Biomass Gasification With Circle Draft� 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
widely used during WWII in Europe for automotive and
Gasification seems to be a promising technology for small
static applications; or   up draft  air blown, directly derived
scale power generation, but while coal gasification has a
long history and many facilities operate the process success- from large coal gasifiers initially used for the production of
fully, biomass hasn t the same success. Biomass gasifiers are   lighting gas. 
usually based on   down draft  or   up draft  configuration People with experience in biomass gasification know very
and are not able to guarantee good quality syngas. Besides, well the limits of those designs: very sensitive to feedstock s
they require very dry and selected feedstock, and the gas size, moisture and composition, poor quality of produced
cleaning process is usually very difficult and complex. Fluid- syngas, tars removals from syngas, just to quote some of the
ized-bed gasifiers are also very promising but they require most common.
feedstock s preparation in terms of size and moisture and A partial solution was attained working extensively on the
they are quite complex to operate in comparison to   down feedstock: regular sizing, shaping, strict moisture control, dry-
draft  or   up draft  configurations. In order to overcome
ing; combining hard effort for syngas cleaning using wet
these limitations INSER has developed a new gasification
scrubbing, fabric filtering, electrostatic separation of tars, etc.
system, air blown and not pressurized: the   Circle Draft�. 
But the overall complexity of the system makes it less attrac-
Ó 2012 American Institute of Chemical Engineers Environ Prog, 31: tive and more fragile.
Other solutions borrow the experience on coal gasifica-
216 218, 2012
tion transforming the biomass into charcoal first and then
Keywords: biofuels, alternative energy
using coal gasifiers technology, accepting a large loss of effi-
ciency.
Fluidized-bed gasifiers are also used in biomass gasifica-
INTRODUCTION
tion. However, if on one side they allow a continuous syngas
This technology is able to provide the same good quality
production, on the other side they also require strict feed-
syngas already experienced in coal gasification (e.g., Tys-
stock s preparation both in terms of size and moisture con-
senKrupp Uhde Prenflo�) but using any kind of biomass and
tent. In some cases, they can use only very standardized
carbonaceous feedstock. Lab analysis on the syngas compo-
sized feedstock like pellets or they even require biomass
sition states that almost 75% of the produced syngas is com-
grinding to suit the feed line. The intrinsic complexity of the
bustible (H2, CO, and CH4) with very little CO2 and N2 even
process limits fluidized bed gasifiers operation in small scale
using air as oxidant. The lower heating value (LHV) is about
applications.
11,500 kJ/nm3 well above the 5000 kJ/nm3 value considered
To those problems and limits must be add the lack of
as reference for biomass air gasification syngas.
experience and engagement of engines and turbines pro-
The potential high percentage of combustible syngas
viders who, despite the high demand worldwide for inno-
achievable by this process is mainly due to the fact that the
vative syngas fueled power generators, are reluctant to
syngas is a mixture of pyrolysis syngas and gasification syn-
leave the well-known   natural gas  field, with very few
gas, the latter obtained with air and steam, so limited quan-
exceptions.
tity of inert N2 is introduced into the vessel.
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  Circle Draft   process is quite different from a standard
Hence, the produced syngas can be used not only for
biomass gasification system. It combines the pyrolysis and
power generation using reciprocating engines or gas tur-
gasification in a single vessel, with a circulation of the syngas
bines, but also for the synthesis of second generation bio-
which enables a self-cleaning of the syngas itself. The results
fuels and bio-methane through Fischer Tropsch reactors or
are very interesting: feedstock does not have to be regular in
similar well experienced technologies.
shape and size, high content of moisture can be accepted,
Besides, the gasifier performs the cleaning of the syngas
and the syngas may even have a higher heating value com-
as an intrinsic step of the gasification process itself, thanks to
pared to standard biomass gasifiers.
the particular circulation of the syngas through the inner
As the syngas composition can be adjusted throttling the
chamber of the reactor. High content of moisture in the feed-
amount of oxidant, it can be used as fuel for power genera-
stock (20 30%) can also be accepted. This syngas circula-
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tion or as raw gas for chemical synthesis.
tion, which gives the process the name of   Circle Draft , 
The pilot plant in Cherasco, Italy, was designed to per-
makes easier the subsequent treatment of the syngas without
form a series of tests on different feedstock and to collect a
the necessity of wet scrubbing or washing.
significant experience on this type of gasification, as well as
testing if and what type of extra equipment would be neces-
DISCUSSION
sary for commercial scale application. It is composed by one
Usually, biomass gasification is performed in gasifiers
gasifier with a screw feeder, two heat exchangers water
based on   down draft  design: like the   Imbert  one, design
cooled for syngas cooling, one test engine (otto cycle), one
torch, and one steam generator. The upper chamber of the
Ó 2012 American Institute of Chemical Engineers gasifiers is equipped with electric heaters. The gasifier can
216 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep
Figure 1. Longitudinal section of the vessel showing two
chambers. [Color figure can be viewed in the online issue,
Figure 2. Syngas circulation inside the vessel. [Color figure
which is available at wileyonlinelibrary.com.]
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
produce from 50 to 150 nm3 per hour of syngas depending
on the working condition.
fication chamber. This   bed of charcoal  is continuously
renewed during the operation as fresh feedstock is converted
into charcoal in the pyrolysis chamber.
CIRCLE DRAFT� PROCESS DESCRIPTION (PCT PATENT PENDING)
The syngas exits the gasifier at about 2508C as result of
The vessel is composed by two chambers.
In the upper one, the feedstock is pyrolyzed and con- heat losses and has to be cooled in a heat exchanger (not
shown). With better insulation the temperature of the syngas
verted into charcoal (yellow in Figure 1).
might be probably higher.
Then the charcoal produced in the pyrolysis chamber
Ash and slag are extracted from the bottom of the vessel.
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 FEEDSTOCKS
in the pyrolysis zone and it must provide seal against leakage Different feedstocks have been successfully gasified like
of syngas. Different solutions can be used to address this woodchips at different moisture, straw, chicken manure, etc.
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problem: lock hoppers with rotating valves, dome valves or Even if the Circle Draft Process has been able to work
cone valves are all solutions commercially available and with all the feedstocks, we experienced some mechanical
widely used in different applications. problems when feeding the vessel, due to mechanical fail-
The upper chamber is equipped with electric heaters in ures.
order to raise quickly the temperature (if required) and to In fact, the gasifier operates at about 50 cm H2O of pres-
ease the process with very humid feedstock. Although elec- sure, so syngas tends to leak from the feeder. Besides, it is
tric heaters are not the most efficient way to raise tempera- essential that the feeder provides seal against the uncontrolled
ture, they allow precise control over it. Besides, as they oper- entrance of air in the pyrolysis chamber. So changing the type
ate only when the temperature in the upper chambers drops and size of the feedstock, although had no or little effect on
below a set point, they are switched off most of the time, the process, as the feeder screw has fixed diameter, in some
making a good technical solution to add energy to the pyrol- cases we experienced leakage of syngas. A future solution will
ysis chamber only when the amount of heat carried by the be to change the feeding line with a better shaped one with a
flux of the gasification syngas is not sufficient. seal valve, e.g., a dome valve or a cone valve.
In the pyrolysis chamber where the temperature can reach Usually when feedstock at 15 20% moisture is used, the
5008C, and no air is present, pyrolysis syngas is produced. water recovered after the syngas cooling was almost the
In the gasification chamber where charcoal and tar are same used to produce the steam necessary for the process.
gasified at temperature of about 900 10008C, gasification syn- In some occasions, water had to be added. Only when using
gas is produced. very humid feedstock at moisture of 30% or more, brown
Both syngases mix together in the pyrolysis chamber and water had to be disposed.
are extracted from the side of the vessel, forced to pass Brown water is water with a variable amount of tars in it.
through a   bed of charcoal  (gray in Figure 2), which acts as Hereunder syngas composition of six samples collected at
a   built-in-filter  trapping the tar which is gasified in the gasi- different air/steam ratio.
Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep July 2012 217
The feedstock was, in all cases, woodchips with moisture The reason because the bed of charcoal is able to trap the
of approximately 20 25% 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
Syngas Composition % v/v
designs where the syngas produced moves away from feed-
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" Sample 1: H2 20.2%; CO 15.9%; CH4 1.9%; CO2 13.4%; N2 stock and leaving it in form of charcoal in Circle Draft
Process the syngas stays in contact with the charcoal and
47.9%; others 0.7% 5 Total 100 LHV 4.86 MJ/nm3, LHV
some absorption phenomenon seems to happen.
1161 kcal/nm3
" Sample 2: H2 13%; CO 14.6%; CH4 1.6%; CO2 10.6%; N2 Further analysis on the content of tar still present in the
syngas will be carried out in order to get more data on the
60%; others 0.2% 5 Total 100 LHV 3.81 MJ/nm3, LHV 911
capacity of the   built-in-filter  to trap the tar and eventually
kcal/nm3
" Sample 3: H2 10.1%; CO 9.7%; CH4 1.3%; CO2 13.4%; N2 to design a better shape of the vessel to enhance even more
this effect and better understand this phenomenon.
