LEC 7.pdf

NATIONAL INSTITUTE OF TECHNOLOGY, WARANGAL Department of Metallurgical and Materials Engineering MM317: Energy Materials 23/08/2024 Dr. Sanghamitra Moharana Assistant Professor  Steam turbine and components...

NATIONAL INSTITUTE OF TECHNOLOGY, WARANGAL Department of Metallurgical and Materials Engineering MM317: Energy Materials 23/08/2024 Dr. Sanghamitra Moharana Assistant Professor  Steam turbine and components Gas turbine and its components Material design requirement considering the properties needed. Challenges in the current materials for the above components 2  Advanced power generation systems: Advanced ultra-supercritical combustion systems (AUSC) Operates at high temperature and high pressure condition. Where water is a super critical fluid. Integrated Gasification Combined Cycle (IGCC) Coverts coal into mix of hydrogen and carbon monoxide through gasification (product is a syn gas). Steam produced in gasification is utilised. Fuel cells Generates electricity through Carbon capture and storage electrochemical reaction. High efficiency turbines (CCS) technology Syn gas can be fed into fuel cell Converts thermal energy from gas To capture CO2 emission from the or steam into electricity. To achieve near net zero emission for combustion of fossil and store it These are driven by both AUSC and power generation. underground. IGCC process 3 High temperature materials are involved. Gasifier: Solid or liquid fuels (coal, petroleum) → gaseous mix called syn gas (synthetic gas) Major components of a gasifier Gasifier vessels Downstream coolers Gas cleaning vessels 4 Gasifier: To remove the impurities The slag is in two forms: Liquid slag that can flow out of the gasifier. Solid slag that solidifies on the refractory lining of the gasifier 5 Gasifier: Vessel Gasifier chamber operates at T = between 1250 and 1550°C and P = 3 MPa or higher. Material requirement: endure high temperatures, resist thermal shock, erosion by particulates, molten slag, corrosive gases, resists corrosion, withstand variable oxidizing/reducing conditions. Refractory materials are used to protect the steel shell from erosion, corrosion, and high temperatures. 6 Gasifier: Various refractory compositions have been used in gasifiers: Fuse-cast materials: Have good chemical resistance but poor thermal shock resistance. Al2O3 and MgO/Al2O3 spinel materials: Offer poor wear resistance. SiC and Si3N4: Also have poor wear resistance. Chromium oxide: Enhances resistance to chemical attack from slag. High chromium oxide refractories (>85 wt% Cr2O3): Are used in severe wear areas, while lower chromium oxide materials are used in less severe areas. A minimum of 75 wt% Cr2O3 is needed for effective performance. High chromium oxide refractory materials are currently preferred, but they don't fully meet industry requirements (high cost, risk of forming hexavalent chromium (Cr⁶⁺) which is toxic and carcinogenic). Future developments may include coatings, monolithic linings, and low-chromium or chromium-free refractory materials like alkali-aluminate. 7 Gasifier: Gasifier is typically cooled by air or water. 1. Water-cooled gasifiers: Use Al2O3-SiC refractory linings. These have a good service life as the slag freezes on the surface, preventing further penetration and corrosion. 2. Air-cooled gasifiers: Use high chromium oxide (Cr2O3) materials, often with alumina and additives like zirconia. These liners have a lifespan of 3 months to 2 years. Reason: water cooling →intense cooling, hence high thermal conductivity, excellent thermal shock needed (can handle rapid T change without cracking). 8 Gasifier: Coolers After generation of syngas, the gases must be cooled and cleaned before being burned in the gas turbine. The raw hot gas consists mainly of carbon monoxide (CO) and hydrogen (H2), with hydrogen sulphide (H2S) as a major corrosive impurity. Cooling and water quenching remove particulates and water-soluble impurities like ammonia (NH3) and chlorides. High-temperature corrosion, particularly sulphidation, is a major issue in syngas coolers, (especially when coal is used as fuel). Operating temperatures: Generally kept below 450°C to mitigate high-temperature corrosion and because higher temperatures are not cost-effective. 