The main operation results of the DeNOx SCR system, including information on the deactivation behaviour of the catalyst, are reported in the following.
4.1 Operation results of the SCR system
The selective catalytic reduction (SCR) system is mainly based on the following reaction:
2NO + 2NH3 + 1/2O2 = 2N2 + 3H2O (1)
The catalysts normally used are titanium-based catalysts such as Ti-V, Ti-W, Ti-Mo, and Ti-V-W even with the presence of SOx. In case of the Chiba GIDE, a Ti-V-W catalyst has been adopted.
One of the main features of this SCR system is that it is pressurised, being installed before the turbo chargers as shown in Fig.2. This is rather suitable for the reaction efficiency. The target value of NOx (29 ppm) is much lower than the regulation value (100 ppm at 13% O2) in order to minimise total NOx emission from the Chiba works.
The reaction temperature and the space velocity, which means the reciprocal of the volume of catalyst, were designed to establish a higher DeNOx rate more than 98.5% at least for the first two years. A duct burner installed before the SCR controls the temperature during low load of the engine. Typical results of SCR operation in the DSS-mode (Daily Start and Stop) are shown in Fig.8. As can be seen, the NOx concentration in the stack is always kept below 5 ppm during the DSS period. Fig.9 shows the change of performance of the SCR catalytic reactor over the 6 years operating period. It is worthwhile mentioning that the high DeNOx rate(over 98.5%) is still maintained now more than six years after start of operation, which is much longer than the expected catalyst life. In addition, the NH3 slip has been kept below 5 ppm. The DeNOx rate was initially increased during the first two years after which it has decreased slightly but steadily, with an approximately linear relationship with time as shown in Fig.9. The exact deactivation behaviour of the catalyst has been investigated by additional examination and analysis of catalyst samples pulled out from the SCR tower (as discussed below). The pressure drop across the catalyst, which is an important factor for the catalyst life, is shown in Fig.9. Though some increase is observed, the pressure drop is still so low that it does not influence the power and efficiency of the engine. The regular inspections have shown that accumulation of dust in the catalyst has been very small.
In conclusion, it has been confirmed that there are no important problems in the SCR system of this GIDE power plant considering all of the above items. Also other items such as corrosion of casing has given no problems. The performance of the SCR system remains on a high level after six years of operation and is expected to remain so in a few years further.
4.2 Deactivation behaviour of SCR catalyst
In order to investigate the DeNOx activity and the deactivation behavior of the SCR tower, micro reactor tests and chemical analyses were done on some test pieces, sampled from part of 1st layer (closer to SCR inlet) catalyst and 4th layer (located at the middle of SCR) catalyst of total 6 layers. In the micro reactor the test conditions were as shown in Fig.10. The load volume of honeycomb catalyst was 11 cm3.
The following chemical analysis was done on the used catalyst: surface area with N2-BET, pore size distribution with Hg porosimeter. Quantitative analysis of Ca, S and Na with Inductively Coupled Plasma and Atomic Absorption Spectroscopy was done to identify the reason of deactivation: the deposit of Ca-components derived from the lubricating oil, or the deposit of S-components derived from fuel oil, or the poison material Na. Fig.10 shows the DeNOx kinetic constant rate (K/K0) of 1st layer catalyst and 4th layer catalyst with time elapsed in the SCR tower.
