Since -99, Puhdas Energia Oy has been developing a pressurized downdraft
gasifier for applications from 500 kW to 3 MW thermal.
Our first standardized product is a 1 MW (gas chemical energy) downdraft
gasifier operating at 1.1-1.5 bars absolute pressure.
The early efforts were focused on the development a gas tight infeed system
to enable forced draught gasification.
We see that pressurized gasifiers are more feasible in various applications
as the gas flows from the gasifier with it's own pressure.
This enables the use of woodgas in industrial applications (heat treatment,
bakeries, dryers etc.), and also in oil boilers which are generally of
forced draught type.
By 2003 we managed to develop a continuous infeed system with no gas
leakages, the ash removal system is similar with the infeed system.
The highest over pressure we can use is 0.5 bars. Technically we can go up
to 4 bars, but the pressure vessel regulations would require us to have the
gasifiers approved as pressurized vessels.
The gasifier has an atmospherical superficial velocity of 3.4 m/s (@650
nM3/h), but at 1.3 bars it is reduced to 2.6m/s.
Our patented refractory reactor has no nozzles, and the air is distributed
in two stages. We can use high superficial velocities as the fuel is not
crushed in front of the air nozzles, and the cross-sectional temperature
profile is very uniform.
The ceramics are based in aluminum oxide as many other refractory materials
will not tolerate reducing atmospheres. The reactor diameter is 500 m with
250 mm throat and 450 mm reduction layer. The nominal pressure drop over the
reactor is 100-200 mbar depending on fuel size.
The gas exits at 600-750 C depending on fuel size and output.
The residual char loss is 5-8% of fuel input (energy content), depending on
The temperature in the combustion zone exceeded 1500 C as we experienced
slagging with finnish softwood ashes (softens at 1400 C).
We dropped the preheat temp of the gasification air from 500 C to 200 C and
no slagging has occurred since.
We also developed our own producer gas burner for thermal applications in
the 1-2 MW range.
The burner has a refractory port-block for stable operation, continuous
electrical ignition and gas conductivity based flame sensing for safety. The
combustion is optimized by residual oxygen control. With woodpellets and
bark free chips the guaranteed emissions are as
CO 20 ppm
NOx 20 ppm
SOx 0 ppm
particulate emission<20 mg/MJ with 80 % boiler efficiency
Prior to combustion, the gas is cleaned with a multicyclone scrubber.
The particulate content before combustion is 150 mg/nM3, mostly char and
soot particles. The carbon in the particles is consumed during combustion,
and particulate levels after combustion are normally between 10-30 mg/nM3
This is a 3-D model and a picture of an installed 1MW gasifier
application what we call the Retrofit-system.
The system includes a gasifier with automatic infeed and ash removal, a
multicyclone and a gas burner with an integrated automation system installed
in a 20-ft container.
The Retrofit is intended for customers with existing oil fired boilers and
directly fired applications like dryers, kilns etc.. One emerging
application is the use of woodgas to produce clean CO2 for greenhouses.
The system can be upgraded for CHP with the addition of gas cleanup and
Our gas cleaning process is based on our indigenous design, and
utilizes a semi-wet Electrostatic precipitator and direct cooling. The
ESP consists of a single 900 mm vessel with a central electrode.
The gas flame becomes invisible at voltages over 135 kilovolts-> the
violet flame reaction from ash derived potash is not visible, nor the
yellow-orange hues from carbon and hydrocarbons. See the pictures of
the flame and the ESP. Massive difficulties were first experienced with
the high voltage insulator before water-acid condensation was
The ESP operates at 80 C, and if fuel quality is constant and humidity
below 20 %, only heavy tars are present in the gas, and can be removed
with the ESP.
After the ESP, we recover the latent heat of the gas with a water
scrubber. Some water soluble condensibles are also separated.
The system is very fuel sensitive, as medium weight tars produced in
process fluctuations will pass the ESP and condense on the high voltage
insulator, resulting in extra cleaning.
The residue from the ESP is a thick but liquid-like slurry, consisting
mostly of fine char and soot, but also a minute fraction of heavy tars.
The water content of the surry is around 70%, and can be disposed of
with a separate combustor, or by returning into the process by mixing
the residue with woodchips before the woodchip dryer.
The condensate after the ESP is clear but has a pungent odor, I'd say
from phenols and acids. The scrubber operates in closed mode, and there
is no liquid discharge from the scrubber if woodchip humidity is below
15%, and the product gas is fed to the engine with a temperature of 53
C and a dew-point of 48 C.
We use our own automation system and a PC-based SCADA-software for
remote operation. It has built in safety features for the ESP and
Gasifier that protect the system and users from damage. It is self
diagnostive, and can report system failures and low fuel levels to the
user. See the screen-shot.
We have one Gasifier with gas cleanup installed at a sawmill in