, chicken manure, etc.

Even if the Circle Draft

Ò

Process has been able to work

with all the feedstocks, we experienced some mechanical
problems when feeding the vessel, due to mechanical fail-
ures.

In fact, the gasifier operates at about 50 cm H

2

O of pres-

sure, so syngas tends to leak from the feeder. Besides, it is
essential that the feeder provides seal against the uncontrolled
entrance of air in the pyrolysis chamber. So changing the type
and size of the feedstock, although had no or little effect on
the process, as the feeder screw has fixed diameter, in some
cases we experienced leakage of syngas. A future solution will
be to change the feeding line with a better shaped one with a
seal valve, e.g., a dome valve or a cone valve.

Usually when feedstock at 15–20% moisture is used, the

water recovered after the syngas cooling was almost the
same used to produce the steam necessary for the process.
In some occasions, water had to be added. Only when using
very humid feedstock at moisture of 30% or more, brown
water had to be disposed.

Brown water is water with a variable amount of tars in it.
Hereunder syngas’ composition of six samples collected at

different air/steam ratio.

Figure 1. Longitudinal section of the vessel showing two
chambers. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]

Figure 2. Syngas circulation inside the vessel. [Color figure
can be viewed in the online issue, which is available at

wileyonlinelibrary.com.]

Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep

July 2012

217

The feedstock was, in all cases, woodchips with moisture

of approximately 20–25%

Syngas’ Composition % v/v

 Sample 1: H

2

20.2%; CO 15.9%; CH

4

1.9%; CO

2

13.4%; N

2

47.9%; others 0.7%

5 Total 100 LHV 4.86 MJ/nm

3

, LHV

1161 kcal/nm

3

 Sample 2: H

2

13%; CO 14.6%; CH

4

1.6%; CO

2

10.6%; N

2

60%; others 0.2%

5 Total 100 LHV 3.81 MJ/nm

3

, LHV 911

kcal/nm

3

 Sample 3: H

2

10.1%; CO 9.7%; CH

4

1.3%; CO

2

13.4%; N

2

62.6%; others 2.9%

5 Total 100 LHV 2.78 MJ/nm

3

, LHV

661 kcal/nm

3

 Sample 4: H

2

8.6%; CO 19.1%; CH

4

1.3%; CO

2

7.6%; N

2

62.7%; others 0.7%

5 Total 100 LHV 3,8 MJ/nm

3

, LHV 909

kcal/nm

3

 Sample 5: H

2

31.3%; CO 33.2%; CH

4

1.1%; CO

2

0.1%; N

2

18.9%; others 15.4%

5 Total 100 LHV 7.95 MJ/nm

3

, LHV

1900 kcal/nm

3

 Sample 6: H

2

33.4%; CO 47.8%; CH

4

3.6%; CO

2

0.1%; N

2

15.1%; others 0

5 Total 100 LHV 10.92 MJ/nm

3

, LHV 2609

kcal/nm

3

Samples 5 and 6 were obtained with very little amount of

air and larger amount of steam, while 3 and 4 were obtained
without steam. Samples 1 and 2 were obtained with a good
air/steam ratio, however, geometry limitations of the vessel
did not allow a complete reduction of CO

2

into CO because

the gasification-syngas’ velocity in the gasification chamber
was too high.

Reducing the amount of air and increasing the amount of

steam (or water) drives to a better quality syngas, reducing
the inert N

2

. However, high volumes of hydrogen is difficult

to burn in reciprocating engines hence, some engines pro-
viders after studying samples 5 and 6, suggested us to dilute
the syngas with CO

2

. We considered this solution not

adequate and we decided to increase the amount of air and
hence the N

2

in order to reduce the quantity of H

2

and get-

ting more syngas at the same time, and we got Sample 1
type syngas.

It is important to notice that the final composition of the

syngas is a mixture of pyrolysis syngas and gasification syn-
gas, and hence the heating value of the syngas is usually
higher than pure gasification syngas.

