Black Water Valves for Gasification Units

Abstract: After nearly 20 years of development, the coal chemical industry has gradually advanced and improved, achieving major breakthroughs in key equipment and technology. This paper focuses on the use of valves in black water systems of gasification units in the coal chemical industry under harsh working conditions. It analyzes the valve performance issues during unit operation, considering the process flow and actual usage, and proposes corrective measures.
 

The coal chemical industry in China has successfully established several modern demonstration projects for industrial upgrading, forming a significant scale in coal indirect liquefaction, coal-to-olefins, coal-to-ethylene glycol, and coal-to-natural gas. The coal chemical industry uses coal as a raw material to produce chemical products, leading to various solid-liquid, gas-liquid, and even three-phase flow media conditions, along with flammable, explosive, toxic, and harmful media. Additionally, the coal chemical process often involves corrosive conditions with complex components and unknown corrosion mechanisms. Therefore, the factors to be considered in the design and selection of pipeline valves in coal chemical units are diverse and complex. Given these complex working conditions, commonly used black water valves include ball valves, globe valves, butterfly valves, and gate valves. Each valve is designed for specific working conditions involving corrosion, pressure, temperature, or multiphase flow media, addressing challenges such as high temperature and pressure, hydrogen corrosion, flammable and explosive materials, toxic substances, and multiphase flow blockage and erosion. This paper analyzes issues in valve selection under harsh working conditions, including black water valves (such as ball valves, globe valves, and black water angle valves. It examines process conditions, identifies existing problems, and proposes methods for valve improvement to ensure the stable and long-term operation of process units.
 

A water-coal slurry gasification process uses water-coal slurry raw materials and high-pressure oxygen from air separation units to burn and react at a temperature of 1400°C and a pressure of 6.5MPa in the gasifier. The reaction products include raw coal gas, liquid slag, and fine solid particles. The high-pressure black water from the gasifier quenching chamber and the bottom of the scrubber is sent to a high-pressure flash tank after decompression. The black water medium has a high solid content, and the opening and closing of the valve can easily cause blockages due to solid materials. To prevent blockages, commonly used black water valves include black water angle valves, ball valves, and butterfly valves. These valves have self-cleaning functions, with the black water angle valve being particularly suitable for multiphase flow treatment due to its side-in, bottom-out structure. Harsh working conditions, such as high temperatures, high pressure differentials, corrosion, and multiphase flow, make the black water angle valve a critical piece of equipment in the gasification unit. Additionally, ball valves and butterfly valves play indispensable roles in controlling large flow volumes and regulating flow. The process flow of the black water angle valve is shown in Figure 1, and the operating parameters of the gasifier black water angle valve are shown in Table 1.
 
 
Figure 1 Black water angle valve process flow chart
 
Table 1 Operating parameters of black water angle valve of gasifier

 

Working conditions for black water systems are extremely harsh. The black water medium contains not only corrosive substances such as hydrogen sulfide and chloride ions, but also a large number of solid particles. The black water valve must not only withstand corrosion from these substances, but also endure severe abrasion caused by the erosion of solid particles at high flow rates. The pressure differential across the black water angle valve is significant, leading to severe flashing. Cavitation caused by flashing also results in significant damage to the valve body. In addition to the primary failure of the valve core and valve seat due to erosion, black water angle valve failures also include valve stem erosion, downstream flange damage, valve stem jamming, and other issues.
 

The black water angle valve is primarily composed of a valve body, a valve bonnet, a valve stem, a valve core, and a valve seat. Black water enters the valve from the side and exits from the bottom. The relative positions of the valve core and valve seat are altered by moving the valve stem up and down, thereby changing the flow area and the medium's flow rate.
Given the corrosive conditions of black water valves, the industry is currently conducting research to improve in the following areas:
(1) Selection of Wear-Resistant and Corrosion-Resistant Materials
Duplex stainless steel possesses good pitting resistance. Corrosive substances such as hydrogen sulfide and chloride ions (Cl⁻) are present in black water. Duplex stainless steel is generally used as the valve body material for black water angle valves. Due to the high flow rate and severe flushing conditions of black water multiphase flow, the valve body flow channel is hardened through chrome plating, and the valve internals are typically treated with solid hard materials or a substrate with a hardened layer. Currently, surfacing or spray welding with Stellite alloy, diamond, and Ni-WC can effectively enhance the surface strength and wear resistance of the valve core and valve seat. The selection of hard solid internal materials primarily focuses on solid sintered tungsten carbide, which has a high density and hardness of no less than HRC 6S, and can resist the erosion caused by ash particles.
(2) Improvement of Valve Flow Channel Structure
Valve manufacturers both domestically and internationally primarily improve the valve flow channel structure by adopting designs with no dead angles and large radius arcs. No dead angles can effectively prevent solid media from accumulating in the valve, reducing the number of valve blockages and valve stem jams. Large radius arcs increase the flow stroke of the medium, effectively reducing the flow speed within the valve, greatly decreasing the erosion of the valve body by high-speed solid particles in the fluid, and extending the service life of the valve.
(3) Improvement of Valve Stem Stability
High-speed black water enters the valve from the side, directly impacting the valve stem, causing it to vibrate, and allowing solid particles to enter the gap between the valve stem and the packing. This can lead to valve stem jamming, material accumulation, and scaling, resulting in valve jams. By improving the connection structure between the valve stem and the valve core and redesigning the valve stem, valve stem stability is enhanced, valve stem vibrations are reduced, and the thrust requirements of the corresponding larger actuators are met.