Gas is the core technology for clean and efficient coal conversion, and its core component is the gasifier. Therefore, the pressure of the gasifier is a crucial monitoring parameter in the coal gasification process. Still, its working conditions are high temperature and high pressure, which will lead to the hydrogen embrittlement phenomenon of the measuring diaphragm of the conventional pressure transmitter. Therefore, we use gold-plated diaphragm-type pressure transmitters, which can prevent the penetration of hydrogen into the diaphragm under high temperature and high-pressure conditions, effectively solving the problem of gasifier pressure measurement.
Coal gasification is a process whereby a solid fuel such as coal or coke is reacted with a gasification agent under high temperature and pressure conditions. It is then converted into synthesis gas (mainly hydrogen and carbon monoxide) and a slight residue. The synthesis gas is purified from hydrogen by variable pressure adsorption, so coal gasification is the core technology for clean and efficient conversion. However, at present, the working conditions of the gasifier are high temperature and high pressure, and the hydrogen content in the synthesis gas is high; therefore, the gas-making conditions can cause hydrogen embrittlement in the measuring diaphragm of the pressure transmitter, which will lead to measurement errors and even damage the instrument. For this reason, we use a new gold-plated diaphragm-type pressure transmitter, which can prevent the influence of hydrogen permeation and effectively solve the above problems.
1. Introduction to the phenomenon of hydrogen embrittlement
Hydrogen is a tiny atom in nature, and it is not inherently corrosive; however, it is highly porous. Since the size of a hydrogen atom is much smaller than that of a metal atom, it can dissociate into hydrogen atoms and penetrate the interstitial positions of the lattice dot matrix of metallic materials under high temperature and pressure. This penetration process takes place in the following steps.
1.1 Hydrogen (H2) collides with the surface of the metallic material (M). At this point, the surface of the metallic material (M) physically adsorbs traces of hydrogen (H2) to form a mixture (H2M), i.e., H2 + MH2M.
1.2 As the mixture (H2M) continues to react with the outer surface of the metallic material (M), they form hydrogen atoms (HadM) adsorbed on the outer surface of the metal, a process known as chemisorption, and which is facilitated by high temperature and pressure conditions, i.e., H2M + M2HadM.
1.3 When the metal material (M) is saturated with adsorbed hydrogen atoms (HadM) on the outer surface, it will gradually dissolve and diffuse, it will form atomic hydrogen (MHad) that permeates the inner part of the material.
2. The effect of hydrogen embrittlement on pressure transmitters
The pressure transmitter is a device that converts the pressure signal into an electric signal for control and remote transmission, the core component of which is a single crystal silicon resonant sensor and a measuring diaphragm. When the pressure transmitter is in operation, the measuring diaphragm of the measuring diaphragm box comes into contact with the measuring medium. We transmit the measuring pressure to the elastic element (resonant beam) of the small vacuum chamber through the silicon oil conductive fluid sealed and filled on the inside of the measuring diaphragm, resulting in a slight deformation displacement of the elastic element, the degree of which is proportional to the pressure.
The pressure transmitter is through the single crystal silicon resonant sensor. The microprocessor will be transformed into the degree of deformation displacement 4 ~ 20mA remote electrical signal, which can measure the medium pressure. We should prevent being corrupted by the measurement medium to reduce the stress in the transmission process. We will use thin-walled metal materials with certain elasticity and anti-corrosion properties (thickness between 40 to 80μm, slightly different for each equipment supplier) to make into a measurement diaphragm, common measurement diaphragm materials are 316L stainless steel, Hastelloy, tantalum, titanium, and many other types.
3. Selection of pressure transmitter measuring diaphragm for coal gasification plant working conditions.
3.1 Process Status Profile
Industrial coal gasification technology is to ensures continuous, uninterrupted production and increases the scale of syngas production. It is usually in the gasifier with water as the gasification agent, at high temperature and high-pressure conditions, the coal and water vapor reaction to produce syngas (the main components are hydrogen and carbon monoxide, in addition to a small amount of carbon dioxide and other trace impurities). As technology continues to improve, there are now three main technical solutions for large-scale coal gasification, one is gasification in a fluidized bed, the other is gasification in a fluidized bed, and the other is gasification in a fixed bed, the core component of which is the gasifier.
The gas bed gasifier is a reliable and mature technology with many advantages such as high conversion efficiency, good continuity of automatic production, stable output, its wide range of coal type adaptation. At present, it is the mainstream trend in the development and application of coal gasification technology. We have many instrument measuring points (e.g. temperature, pressure, flow rate, etc.) on the gasifier according to the automatic control scheme of the relevant enterprises, which need to collect the pressure parameters and transmit them to the automatic control system of the gas production unit.
3.2 Selection of pressure transmitter measuring diaphragms
The gasifier pressure parameter is directly related to the gas production section’s stable operation and safe production. It is a crucial process indicator in the automatic control of the operational process. The gasifier pressure parameter is directly related to the gas production section’s stable operation and safe production. It is a crucial process indicator in the automatic control of the operational process. Furthermore, it is directly involved in the gasifier safety interlock stopping action, so the pressure instrument selection needs to be taken seriously. Choose our longer-life instrument to ensure the collection of practical and accurate process parameters.
According to the relevant information, at present, our common gas making furnace measuring point at the working temperature parameter is 250 ℃, the working pressure parameter is 3.75 MPa, it is with the synthesis gas in the hydrogen content of not less than 30% (molar fraction), the synthesis gas in the hydrogen partial pressure is not less than 1.125 MPa (3.75 MPa 30% = 1.125 MPa). When we are in the presence of high-temperature hydrogen, metal equipment is very susceptible to hydrogen embrittlement. As the ambient temperature, pressure, and hydrogen concentration increase, the hydrogen embrittlement phenomenon will become more significant (the ambient temperature is a critical influence). Therefore, the measuring diaphragm of a gas furnace pressure transmitter needs to be protected against hydrogen penetration to reduce the effects of hydrogen embrittlement.
4. Conclusion
We should choose the pressure instrument reasonably, and it is an essential means to ensure the safety of production. In this paper, we mention gold-plated diaphragm-type pressure transmitters suitable for high-temperature pro-hydrogen applications in the petrochemical industry (e.g., hydrogenation and hydrogen production units), providing some reference and learning for the selection and design of pressure instruments for similar projects.
