A novel fluidized bed gasification reactor has been developed to get a
product gas with a high calorific value (up to 15 MJ/Nm³) and nearly
free of nitrogen. The gasification process is based on an internally
circulating fluidized system and consists of a gasification zone fluidized
with steam and a combustion zone fluidized with air. The circulating bed
material acts as heat carrier from the combustion to the gasification
zone. Gas mixing between these two zones is avoided by construction
measures. Furtheron, the apparatus is characterized by a very compact
design. This process was developed by the Institute of Chemical
Engineering, Fuel and Environmental Technology in cooperation with AE
Energietechnik (A patent for this system with the Number 405937 was
granted).
The development of the gasification reactor has been carried
out step by step. First, a cold flow model was operated to study the fluid
mechanics of the fluidization system. The second step was a laboratory
scale test rig to study the main features of the reactor by varying
different operating and geometrical parameters. After this step a pilot
plant was constructed and has been successfully operated. The results
attained came fully up to the expectations.
Keywords: Biomass, steam
gasification, fluidized bed, compact reactor design, medium energy gas,
low nitrogen content.
In Europe, Austria is one of the leading countries in using bioenergy.
The most common utilization of biomass for energy is the combustion for
heating applications. Gasification could become a second important route
especially for power production.
Usually, biomass gasification is
carried out using fixed or fluidized beds. As the overall gasification
reactions are endothermic, the gasification process must be supplied with
heat. The easiest way is to use air as gasification agent and to burn the
biomass partially within the gasification reactor. In this case the
product gas has a low calorific value (around 4-6 MJ/Nm³) and a high
nitrogen content of 45-55 %.
A gas with a low nitrogen content and a
higher calorific value (about 12 MJ/Nm³) can be produced with pure
oxygen as gasification agent but the costs for the oxygen production are
high. Another possibility is to supply heat with heat exchangers but here
material problems due to the high temperature level will arise. The
dilution of the product gas by nitrogen can also be avoided by using a
dual fluidized bed system. In this case no oxygen generator is necessary
and also no serious material problems due to high temperatures will
appear..
The gasification process has been developed step by step. Five steps are carried out already and a demonstration step is in operation at the Biomass-CHP in Guessing:
At the beginning of the development a cold flow model was built to study and optimize the fluid mechanics of an internally circulating fluidized bed with a draught tube and a surrounding annular bed. Circulation rates of the bed material and gas leackage between the two zones were measured. The circulation rates are important for the heat transport. Furtheron the gas leackage must be minimized. The results were very promising therefore a laboratory scale test rig for gasification tests was constructed.
The laboratory test rig was designed for a thermal output of 10 kW. Fuel is fed into the annular bubbling bed by a double screw feeding system. The annular bed is fluidized with steam which also acts as gasification agent. The bed material together with the charcoal moves down towards a riser which is situated in the centre of the reactor. The particles are transported up by air through the riser where charcoal is partly burned. At the top of the riser the particles are separated from the gas. The particles fall down and come back to the gasification zone via an annular gap. This gap and the location of the steam and air inlets ensure, that the gas leackage between the two zones is lower than 5 % of the total gas input. Both zones have separated gas exits.
The next step was the construction of a pilot plant with a thermal
output of about 100 kW based on the experience of the laboratory test rig.
The circular-symmetric geometry was changed to a rectangular cross-section
because of scale up consideration. The riser and the bubbling gasification
bed were arranged side by side. To avoid large amounts of gas mixing a
siphon was introduced in the line from the combustion zone to the
gasification zone. The bed particles were splitted from the riser gas
stream using a U-beam separator. The fuel feeding system consisted of a
hopper and a multi-screw-conveyor. Air was supplied by blowers into the
riser and during the start up period also into the gasification zone.
Steam was produced by an electrical steam generator and overheated by an
electrical heater. The product gas, which is cleaned by a cyclone and
afterwards cooled from 700°C to 200°C by an heat exchanger, and
the flue gas had separated exits from the reactor. They were mixed
together after taking gas samples for analyzing. A gas burner ensured that
the product gas was completely combusted before entering a cyclone and a
flue gas cooler. The flyash could be returned continuously into the
gasifier with the aid of a pneumatic flyash recycle system.
This
pilot plant was used for experiments from May 1995 till August 1999.
From the results of the 100kW pilot plant a second pilot plant with an
improved operational performance and a thermal input of also 100kW was
designed and constructed. This pilot plant was put into operation in
November 1999.
This pilot plant was manily used to investigate different gas cleaning systems. With the results of the second pilot plant the design of the 8MW fuel input demonstration plant was done.
The 3rd pilot plant is very similar to the second one. The main difference is that the separation at the bottom between gasification and combustion zone now is done by a siphon.
A novel gasifier was presented which has the following main advantages compared with an air blown gasifier:
The product gas, which is produced, by this novel gasification system can be used in the following applications: