Introduction

In the year 2002 an 8 MW CHP plant based on a circulating fluidized bed steam blown gasifier producing heat and power (4.5 MWth, 2 MWel) with a gas engine went into operation in Guessing, Austria. At the middle of 2002 the gasifier and the gas cleaning system was coupled with the gas engine. Renet-Austria, a competence network on energy from biomass, consisting of experts from universities and industry started to develop this process further to a commercial stage. During the last years a lot of improvements could be reached. These improvements were connected on the one hand with changes in construction (e.g. feeding system, online particle separation) and on the other hand with advances in the operation performance. The latter will be the main content of this paper.
Due to the excellent performance that was reached during the last years, several additional research projects could be started in Güssing. The producer gas from the circulating allothermal fluidized bed gasifier is nearly free of nitrogen and has an high hydrogen content. For this reason it is well suited for fuel cells as well as several synthesis products. Therefore, projects aiming at the development of processes for the production of synthetic natural gas and Fischer Tropsch liquids are currently carried out. First results and further planning are shown in the second part of this paper.

Description of the Biomass CHP Güssing

In Guessing an innovative process for combined heat and power production based on steam gasification has been successfully demonstrated. Biomass is gasified in a circulating dual fluidised bed reactor. The producer gas is cooled, cleaned (2 stages) and used in a gas engine. Wood chips from forestry are the main fuel for the demonstration plant. The wood trunks are dried naturally by storage of about 1-2 years in the forest. Then they are delivered to the CHP-plant and chipped there. When the biomass is used, it has a water content of about 25-40%.
The heat produced in the process is partly used internal, e.g. for air preheating, steam production, etc., and the rest is delivered to an existing district heating system. The net electricity produced is delivered to the electrical grid. The feed in rate in Austria is regulated by law and depends on the type of biomass used and on the size of the plant (13-16 Cents/kWh).

Biomass chips are transported from a daily hopper to a metering bin and fed into the fluidised bed reactor via screw feeders. The fluidised bed gasifier consists of twozones, a gasification zone and a combustion zone. The gasification zone is fluidised with steam which is generated by waste heat of the process, to produce a nitrogen free producer gas. The combustion zone is fluidised with air and delivers the heat for the gasification process via the circulating bed material.
A water cooled heat exchanger reduces the temperature from 850°C – 900°C to about 150°C – 180°C.The producer gas is cooled and cleaned by a two stage cleaning system. The first stage of the cleaning system is a fabric filter to separate the particles and some of the tar from the producer gas. These particles are recycled to the combustion zone of the gasifier. In a second stage the gas is liberated from tar by a scrubber.
Spent scrubber liquid saturated with tar and condensate is vaporized and fed for thermal disposal into the combustion zone of the gasifier. The scrubber is used to reduce the temperature of the clean producer gas to about 40 °C. The clean gas is finally fed into a gas engine to produce electricity and heat. If the gas engine is not in operation the whole amount of producer gas can be burned in a backup boiler to produce heat.
The flue gas of the gas engine is catalytically oxidised to reduce the CO emissions. The sensible heat of the engine’s flue gas is used to produce district heat. The flue gas from the combustion zone is used for preheating air, superheating steam as well as to deliver heat to the district heating grid. A gas filter separates the particles before the flue gas of the combustion zone is released via a stack to the environment.
The plant fulfils all emission requirements. The operation experience shows that there is only one solid residue which is the fly ash from the flue gas. This fly ash fully burned out, the loss of ignition is lower than 0.5 w- %. The plant produces no condensate which has to be disposed externally.

Operation Performance of the Biomass CHP Güssing

Overall operation performance

The Güssing plant is in operation continuously since the middle of the year 2002. Of course during this time there were several periods of maintenance and also periods for improvement of the construction. Heat is fed into an existing district heating system and electricity into the power grid. Fig. 2 shows the cumulative production of heat and power since January 2002. The production of electricity started in the middle of the year 2002. From the following figure it can be seen clearly that there were only a few periods were the plant was not in operation. Furthermore, also an increase of the heat and power output can be observed.