62.6%; others 2.9% 5 Total 100 LHV 2.78 MJ/nm3, LHV
661 kcal/nm3
" Sample 4: H2 8.6%; CO 19.1%; CH4 1.3%; CO2 7.6%; N2
62.7%; others 0.7% 5 Total 100 LHV 3,8 MJ/nm3, LHV 909
kcal/nm3
CONCLUSIONS
" Sample 5: H2 31.3%; CO 33.2%; CH4 1.1%; CO2 0.1%; N2 After several tests, some of them last over 100 hr continu-
18.9%; others 15.4% 5 Total 100 LHV 7.95 MJ/nm3, LHV
ously, we solved most of the problems. We still need to
1900 kcal/nm3
improve mechanics of the feeder as well as the levels meas-
" Sample 6: H2 33.4%; CO 47.8%; CH4 3.6%; CO2 0.1%; N2 urements inside the vessel. Because of geometry constraints,
15.1%; others 0 5 Total 100 LHV 10.92 MJ/nm3, LHV 2609
especially in the lower section of the vessel, we designed a
kcal/nm3
new gasifier.
Samples 5 and 6 were obtained with very little amount of
We run successfully the test engine and we produced
air and larger amount of steam, while 3 and 4 were obtained
about 50 kW power. We are now looking for small scale
without steam. Samples 1 and 2 were obtained with a good
reactors for substitute natural gas synthesis on R&D basis.
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air/steam ratio, however, geometry limitations of the vessel
Circle Draft Process seems to be a very effective way to
did not allow a complete reduction of CO2 into CO because
gasify a wide range of feedstock. Without the need of syngas
the gasification-syngas velocity in the gasification chamber
treatment and feedstock preparation, almost all the plant is
was too high.
essentially composed by the vessel.
Reducing the amount of air and increasing the amount of
Compared with   Down Draft  or   Up Draft  the syngas
�
steam (or water) drives to a better quality syngas, reducing
produced in Circle Draft Process is even visually much
the inert N2. However, high volumes of hydrogen is difficult
cleaner without any filter. Besides, throttling the amount of
to burn in reciprocating engines hence, some engines pro-
air and steam the syngas s composition can be adjusted to
viders after studying samples 5 and 6, suggested us to dilute
meet end user requirement, which is impossible with   Down
the syngas with CO2. We considered this solution not
Draft  or   Up Draft  design. Unlike   Down Draft,    Up
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adequate and we decided to increase the amount of air and
Draft  or Fluidized bed reactors, Circle Draft does not
hence the N2 in order to reduce the quantity of H2 and get-
require specific feedstock preparation, hence high variability
ting more syngas at the same time, and we got Sample 1
in size and energy density can be accepted. Using   Down
type syngas.
Draft  design in particular, the size of the feedstock is critical
It is important to notice that the final composition of the
as, e.g., sawdust can cause the blockage of the air flux, while
syngas is a mixture of pyrolysis syngas and gasification syn-
using   Up Draft  design the flux of the air/syngas carries a
gas, and hence the heating value of the syngas is usually
lot of volatile compounds that have to be removed before
higher than pure gasification syngas.
usage.
Compared to Fluidized bed gasifier the operation of the
SELF-CLEANING PROPERTIES OF CIRCLE DRAFT� PROCESS system is simpler and scaling up or down the vessel for com-
The syngas, as mixture of gasification syngas generated in mercial use is also foreseen simpler.
the lower chamber and the pyrolysis syngas generated in the We consider of great importance the possibility to use the
upper chamber, is extracted from the side of the vessel after feedstock without specific pre-treatments as those pre-treat-
being forced to pass through a   bed of charcoal  which acts ment, e.g., pelletizing, torrefaction, etc., are energy intensive
as a   built-in-filter  which removes most of the tar. This   bed activities.
of charcoal  is continuously renewed by new charcoal com- We never experienced big problems related to low tem-
ing from pyrolysis of feedstock as it descends the vessel to- perature melting residues.
ward the bottom. As a consequence, the feedstock is at the The possibility to substitute the steam with simple water
same time fuel and filter. for the process, would ease even more the operation.
Analyzing the composition of a sample of this   built-in-fil- Further steps will be to manufacture a new and bigger
ter  we found that it does act as a filter, please note the com- gasifier without the geometry constraints and mechanical
position: weaknesses we experienced so far; purchase a new engine
" Ash 3.79% mass for grid connection, and get full permits from the Italian
" Dry solid charcoal 42.4% mass authority for commercial scale units.
" Tar 53.81% mass Further analysis on the amount of residual tar still present
It is interesting to notice that the matter shown above is in the syngas will be carried on different working conditions
what is gasified in the lower chamber of the vessel. Unlike and different air/steam ratio.
other technologies, even coal gasifiers, where the charcoal is A complete energy balance will be carried on the new
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very dry, in the   Circle Draft Process  the lower chambers vessel where most parasitic load and energy loss will be
perform an air/steam gasification of a very oily matter. eliminated.
218 July 2012 Environmental Progress & Sustainable Energy (Vol.31, No.2) DOI 10.1002/ep