Ternary molten salt coking tendency evaluation is a critical thermal stability assessment for nitrate, carbonate, and chloride salt systems used in concentrated solar power, thermal energy storage, and high-temperature process industries. By quantifying decomposition onset temperature, isothermal mass loss rate, insoluble residue formation, and gas evolution behavior, this evaluation establishes the safe upper operating temperature and long-term service reliability of ternary salt mixtures. This article explains the degradation chemistry behind salt coking, the sample types, testing items, laboratory methods, acceptance criteria, and typical industrial applications of a professional coking tendency assessment.
Technical Overview
Ternary molten salts are three-component eutectic or near-eutectic mixtures engineered to achieve a low melting point, low vapor pressure, and high heat capacity for heat transfer fluid (HTF) and thermal energy storage (TES) duties. The most widely deployed systems include nitrate-nitrite mixtures of the Hitec family, calcium-nitrate-rich Hitec XL formulations, and ternary carbonate or chloride salts for next-generation high-temperature service. "Coking tendency" in this context refers to the propensity of a molten salt to chemically degrade when held at elevated temperature, generating gaseous decomposition products and insoluble residues such as carbonates, oxides, and corrosion-derived particles that precipitate on vessel walls, heater surfaces, and cold spots as a hard, coke-like crust.
- Nitrate-nitrite ternary system (Hitec type): approximately 53 wt% KNO₃, 40 wt% NaNO₂, 7 wt% NaNO₃, melting point near 142 °C
- Calcium nitrate ternary system (Hitec XL type): Ca(NO₃)₂–KNO₃–NaNO₃, melting point near 120–130 °C
- Ternary carbonate system: Li₂CO₃–Na₂CO₃–KCO₃-type mixtures, melting point near 400 °C, service up to approximately 650 °C
- Ternary chloride system: MgCl₂–KCl–NaCl eutectics for 700 °C-class CSP and molten salt reactor applications
The underlying degradation chemistry proceeds through a sequential chain. Nitrate first decomposes to nitrite with oxygen release (2NO₃⁻ → 2NO₂⁻ + O₂) at roughly 430–500 °C, depending on atmosphere. Nitrite then disproportionates (5NaNO₂ → 3NaNO₃ + Na₂O + N₂) at higher temperature, and the resulting alkali oxide absorbs CO₂ from air or cover gas to form carbonate. Carbonate, together with iron, chromium, and nickel oxides produced by structural corrosion, has limited solubility in the melt and accumulates as sludge or bonded deposits. Local overheating on electrical heater sheaths and thermal gradients accelerate this crust formation, which reduces heat transfer, blocks channels, and can force emergency shutdown and costly salt replacement.
Why Coking Tendency Must Be Evaluated
A molten salt that appears stable in a short laboratory screening may still coke after hundreds or thousands of hours in service. Quantifying the coking tendency before plant deployment allows engineers to set a defensible maximum film temperature, define purge and atmosphere control strategies, select compatible structural alloys, and establish salt replacement or purification intervals. It also supports incoming-batch quality control, supplier comparison, and lifetime extension studies for operating facilities.
Testing Samples
Nitrate-Nitrite Ternary Salts (Hitec-Type)
Samples of virgin Hitec salt, plant-aged salt, and reclaimed salt are evaluated to benchmark thermal stability. Particular attention is paid to the nitrate/nitrite ratio, moisture content, and carbonate accumulation, because nitrite depletion directly governs the onset of oxide generation and subsequent coking.
Ternary Carbonate Salts
Lithium-sodium-potassium carbonate mixtures intended for advanced power cycles and high-temperature TES are tested for carbonate decomposition, CO₂ loss, and oxide slag formation up to approximately 800 °C.
Ternary Chloride and Fluoride Salts
Chloride and fluoride ternary systems for Generation-3 CSP and molten salt reactors are assessed with strict atmosphere control, since hydrolysis and oxidation impurities (MgOHCl, oxide ions) drive sludge formation far more strongly than in nitrate systems.
Sample Preparation and Quantity
Representative samples of 100–500 g are typically sufficient for a full test program. The salt is dried under vacuum at 105–150 °C to remove moisture, homogenized, and optionally pre-melted under the intended cover gas. Moisture content, particle size, and homogeneity are documented before testing because hygroscopic calcium nitrate and hydrous chlorides can distort mass loss and gas evolution results.
Testing Items
A comprehensive coking tendency evaluation typically includes the following measurable parameters:
- Decomposition onset temperature (Tonset) and peak decomposition temperature by dynamic thermogravimetry
- Isothermal mass loss rate (wt% per 100 h or per 1000 h) at candidate operating and upset temperatures
- Gas evolution rate and composition (O₂, N₂, NO, NO₂, CO₂) by evolved gas analysis
- Nitrate/nitrite molar ratio drift and carbonate content increase after aging
- Insoluble residue content (wt%) in the aged salt, the primary quantitative coking indicator
- Deposit morphology, crystalline phase identification, and layer thickness on test coupons
- Metal ion contamination (Fe, Cr, Ni, Ca, Mg) and corrosive impurities (Cl⁻, SO₄²⁻) by ICP-OES and ion chromatography
- Changes in melting point, heat capacity, and solidification behavior after thermal aging
Commonly applied engineering criteria classify the coking tendency as low when aged insoluble residue remains below approximately 0.05 wt% with mass loss below 0.1 wt% per 100 h, moderate between 0.05 and 0.5 wt%, and high above 0.5 wt% or when a bonded, difficult-to-remove crust forms on test surfaces. The classification is always reported together with the test temperature, atmosphere, and exposure duration so that results can be extrapolated responsibly.
Testing Methods
Thermogravimetric Analysis (TGA)
Dynamic TGA scans (typically 5–10 mg samples at 5–10 °C/min under air or inert gas, consistent with the approach of ASTM E2550) locate the decomposition onset temperature. Follow-up isothermal holds at temperatures 20–80 °C below Tonset measure the steady-state mass loss rate, which is the single most direct screening index of coking tendency.
Differential Scanning Calorimetry (DSC)
DSC verifies melting point, phase change enthalpy, and heat capacity before and after aging. A rising liquidus or the appearance of new endothermic peaks after aging signals carbonate or oxide accumulation and confirms the chemical alteration indicated by TGA.
Static Isothermal Capsule Aging
Gram-scale salt charges are sealed in quartz ampoules or inert-atmosphere 316L/310S stainless capsules and held at target temperatures (typically 450–600 °C for nitrate systems) for 100 to 1000 hours. Post-test salts are dissolved and filtered to determine insoluble residue, reanalyzed for ion speciation, and compared with unaged reference material.
Dynamic Stirred Reactor and Thermal Cycling Tests
Kilogram-scale stirred autoclave tests with immersed heater rods and metal coupons replicate forced-convection and wall-superheat conditions. Torque rise, deposit mass on the heater surface, and coupon inspection quantify crust adhesion. Repeated freeze–melt and thermal shock cycling (for example, 300–550 °C) further reveals whether thermal fatigue accelerates residue generation and channel blockage.
Evolved Gas and Deposit Characterization
TGA coupled with mass spectrometry or FTIR identifies the decomposition gases (O₂, NOx, N₂, CO₂) and their release sequence. Recovered deposits are examined by XRD for phase identification and by SEM-EDS for morphology and elemental mapping, linking the coking mechanism to its chemical origin, such as nitrite disproportionation versus corrosion product precipitation.
Applications
Coking tendency evaluation directly supports design, procurement, and operations decisions across multiple industries:
- Concentrated solar power plants: qualification of Hitec, Hitec XL, and solar-salt-variant ternary mixtures for TES tanks and steam generation duty
- Generation-3 CSP and molten salt reactors: screening of ternary chloride and fluoride salts for 700 °C-class service under purified inert atmospheres
- Industrial process heat: salt selection for heat treatment salt baths, thermochemical reactors, and waste heat recovery loops
- Grid-scale and cogeneration thermal storage: lifetime prediction, salt maintenance scheduling, and reclamation validation
- Supplier qualification and failure analysis: batch acceptance testing and root-cause investigation of in-service deposit formation and heater fouling
FAQ
At what temperature do ternary nitrate salts begin to coke?
For Hitec-type salts, measurable nitrite degradation begins near 450 °C in air-contacted systems, while sealed or inert-blanketed operation can extend stable service to approximately 535 °C. Rapid decomposition with nitrogen evolution and heavy residue formation generally accelerates above 600 °C. A material-specific TGA and aging program is required to set exact limits for a given formulation and atmosphere.
How is coking different from freezing?
Freezing is a fully reversible physical phase change that remelts on reheating with no chemical change. Coking is irreversible chemical decomposition that consumes the salt, generates gas, and produces insoluble carbonate and oxide residues that do not redissolve and progressively foul heat transfer surfaces.
How long does a complete evaluation take?
TGA and DSC screening requires only a few days. Static capsule aging programs typically run two to twelve weeks depending on the exposure temperature and duration specified, and dynamic stirred-reactor or loop tests may extend over several months to capture long-term deposit behavior.
What sample size and condition are required?
A representative sample of 100–500 g of dry, homogeneous salt is normally sufficient for the full test matrix. Hygroscopic or hydrous salts must be pre-dried, and the sampling point, batch identification, and storage history should be documented so results are traceable.
How are results reported and used?
The final report presents Tonset, isothermal mass loss rates, insoluble residue data, gas analysis, deposit characterization, and a clearly justified coking tendency classification. Design engineers use these outputs to define the maximum salt and film temperatures, cover gas specifications, alloy selection, filtration strategy, and salt replacement intervals for the application.