By-Product Hydrogen Purity Testing: Standards, Methods, and Industrial Applications

By-product hydrogen purity testing is the systematic analytical evaluation of hydrogen recovered from industrial processes such as chlor-alkali electrolysis, propane dehydrogenation, methanol synthesis, and coke oven gas upgrading. Accurate determination of hydrogen purity and trace impurities such as oxygen, carbon monoxide, moisture, and sulfur compounds determines whether this hydrogen can be valorized in fuel cells, electronics manufacturing, and chemical synthesis. This article explains the applicable international and national standards, sampling practices, testing items, and analytical methods used by accredited third-party laboratories for by-product hydrogen quality verification.

Technical Overview

By-product hydrogen refers to hydrogen that is co-generated during industrial chemical processes rather than being intentionally produced through dedicated routes such as steam methane reforming or water electrolysis. Large volumes of by-product hydrogen arise from chlor-alkali electrolysis, propane dehydrogenation (PDH), ethylene cracking, methanol synthesis purge streams, ammonia plant purge gas, and coke oven gas. As industries pursue decarbonization and circular resource utilization, this hydrogen stream has become a strategically important feedstock, provided its quality can be reliably demonstrated.

The raw purity of by-product hydrogen varies dramatically depending on the source process. Chlor-alkali hydrogen can exceed 99% after dechlorination and drying, while coke oven gas contains only 55–60% hydrogen before pressure swing adsorption (PSA) upgrading. Regardless of the source, downstream users impose strict limits on individual impurities because even trace-level contaminants can poison catalysts, degrade proton exchange membrane (PEM) fuel cells, or create safety hazards. Purity testing therefore serves three purposes: verifying the hydrogen fraction itself, quantifying every relevant impurity against specification limits, and validating that purification systems such as PSA units, deoxidizers, and dryers are performing correctly.

An important technical principle is that hydrogen purity at levels above 99.99% is rarely measured directly. Instead, it is determined "by difference": the mole fractions of all specified impurities are measured individually, summed, and subtracted from 100%. This approach is explicitly recognized in ISO 21087, which defines the analytical protocols and method validation requirements for hydrogen fuel impurity analysis. Consequently, the reliability of a purity value depends entirely on the detection limits, calibration quality, and completeness of the impurity test panel.

Testing Samples

By-product hydrogen samples are classified according to their generation process, because each process creates a characteristic impurity profile that dictates the analytical test panel.

Chlor-Alkali By-Product Hydrogen

Hydrogen liberated at the cathode of chlor-alkali membrane or diaphragm cells typically contains oxygen, nitrogen, and moisture, with possible traces of chlorine, hydrogen chloride, and sodium hydroxide mist carried over from the electrolyzer. After dechlorination, deoxygenation, and drying, this stream can reach very high purity, making it one of the most promising sources for fuel cell applications.

Propane Dehydrogenation Off-Gas Hydrogen

PDH units release a hydrogen-rich tail gas containing methane, ethane, ethylene, propane, propylene, carbon monoxide, and carbon dioxide. Following PSA purification, hydrogen purities above 99.99% are routinely achieved, but verification of residual light hydrocarbons and carbon oxides remains essential.

Coke Oven Gas Derived Hydrogen

Coke oven gas is a complex mixture of hydrogen, methane, carbon monoxide, carbon dioxide, nitrogen, oxygen, ammonia, hydrogen sulfide, organic sulfur species, naphthalene, and aromatic hydrocarbons. Hydrogen extracted from this stream through PSA requires comprehensive testing for sulfur compounds, oxygen, nitrogen, and carbon oxides, since coke oven gas carries the most demanding impurity burden of any common by-product source.

Methanol and Ammonia Purge Gas Hydrogen

Purge streams from methanol synthesis loops contain hydrogen together with methane, nitrogen, carbon monoxide, carbon dioxide, and traces of methanol, while ammonia plant purge gas additionally carries ammonia and argon. Recovered hydrogen from these sources must be screened for nitrogen, argon, and the specific process-related contaminants before reuse or sale.

Ethylene Cracker and Other Tail Gas Hydrogen

Ethylene cracking tail gas, formaldehyde plant off-gas, chlorate electrolysis gas, and sodium methoxide process gas represent additional by-product hydrogen sources. Each requires a source-specific test panel covering light hydrocarbons, carbon oxides, halogenated species, oxygenates such as formaldehyde and formic acid, or chlorinated contaminants.