Reduction of operation means

Since the beginning of operation of the demonstration plant the improvement and optimisation of the operation performance was a permanent task. This optimisation work carried out by Renet-Austria leads to a reduced amount of operation means and also a reduction of waste material which means fly ash from the flue gas line.
The following figure shows the development of the main operation means over the last years.

Nitrogen is used as purge gas at all entrances to or exits from the plant. Furthermore, nitrogen is used also for removing the dust from the fabric filter in the producer gas line. From 2003 to 2005 the nitrogen consumption could be reduced to about 50 % of the original amount.
As gasifier a circulating fluidized bed technology is applied. This means that a bed material is necessary which has a certain attrition rate during operation. This attrition rate and therefore the loss of the bed material depends mainly on the kind of the material, the velocities of the riser, and also the separation efficiency of the cyclone. On the base of the operation experience, some simulation work, and optimisation of these parameters a reduction of the bed material loss by more than 70 % could be obtained.
Another material necessary for the operation is the so-called precoat material. This precoat material is necessary to avoid condensation of tar compounds directly on the filter bag which could lead to plugging or even to damage of the filter cloth. At the beginning a swing operation with two filters (one in operation mode, on-line, another in pre-coating mode, off-line) are applied. Now this operation mode has been change to an online operation which works without any problems. This leads to a dramatic reduction in the need of the precoat material down to about 20 % of the original amount and also to a reduction in the nitrogen consumption (see obove).
The scrubber for the tar separation is operated with biodiesel. The spent biodiesel together with some condensate is fed into the combustion chamber of the gasifier. With this scrubber the overall tar in the producer gas can be removed over 90 %. The operation experience leads to a slight reduction of the biodiesel of about 25 %.
The general aim of all these efforts is the reduction of the costs and therefore the improvement of the economy of the plant.

One further cost effective operation means is the engine oil. The experience of previous plants shows short operation times of about 500 hours which means high costs. With the aid of additives an essential increase of operation times without change of the engine oil could be obtained.
the following figure shows the increase of the total acid number with time for four different engine oils (different additives). If the limit of the total acid number (2.5 mg KOH/g) is exceeded the oit has to be changed. The current status is that operation hours of about 4000 hours are possible with the same engine oil without any serious problems.

Availability of the plant

It is clear that the availability of any demonstration plant can not be as high as for a plant which uses an already mature technology. For such an innovative technology as it is used in the case of the CFB allothermal steam gasification plant and the gas cleaning system several years of operation experience are necessary to remove all the weak points within the plant.
The following figure shows increase of the availability for the gasifier and also for the gas engine for the years from 2002 until 2006. It can be seen that the availability could be increased essentially over time and it reaches now more than 90 % for the gasifier and more than 85 % for the gas engine in the year 2006.

Research Projects

The favourable characteristics of the product gas (low nitrogen content, high hydrogen content, H2:CO ratio of 1.6 – 1.8) allow also other applications of this producer gas. Research projects concerning the production of electricity in a SOFC (solid oxide fuel cell), the synthesis of SNG (synthetic natural gas), and Fischer-Tropsch liquids and have been started.
The following figure gives an overview about possible applications of the producer gas from a steam blown gasifier.

In principal, all products can be obtained from the synthesis gas as this is the case for coal or crude oil. All the necessary chemical pathways are well known since many decades. Therefore, in analogy to coal or oil chemistry one can say now “green chemistry” if the original material is renewable (e.g. biomass). All this advanced applications need an ultra-clean synthesis gas. To cover with these requirements further cleaned up and conditioning steps are necessary. For this purpose a slip stream of the synthesis gas is taken, treated in a suitable way, and fed to the research installations. The following figure shows a principle scheme of this arrangement.