9 Gasifier: Coolers Material selection for syngas coolers: Carbon and low-alloy steels: Have high rate of corrosion (Corrosion rates: Are influenced by the chloride content of the gas). Alloys with low chromium content (e.g., Alloy 800 and Inco 82): Alloy 800 is commonly used due to its adequate properties at an economic price, but are more susceptible to corrosion. Stainless steel 310: Has adequate corrosion resistance but is not approved due to waterside corrosion issues. High chromium materials with aluminium and silicon: Are less affected by HCl presence. Sanicro 28 (highly alloyed austenitic stainless steel): Is currently the favoured material for use in these conditions. Properties: excellent resistance to pitting, crevice corrosion, and general corrosion in environments with high levels of chlorides, sulfuric acid, and phosphoric acid. The high nickel content provides good resistance to stress corrosion cracking, while chromium and molybdenum enhance the material's resistance to both oxidizing and reducing environments. 10 Gasifier: Coolers during downtime Aqueous corrosion during downtime increases corrosion rates, especially in the presence of chloride-containing deposits. Reason: Chlorides penetrate cracks in the oxide scale and attack the metal at the oxide/metal interface during downtime. This leads to the formation of iron chlorides, causing spallation of the oxide scale during start-up and exposing fresh metal to further corrosion. Material selection for gas coolers: Stainless steels (>20 wt% Cr): Adequate for use between 300 and 400°C if spallation does not occur. Molybdenum-containing materials (e.g., Sanicro 28): Offer better protection where spallation occurs. Inconel Alloy 625: Preferred for gas with high chloride levels due to its molybdenum content and high nickel level. A major challenge in gasification systems is developing a reliable and cost-effective cooling/cleaning path.This includes the development of syngas coolers with novel alloys and manufacturing processes to improve corrosion resistance and reduce costs. Improved hot gas cleanup systems could lower the cost of Integrated Gasification Combined Cycle (IGCC) by providing a cheaper alternative to current low-temperature processes. 11 Synthetic gas – fired turbine: Gasification plants use multi-stage cleanup processes for syngas, but some impurities still enter the gas turbine hot section after combustion. These impurities pose a risk of damage to the turbine blades. Syngas-fired turbines face similar issues as conventional gas turbines, with added complexity due to fuel and impurities. Currently, syngas-fired turbines operate at similar firing temperatures as natural gas turbines, with about 14% increased mass flow through the turbine. This increased mass flow leads to higher turbine outputs but also greater heat transfer to the hot section vanes and blades, necessitating higher temperature materials. Even very low levels of gas stream ash and impurities can cause significant degradation through corrosion, deposition, and erosion. As turbine gas inlet temperatures are expected to rise to improve efficiency, this will affect the use of high-temperature alloys and worsen the impact of molten salts and deposits. 12 CO2 Capture and storage: CO2 Capture and Storage (CCS) is a technology designed to reduce carbon dioxide (CO2) emissions from large-scale industrial processes and power generation. The primary goal of CCS is to prevent CO2 from being released into the atmosphere, thereby mitigating the impact of greenhouse gases on climate change by three main steps i.e., capturing CO2, transporting it, and storing it safely. Post-combustion: CO2 is separated from the process stream after the fuel has been burned. Pre-combustion: Carbon-bearing compounds are removed from the gas stream before the combustion of fuel constituents. Oxy-fuel: Fuel is burned in an oxygen-enriched gas, resulting in an exit gas with high levels of CO2 and steam, which are easily separated through condensation. CO2 transport and storage: The captured CO2 is then transported and stored. 13 CO2 Capture and storage: post-combustion Low-temperature liquid scrubbing with amine-based solvents is the most likely technology, widely used in the oil/gas sector for CO2 separation from natural gas. An amine scrubber consists of two units: Absorber: CO2-lean solvent reacts with flue gas CO2 at 40–60°C. (product: CO2-enriched molecule, that increases the energy of the molecule) Stripper (or regenerator): CO2-rich solvent is heated to 100–140°C with steam to strip the gas at near- atmospheric pressure. (Steam provide necessary heat to solvent to help in displacing CO2) 14 CO2 Capture and storage: post-combustion Critical materials issues in amine scrubbing: Corrosion resistance in scrubber environments. Performance of corrosion inhibitors. Potential surface treatments and coatings for protection/repair. Assessing critical fluid velocities that could cause significant erosion-corrosion. Lack of knowledge about service corrosion mechanisms (e.g., pitting, localized corrosion at welds, stress corrosion cracking). Effects of different amine solvents, concentrations, and degradation products on corrosion. Alternative post-combustion CO2 separation approaches include: Use of ammonia. Solid sorbents (e.g., lime or alkali compounds). Adsorption on molecular sieves or active carbons using pressure, temperature, or electrical swing systems. Cryogenics (uses extremely low T such as -100°C, CO2 transition to liquid or solid phase, once solidified, it can be physically separated) 15 CO2 Capture and storage: pre-combustion Pre-combustion carbon capture is commonly achieved using Integrated Gasification Combined Cycle (IGCC) technology. CO2 capture in an IGCC plant involves removing CO2 from the syngas within the gasifier (before combustion). Advanced gasification systems are being developed for power generation, chemical feedstock, liquid fuels, and hydrogen production, with hydrogen requiring CO2 capture. High-pressure, oxygen-blown gasifiers are most suitable for CO2 capture. To capture CO2, fuel gas is fed to a catalytic shift reactor where CO reacts with steam to produce H2 and CO2 (called shifted gas). CO2 in the shifted fuel gas is relatively concentrated (around 50 vol%) and at high pressure, enabling lower CO2 capture costs. CO2 separation approaches from shifted fuel gas include: Physical solvents, e.g., Rectisol process using cold methanol. Selexol process using dimethyl ether of polyethylene glycol. (well-established in ammonia production but have a high efficiency penalty) Solid sorbents or adsorbents (Materials that capture CO2 by binding it to their surface or within their structure.) Cryogenics. Advanced separation membranes. (selectively allow CO2 to pass through while blocking other gases), 16 requires less energy. CO2 Capture and storage: oxy-combustion Oxyfuel combustion is an experimental approach to post-combustion carbon capture. It involves burning coal in an oxygen and CO2-rich mixture instead of air, producing a waste gas stream rich in CO2, making CO2 capture easier. Main drawback: Requires an expensive, energy-intensive air separation unit. Combustion in an oxygen-enriched/low nitrogen environment results in gaseous products mostly of CO2 and steam, which can be separated using a condenser. Materials challenges include changes to the boiler environment due to flue gas recycling, as well as potential issues in the steam condenser/CO2 separator. SOx levels can be up to 5 times higher, depending on where recycled flue gas is taken from. Cleanest option: Recycle flue gas after desulfurization, reducing SOx but increasing the size of the flue gas desulfurization (FGD) plant. Cheapest option: Recycle flue gas with minimal pre-cleaning, leading to higher SOx levels and increased fouling and corrosion. Water-wall (tubes that lines the furnace wall of boiler) and superheater (Superheaters are components in the boiler that heat the steam generated in the water-walls to a higher temperature before it enters the turbine) corrosion are major concerns, especially under advanced supercritical plant conditions with higher contaminants and temperatures. 17 CO2 transport and storage: transport and challenges CO2 captured will be impure, containing various gases depending on the power plant type and capture technology used. Contaminants may include SOx, HCl, NOx, and trace metal compounds due to inefficiencies in gas cleaning technologies. Contaminated CO2 creates an aggressive environment for pipelines or transport systems. Hence, special cleanup measures would be needed, potentially introducing additional unspecified species. Current specification for CO2 transport (>95% CO2,