SELF-CLEANING PROPERTIES OF CIRCLE DRAFT

Ò

PROCESS

The syngas, as mixture of gasification syngas generated in

the lower chamber and the pyrolysis syngas generated in the
upper chamber, is extracted from the side of the vessel after
being forced to pass through a ‘‘bed of charcoal’’ which acts
as a ‘‘built-in-filter’’ which removes most of the tar. This ‘‘bed
of charcoal’’ is continuously renewed by new charcoal com-
ing from pyrolysis of feedstock as it descends the vessel to-
ward the bottom. As a consequence, the feedstock is at the
same time fuel and filter.

Analyzing the composition of a sample of this ‘‘built-in-fil-

ter’’ we found that it does act as a filter, please note the com-
position:

 Ash 3.79% mass

 Dry solid charcoal 42.4% mass

 Tar 53.81% mass

It is interesting to notice that the matter shown above is

what is gasified in the lower chamber of the vessel. Unlike
other technologies, even coal gasifiers, where the charcoal is
very dry, in the ‘‘Circle Draft

Ò

Process’’ the lower chambers

perform an air/steam gasification of a very oily matter.

The reason because the bed of charcoal is able to trap the

tar should be found studying the flux of the syngas. In the
upper chamber, where pyrolysis occurs, tars are formed.
However, unlike other pyrolysis and/or gasification reactor
designs where the syngas produced moves away from feed-
stock—and leaving it in form of charcoal—in Circle Draft

Ò

Process the syngas stays in contact with the charcoal and
some absorption phenomenon seems to happen.

Further analysis on the content of tar still present in the

syngas will be carried out in order to get more data on the
capacity of the ‘‘built-in-filter’’ to trap the tar and eventually
to design a better shape of the vessel to enhance even more
this effect and better understand this phenomenon.

CONCLUSIONS

After several tests, some of them last over 100 hr continu-

ously, we solved most of the problems. We still need to
improve mechanics of the feeder as well as the levels meas-
urements inside the vessel. Because of geometry constraints,
especially in the lower section of the vessel, we designed a
new gasifier.

We run successfully the test engine and we produced

about 50 kW power. We are now looking for small scale
reactors for substitute natural gas synthesis on R&D basis.

Circle Draft

Ò

Process seems to be a very effective way to

gasify a wide range of feedstock. Without the need of syngas
treatment and feedstock preparation, almost all the plant is
essentially composed by the vessel.

Compared with ‘‘Down Draft’’ or ‘‘Up Draft’’ the syngas

produced in Circle Draft

Ò

Process is even visually much

cleaner without any filter. Besides, throttling the amount of
air and steam the syngas’s composition can be adjusted to
meet end user requirement, which is impossible with ‘‘Down
Draft’’ or ‘‘Up Draft’’ design. Unlike ‘‘Down Draft,’’ ‘‘Up
Draft’’ or Fluidized bed reactors, Circle Draft

Ò

does not

require specific feedstock preparation, hence high variability
in size and energy density can be accepted. Using ‘‘Down
Draft’’ design in particular, the size of the feedstock is critical
as, e.g., sawdust can cause the blockage of the air flux, while
using ‘‘Up Draft’’ design the flux of the air/syngas carries a
lot of volatile compounds that have to be removed before
usage.

Compared to Fluidized bed gasifier the operation of the

system is simpler and scaling up or down the vessel for com-
mercial use is also foreseen simpler.

We consider of great importance the possibility to use the

feedstock without specific pre-treatments as those pre-treat-
ment, e.g., pelletizing, torrefaction, etc., are energy intensive
activities.

We never experienced big problems related to low tem-

perature melting residues.

The possibility to substitute the steam with simple water

for the process, would ease even more the operation.

Further steps will be to manufacture a new and bigger

gasifier without the geometry constraints and mechanical
weaknesses we experienced so far; purchase a new engine
for grid connection, and get full permits from the Italian
authority for commercial scale units.

Further analysis on the amount of residual tar still present

in the syngas will be carried on different working conditions
and different air/steam ratio.

A complete energy balance will be carried on the new

vessel where most parasitic load and energy loss will be
eliminated.

218

July 2012

Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep