Several pilot plants were used for research at the Institute of Chemical Engineering, Vienna, University of Technology. After intensive work on different cold flow models the first 10kW pilot plant was build in 1993. As the first results were very positive, it was decided to build together with industry the first 100kW pilot plant. This plant was startet in 1995 and was in operation till 1999. In 1999 it was decided to install a new 100kW pilot plant with the same prinzilpe of gasification, but an improved operating performance.
|10kW pilot plant||1st 100kW pilot plant||2nd 100kW pilot plant||3rd 100kW pilot plant|
Here only the results of the new improved FICFB-gasifier are described. Information about the results of the previous pilot plant can be found in the publications: "The FICFB gasification process" and "Two Years Experience with the FICFB-Gasification Process"
Gasification tests were carried out using wood pellets as fuel.These have the advantage, that the water contents is very low (<10%) and it is a standardised fuel with nearly the same composition over the whole year. The following table contains the analysis of the wood pellets.
Table 1. Fuel analysis (Analysis after DIN standards, dm ... dry matter)
|S||w.-%, dm||< 0.05|
In table 2 the operation parameters are described.
Table 2. operation parameter
|type of fuel||-||wood pellets|
|circulation rate||kg/kg fuel||40-50|
In table 3 a typical gas composition is shown.
Table 3. Example of detailed analysis of the product gas (dry) using gas chromatography
|calorific value [kJ/Nm³]||12614|
In the following diagramm the temperatures of a typical experiment are shown.
Table 4. description of the temperatures
|inlet of scrubber||SC1|
|outlet of scrubber||SC2|
The aim of this work was to find a catalytic bed material to reduce the tar contant of the product gas direct in the gasifier. In the 1. pilot plant the following bed materials (except medium 7) were tested:
It was shown in the experiments, that medium 2 and medium 6 have a similar catalytic activity, but only medium 6 was suitable for a fluidised bed. Medium 2 and medium 5 could be a alternative if they are fed continously into the gasifier. Medium 3 and 4 had no catalytic activity or they were not suitable for the use in a fluidised bed. Medium 7 was tested in the 2. pilot plant and showed a very good catalytic activity. The tar content was reduced by about 80% compared to medium 1. Also the content of hydrogen increased by using medium 7 as bed material. After tests of about 50 hours no deactivation of medium 7 could be measured.
As result from these tests it was decided to use only medium 6 as bed material for the 2. pilot plant.
The aim of this work was to evaluate which fuels are suitable for the FICFB-gasifier. To estimate the usability of the fuels on the usage in the gasifier the gas composition, the heating value of the gas and the tar content was measured. Before the experiments the water content, the ash content, the heating value and the ash melting behaviour of the fuels was measured. The following fuels were tested:
The heating values of the dry fuels are all in the area of wood chips (19100 kJ/kg), except brown coal (23150 kJ/kg)and sewage sludge (10600 kJ/kg). The high ash content (ca. 40%) and the low ash melting point of clover pellets can be pointed out. Also has brown coal a higher carbon content than wood chips.
All fuels except clover pellets could be gasified almost without problems. (wet sawdust and brown coal caused problems in the fuel feeding system). At the experiments with clover pellets there were the same problems with ash melting behaviour as known from fluidised bed combustion. After a short time there were deposits at the U-beams which reduced the circulation rate. Because of these deposits the experiments has to be stopped and the U-beams and the syphon has to be cleaned. It was not possible to gasify clover pellets in the existing FICFB-gasifier.
All other fuels could be gasified without problems in the gasifier. Also the high ash content of sewage sludge caused no problems, because the ash was separated at the U-beams from the bed material and left the gasifier together with the flue gas.
In the following diagrams only the results of the new improved gasifier are presented. Wood pellets were used as fuel and medium 6 was used for all experiments as bed material. Wood pellets have the advantage, that the water contents is very low (<10%) and it is a standardised fuel with nearly the same composition over the whole year. The steam-fuel ratio was varied from 0.18-0.8 kg steam per kg dry fuel. The steam-fuel ratio is calculated in the following way:
Sum water input is the sum of steam, which is used in the gasifier (for fluidisation of the gasification zone and siphon) and the water content of the fuel. The gasification temperature during the experiments was varied from 750 to 900°C.
As known from literature, the gas composition depends mainly on the used fuel, on the temperature and on the steam-fuel ratio. The gas composition depends also on the residence time, but in all experiments the residence time was kept as constant as possible. Therefore in the following diagrams the dependency of the dry product gas composition to these parameters is shown. The nitrogen content for all experiments was below 5 vol% and is not shown in the diagrams. The rest to 100 % is nitrogen and higher hydrocarbons. From the gaschromatographic analysis it can be seen, that the main component of the these hydrocarbons is ethene.
From the diagram "product gas composition dependency of temperature" it can be seen, that with increasing temperature the hydrogen and carbon monoxide concentrations are increasing and the carbon dioxide and methane concentrations are decreasing with increasing temperature. The reasons for these dependencies are, that the reactions at higher temperatures are faster and the gas composition is nearer to equilibrium.
In the next diagram the gas composition in dependency on the steam-fuel ratio is shown. In the temperature area between 800 and 850°C the hydrogen and carbon dioxide contents are increasing with a higher steam-fuel ratio. Carbon monoxide and methane are decreasing.
With the results of these measurements the gas composition of the product gas can be calculated for different steam-fuel ratios and temperatures. The next step will be to improve the model of the gasifier on basis of this measurements.
As known from previous experiments (with the 1. pilot plant) the tar content depends strongly on the gasification temperature. Here the dependency of the tar content to the steam-fuel ratio was studied. In the next diagram the tar dependency on the temperature is shown at different steam-fuel ratios. For this diagram two steam-fuel ratios were used to show the dependency of the tar content on the temperature.
It can be seen from the diagram, that the dependency on the temperature is higher at a low steam-fuel ratio. It is also shown in this diagram, that a higher steam-fuel ratio causes a lower tar content in the product gas. From these results it was realised, that the optimal steam-fuel ratio for a low tar content is higher than 0.5 kg steam per kg dry fuel. The optimal temperature for a low tar content is higher than 850°C.
In the next diagram the water conversion is shown in dependency on the temperature at two different steam-fuel ratios. The water conversion is a important figure to estimate the efficiency of the steam gasification system. The water conversion was calculated in the following way:
It can be seen, that the water conversion increases with increasing temperature. From these results it can be estimated, that the optimal steam-fuel ratio is below 0.5 kg steam per kg dry fuel. Above this ratio the water conversion is almost constant and more steam causes only more steam content in the product gas, without shifting the reactions to the desired side.
The cold gas efficiency is the ratio between the chemical energy content in the product gas compared to the chemical energy in the fuel. (based on the lower heating value) The cold gas efficiency was calculated in the following way:
In the following diagram the dependency of the cold gas efficiency on the steam-fuel ratio at different temperatures is shown.
It can be seen in this diagram, that the steam-fuel ratio has a high influence on the cold gas efficiency. Generally it has to be taken into account, that these are measured results from the 100kW pilot plant. This pilot plant has a heat loss of about 15%. In larger scale the cold gas efficiency will be higher. This is also shown in the simulation.
It was shown in this work, that the product gas composition, the tar content, the water conversion and the cold gas efficiency depends strongly on the temperature and the steam-fuel ratio.
The dependency of the product gas composition on the temperature and on the steam-fuel ratio was investigated. With this results the model of the gasifier will be improved and a basis for scaling up was created. Also the dependency of the tar content on the temperature and the steam-fuel ratio was investigated. By using a low steam-fuel ratio a high tar content was produced. Increasing the steam-fuel ratio the tar content could be reduced essentially. As known from previous experiments a high gasification temperature causes a low tar content in the product gas. The influence of the steam-fuel ratio and temperature on the water conversion were also investigated in the experiments. It was shown that a steam-fuel ratio above 0.5 kg/kg has only minor influence on the water conversion. Above this steam-fuel ratio only the steam content in the product gas increases, without a major influence on the equilibrium on the gasification reactons. It was also shown in the experiments, that hte steam-fuel ratio has a important influence on the cold gas efficiency.
In the 100kWth pilot plant two steam-fuel ratio for all further experiments could be defined. One is 0.25 kg/kg, which causes a high tar content, but also a low steam content in the product gas, the second is 0.5 kg/kg, which causes a lower tar content, but also a higher steam content in the product gas than the first one. With this two steam-fuel ratios all further experiments will be done and the separation efficiencies of the gas treatment system will be investigated. In this work all necessary investigations referring the temperature and steam-fuel ratio were done to improve the model of the FICFB-gasifier and to have a basis for the scaling up the pilot plant to demonstration plant.