Start up and tests
Plant start-up and tests
The first start-up was done without external combustion using the MGT in simple cycle. During this test the proper flame light on inside the MGT combustion chamber, was verified. Then, in order to ensure a smooth start-up, in particular a smooth increase of the HT-HE inlet temperature, the temperature of flow approaching the HTHE is controlled by dilution of the TEG with ambient air during MGT start-up. Once verified the trend of temperature increase was satisfactory and complied with the simulation, the MGT with external regeneration (by means of the HT-HE) was tested. As expected, the MGT run properly. It was observed that the gradient on the HT-HE tubes side went out of the specifications just for a few minutes in a very low temperature range (practically when the MGT performs a steep rotational speed increase to begin to supply power) so not to big thermal stressed on HT-HE are expected. Out of this narrow range, it was possible to perform very smooth temperature ramps, by adjusting the TOT increase and the Fan1 flow rate by means of a proper butterfly valve closure.
Before the biomass furnace can be started, a stable operation of the MGT and a certain temperature of the oxidising agent at the furnace inlet is required to ensure, that the biomass gets ignited automatically (no separate automatic ignition is installed). As soon as a stable MGT operation is achieved, the fuel feeding system can be started. The biomass ignites automatically and the temperatures both in the primary and the secondary combustion zones start to increase. The increase at the outlet of the biomass furnace (FT1) follows a pre-defined temperature ramp. As already mentioned, the control system for the biomass furnace in this configuration is different compared to conventional biomass furnaces, as the fuel feed has to control the load. It turned out, that the control system designed is able to ensure the load control both during transient operation (start-up and shutdown) as well as during stable operation at a certain load with acceptable temperature fluctuations. The furnace reached already its design outlet temperature of 850°C.
Once the MGTīs stable operation has been checked and the start-up procedure has been tested and tuned, long-term tests have been started in BIO_MGT mode (with running biomass furnace). Maintaining the power output constant (TOT also constant) the natural gas contribution decrease as the furnace exhaust temperature (FT_1) increases and consequently the pre-heated air temperature (CCIT) from the HT-HE to the turbine increases too. During this test the turbine reached an electric power of up to 82 kW.
The HT-HE heating needs a rather long time to reach steady conditions because of its large mass and the imposed heating gradients. The HT-HE starts to transfer energy to the MGT after about 200 min (T03 > T02). The air outlet temperature (T03) follows the flue gas inlet temperature (T13b), at steady state of MGT alone operation (before furnace start). During the first 3.3 h the HT-HE works as a cooler. The plant is designed to operate continuously, so that the start-up phase procedure is not expected to be performed frequently. One more conclusion can be taken from this analysis: if a MGT shut down occurs, a new start-up is allowed if it is performed in a short time (few less than 20 minutes).
Concerning on the Kays effectiveness the measured data confirm the estimation because in the significant range the value is constant around 0.80-0.82. This value is very close to that one used for the calculations - AMOS simulations were carried out with Epsilon=0.799.
Finally, visual inspections were performed after more than 700 working hours. Looking at the biomass furnace refractory coating, no hot spots or damaged surfaces or simply dirty points in both primary and secondary zones have been found. The same analysis was carried out in the turning chamber of the HT-HE. Only small dust depositions were observed.