A common restriction in the energy field is the availability of proper materials that can sustain and handle the extreme or aggressive applications they are put in, e.g., exposure to dirty fuels, high temperatures, high pressures, or aggressive environments. Poor material selection can cause a range of failures spanning from problems such as leakage, fouling, cracks, breakage, to full ruptures, fatigue failure, and material degradation. Altogether can create the need for risk management, cost in time and money due to downtime to repair or replace parts, or can cause safety concerns and unsafe work environments. In view of this, the studies conducted in the Corrosion Laboratory at KAUST aim at developing a fundamental understanding of the materials behavior when exposed to aggressive and corrosive environments. Such a knowledge is expected to aid in decision making for different energy related applications.
Fig. 1 shows the hot corrosion rig currently utilized for corrosion studies at the Clean Combustion Research Center. This rig was designed/constructed to study and understand in detail the corrosion process of gas turbine hot gas path (HGP) materials and coatings at extreme conditions, i.e., high temperatures (900-1000 oC) and pressures (10-15 bar) for prolonged hours (e.g., 1000 hours). The dimensional characteristics of the rig isolate the effect of corrosion from erosion by keeping overall low flow velocities (<1m/s).
Fig.1. KAUST hot corrosion rig
The experimental facility consists of five main parts: (i) burst disc (rupture disc) section, (ii) preheat section, (iii) burner, (iv) sample section, and (v) exhaust, as shown in Fig. 2. The Burst Disc (1) or rupture disc is a safety element that protects the rig from over pressurization or potentially damaging conditions. It is held on a vertical cylinder upstream of the preheat section. The Preheat Section (2) is a temperature-controlled zone in which the entering flows are preheated at temperatures ranging from 650-800 oC.
Fig. 2. Schematic of the hot corrosion rig
The preheat section is made of a 12" SCH80 316L stainless steel pipe with an inside electric heater (30 kW) embedded in an alumina-silica insulation matrix. The preheat temperature is an essential parameter during operation since it aids in reaching a homogeneous temperature profile in the sample section. The Burner Section (3) consists of a fuel and secondary air supply lines. A swirl is placed at the entry of the burner section to give the primary air flow an angular momentum and to aid in flame stabilization, and a siphon air-assisted atomizer is capable of delivering low fuel-flow rates, making it suitable for continuous operation, all shown in Fig. 3.
Fig. 3. Burner Section components from left to right: the swirler, the modified siphon-nozzle providing atomized fuel delivery, and the placement in the burner section.
The Sample Section (4) is similarly made of a 12" SCH80 316L stainless steel pipe holding an internal alumina tube inside an insulation-embedded three-zone electric heater. Inside the sample section, a sample tray made of Inconel 625, capable of holding 64 - 5mm diameter and 12 - 10mm diameter cylindrical coupons, is placed (Fig. 4). The coupons are arranged with their lengths perpendicular to the flow.
Fig. 4. Sample section
During operation, the sample section is continuously provided with gases held at temperatures between 700 and 1000 oC and elevated pressures (5 – 15 bar) depending on the experimental parameters of interest. The three-zone heater is used to ensure a uniform temperature along the length of the sample tray. The residence time of the entering gases in the sample section is approximately 1 second. After the gases pass through the sample section, they move towards the Exhaust (5), which directs the gases towards the venting system. Two tested protocols have been used in this rig, one for 2000 hours in 500-hour increments and another for 300 hours in 100-hour increments. At each increment, predetermined samples are taken out of the sample tray and the test continued. The samples taken out from the rig are subjected to different laboratory analysis such as SEM, XRD, EDX, weight loss/gain measurements for corrosion rates, tensile testing, etc.
In line with the current decarbonization initiative of the energy sector, ammonia is proposed as a promising energy carrier and is expected to play a resilient and sustainable role in future energy scenarios worldwide. For instance, NH3 has been perceived as an excellent candidate for hydrogen storage and transport since it enables liquid-phase H2 storage under mild conditions. Nonetheless, all these promising potentials get slightly blurred as ammonia is incompatible with various industrial materials. For instance, nitridation is one of the most challenging factors faced in the design and operation of ammonia plants. Sometimes material nitridation is desirable and is introduced during certain commercial applications to increase hardness and resistance. However, in many other industrial processes, the increased brittleness would negatively impact the longevity and reliability of the equipment. Thus, aside from hot corrosion studies, the current investigations conducted at CCRC aim to provide important insights into the nitridation process of different metals when exposed to ammonia at high temperatures and pressures during prolonged periods.
Fig. 5 shows the experimental facility currently utilized for materials nitridation studies.
Fig.5. Nitridation studies rig
The experimental facility consists of five main parts: (i) liquid ammonia feeding system, (ii) burst disc (rupture disc) section, (iii) sample section, (iv) gas samples characterization section, and (v) abatement system. The Liquid Ammonia Feeding System (1) consists of an ammonia condenser, an HPLC pump, and a vaporizer (Fig. 6). This system can supply ammonia in liquid phase at flow rates in the range of 0 - 50 ml/min and pressure up to 70 bar. Liquid ammonia is vaporized in the heater before entering the samples section.
Fig. 6. Liquid ammonia feeding system
Similar to the hot corrosion rig, the Burst Disc (2), or rupture disc, is a safety element that protects the rig from over pressurization or potentially damaging conditions. The Sample Section (3) is a temperature-controlled zone in which the samples are placed. It is made of a 12" SCH80 316L stainless steel pipe with an inside electric heater (30 kW) embedded in an alumina-silica insulation matrix. The sample section holds an internal alumina tube inside, which contains the sample tray (Fig. 7).
Fig.7. Sample holder for nitridation tests
The Gas Samples Characterization Section (4) consists of a micro-GC capable of detecting the reaction products: NH3, H2, and N2. The micro-GC is connected on line directly downstream the sample section (Fig. 8); thus, it is possible to determine in real time the composition of the gaseous products leaving the reaction zone.
Fig. 8. Online measurement of gas products
Finally, the Abatement System (5) is utilized to burn the reaction products before they are released through the exhaust (Fig. 9). A propane pilot flame is used to initiate the ammonia flame.
Fig. 9. Abatement system