Testing Items

A complete by-product hydrogen characterization combines the hydrogen purity determination with a defined panel of impurity measurements. Typical testing items include:

• Hydrogen purity (mole fraction, calculated by difference or verified directly)
• Oxygen (O2) — safety-critical and fuel cell limiting parameter
• Nitrogen (N2) and argon (Ar), individually or combined
• Carbon monoxide (CO) — principal PEM fuel cell catalyst poison
• Carbon dioxide (CO2)
• Methane (CH4) and total hydrocarbons (THC)
• Moisture content and pressure dew point
• Total sulfur compounds (H2S, COS, mercaptans, thiophenes)
• Ammonia (NH3) — relevant to purge gas and coke oven gas sources
• Halogenated compounds (Cl2, HCl, organic chlorides) — relevant to chlor-alkali and PDH sources
• Oxygenates such as formaldehyde and formic acid — relevant to methanol and formaldehyde plant sources
• Oil mist, particulates, and aerosols introduced by compression or purification equipment

The applicable specification depends on the intended end use. GB/T 3634.1 defines industrial hydrogen requirements, typically at purity levels of 99% and above. GB/T 3634.2 specifies pure hydrogen (≥99.99%), high-purity hydrogen (≥99.999%), and ultra-pure hydrogen (≥99.9999%) with corresponding limits for O2, N2, CO, CO2, CH4, and moisture. ISO 14687-1 establishes quality grades for all applications except PEM fuel cells for road vehicles, while ISO 14687-2 defines the fuel cell vehicle grade, which requires a hydrogen fuel index of at least 99.97% combined with strict individual impurity limits.

Key limits from ISO 14687-2 for PEM fuel cell road vehicle applications include the following, expressed as maximum mole fractions:

• Carbon monoxide: 0.2 µmol/mol (ppm)
• Total sulfur compounds: 0.004 µmol/mol
• Water: 5 µmol/mol
• Oxygen: 5 µmol/mol
• Carbon dioxide: 2 µmol/mol
• Total hydrocarbons as methane: 2 µmol/mol
• Ammonia: 0.1 µmol/mol
• Halogenated compounds: 0.05 µmol/mol
• Helium, nitrogen, and argon combined: 100 µmol/mol

Testing Methods

Gas Chromatography

Gas chromatography is the backbone of hydrogen purity analysis. Thermal conductivity detection (GC-TCD) quantifies percent-level permanent gases such as O2, N2, CH4, CO, and CO2 in accordance with methods such as ASTM D1946 and the GB/T 8984 series for hydrogen gas analysis. For high-purity and ultra-high-purity grades, pulsed discharge helium ionization detection (GC-PDHID) or pulsed discharge photoionization detection achieves parts-per-billion detection limits for trace O2, N2, Ar, CO, CO2, and CH4. Flame ionization detection (GC-FID), optionally with a methanizer, provides sensitive measurement of carbon oxides and hydrocarbon speciation following the analytical philosophy of ASTM D1945 and ISO 6974 series gas analysis methods.

Trace Oxygen Analysis

Dedicated oxygen analyzers based on paramagnetic, electrochemical (fuel cell), or galvanic sensor principles measure trace oxygen down to approximately 0.1 ppm, in accordance with GB/T 6285. Because oxygen governs both safety classification and ISO 14687 compliance, cross-verification between the analyzer and GC-PDHID results is a common laboratory practice.

Moisture and Dew Point Determination

Water content is measured using electrolytic phosphorus pentoxide cells, chilled-mirror dew point hygrometers, or capacitive aluminum oxide sensors. Chilled-mirror measurement of the water dew point is covered by ASTM D1142, while the GB/T 5832 series defines electrolytic and other moisture determination procedures for gases. Modern instruments resolve dew points below –80 °C, corresponding to sub-ppm moisture levels.

Sulfur and Halide Analysis

Total sulfur and individual sulfur species are determined by gas chromatography coupled with sulfur chemiluminescence detection (GC-SCD), following the approach of ASTM D5504 and ASTM D7652 for trace sulfur compounds in hydrogen fuel. Halogenated species and hydrogen chloride are quantified by GC with electron capture or mass spectrometric detection, or by ion chromatography after controlled impinger absorption, as recommended in the ISO 21087 analytical protocol.

FTIR and GC-MS Screening

Fourier transform infrared spectroscopy enables simultaneous multi-component screening of CO, CO2, CH4, formaldehyde, formic acid, ammonia, and halogenated impurities in hydrogen fuel, as standardized in ASTM D7653. Gas chromatography–mass spectrometry (GC-MS) provides confirmatory identification of unknown organic contaminants at trace levels, which is particularly valuable when characterizing by-product streams with complex process chemistries.

Sampling and Calibration

Representative sampling is critical for meaningful results. High-pressure hydrogen is collected into passivated stainless steel or aluminum cylinders following practices such as ASTM D7606, with repeated purging, inert interior surfaces, minimal elastomer exposure, and rapid analysis of reactive species such as sulfur compounds and ammonia. All instruments are calibrated with certified reference gas mixtures prepared or verified per ISO 6142 and ISO 6143, and test reports from ISO/IEC 17025 accredited third-party laboratories include measurement uncertainty for each reported parameter.

Applications

Verified by-product hydrogen purity opens access to high-value markets and enables internal process optimization across multiple industries:

• Fuel cell mobility: hydrogen meeting ISO 14687-2 and ISO 19880-1 requirements can be supplied to refueling stations for passenger vehicles, buses, and heavy trucks
• Petrochemical and refinery processes: hydrodesulfurization, hydrocracking, and hydrotreating units require hydrogen with controlled CO, CO2, and sulfur content to protect catalysts
• Chemical synthesis: ammonia and methanol plants consume hydrogen with strict limits on oxygen, moisture, and catalyst poisons
• Electronics and semiconductor manufacturing: epitaxy, annealing, and thin-film processes demand high-purity or ultra-pure hydrogen per GB/T 3634.2 specifications
• Fine chemical hydrogenation: pharmaceutical and specialty chemical production relies on hydrogen free of sulfur, carbon monoxide, and oil mist contamination
• Hydrogen peroxide production: the anthraquinone process consumes hydrogen with tight controls on oxygenates and catalyst poisons
• Energy applications: hydrogen turbine blending and injection into natural gas grids require purity verification to protect infrastructure and end-use equipment

Beyond market qualification, routine purity testing supports PSA unit optimization, deoxidizer and dryer breakthrough monitoring, custody transfer agreements, and regulatory reporting for carbon footprint and circular economy programs.

FAQ

Why is by-product hydrogen purity usually determined by difference instead of measuring hydrogen directly?

At purities above 99.99%, direct hydrogen measurement cannot resolve small impurity contributions with sufficient accuracy. Laboratories instead measure every specified impurity with calibrated instruments, sum the mole fractions, and subtract from 100%. The purity result is therefore only as complete as the impurity test panel, which is why ISO 21087 emphasizes method validation and appropriate detection limits for each parameter.

Which by-product hydrogen source is easiest to upgrade to fuel cell grade?

Chlor-alkali hydrogen is generally considered the most favorable source because its raw gas is already free of hydrocarbons and sulfur species, and remaining impurities such as oxygen and moisture are removed by well-established deoxidization and drying steps. Coke oven gas and PDH off-gas require more extensive purification and testing due to their sulfur, hydrocarbon, and carbon oxide burdens.

What are the most harmful impurities for PEM fuel cells?

Carbon monoxide above 0.2 ppm adsorbs onto platinum catalyst sites and reduces cell performance, while sulfur compounds at parts-per-billion levels cause irreversible catalyst damage. Ammonia, formaldehyde, and formic acid degrade the membrane electrode assembly, and excess moisture or oxygen affects efficiency and safety margins.

How often should by-product hydrogen be tested?

Initial qualification testing is required whenever a new source, purification unit, or operating condition is introduced. Routine verification testing is then performed at a frequency proportional to process variability and contractual requirements, with continuous online analyzers typically monitoring oxygen and moisture between laboratory campaigns.

Can by-product hydrogen meet the 99.97% fuel cell specification?

Yes. With properly designed and well-maintained purification trains, particularly for chlor-alkali and PDH sources, by-product hydrogen routinely achieves hydrogen fuel indices of 99.97% and above, and full compliance is demonstrated through complete impurity testing against every ISO 14687-2 limit by an accredited third-party laboratory.

What sample conditions are required for reliable testing?

Samples should be collected in passivated high-pressure cylinders at representative line conditions, with thorough purging and avoidance of elastomer seals that leach moisture or organics. Reactive impurities such as sulfur compounds and ammonia should be analyzed promptly after sampling to prevent adsorption losses on cylinder walls.