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.
Beside the biomass CHP Güssing, there is actually the second biomass CHP based on FICFB gasification in Oberwart in operation. Another CHP in Villach is currently in the optimisation phase and one in Ulm, Germany is under construction.
|
Location |
Gas utilisation / Product |
Fuel / Product MW, MW |
Start up |
Status |
Supplier |
|
Güssing, AT |
Gas engine |
8.0fuel / 2.0el |
2002 |
Operational |
AE&E /
Repotec |
|
Oberwart, AT |
Gas engine / ORC |
8.5fuel / 2.8el |
2008 |
Operational |
Ortner Anlagenbau |
|
Villach, AT |
Gas engine |
15fuel / 3.7el |
2010 |
commissioning |
Ortner Anlagenbau |
|
Klagenfurt, AT |
Gas engine |
25fuel / 5.5el |
2011 |
planing |
Ortner Anlagenbau |
|
Ulm, DE |
Gas engine / ORC |
14fuel / 5el |
2011 |
Under construction |
Repotec |
|
Göteborg, Sweden |
BioSNG |
32fuel/20 BioSNG |
2012 |
planing |
Repotec |
|
Vienna, OMV |
Hydrogen |
50fuel/30
hydrogen
|
2015 |
planing |
Repotec |
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: