Heat recovery in hydrogen production: heat exchangers, recuperators and condensing economisers
Finned-tube heat exchangers and gas-gas recuperators are key equipment in the energy balance of hydrogen production plants, both in reforming processes and electrolysis installations.
Hydrogen production —both by steam methane reforming and by electrolysis using renewable energy— generates high-temperature heat streams that represent real energy recovery opportunities. Finned-tube heat exchangers, gas-gas recuperators and condensing economisers are the reference technical solutions for harnessing these streams under the process and regulatory conditions specific to the sector.
1. Heat recovery opportunities in hydrogen plants
In a hydrogen production plant, the heat streams available for recovery appear at several points in the process. Identifying and harnessing these streams —by means of finned-tube heat exchangers or gas-gas recuperators conceived for the specific conditions of each point— is one of the main vectors for improving the plant's overall energy performance.
2. The flue gas recuperator: the equipment with the greatest efficiency impact
In reforming installations, the recuperator or economiser that cools the furnace combustion gases —preheating the combustion air or process water, or generating steam— is usually the heat transfer equipment with the greatest impact on the plant's overall energy performance. The way this equipment is conceived with respect to the acid dew point of the combustion gas determines how much energy can be recovered.
A conventional recuperator operates with the wall temperature above the acid dew point, recovering only the sensible heat of the gases. A condensing economiser operates deliberately below the dew point, also recovering the latent heat of the water vapour —which in natural gas combustion gases represents a significant fraction of the total available energy. The result is a lower gas outlet temperature and a higher overall thermal efficiency. BOIXAC can supply both solutions; the choice between them depends on the combustion gas composition, the temperature of the available cooling fluid and the project's efficiency objectives.
3. Materials for heat exchangers in hydrogen service
Hydrogen presents attack mechanisms on metallic materials that do not exist with other conventional fluids. Its high diffusivity in metals activates specific phenomena that must be considered when conceiving heat exchangers for this service.
- HTHA (High Temperature Hydrogen Attack): at elevated temperatures and H₂ partial pressures, atomic hydrogen diffuses into the steel and reacts with the carbon in the material to form methane, causing loss of strength and intergranular cracking. The reference standard is API 941, which defines the so-called Nelson curves: for each type of steel, they establish the maximum allowable combination of temperature and H₂ partial pressure in continuous service. Low-alloy Cr-Mo steels withstand more severe conditions than carbon steels.
- Hydrogen embrittlement (HE): at ambient or low temperatures, absorbed hydrogen can reduce the ductility of certain high-strength steels, increasing the risk of fracture under stress. Particularly relevant in high-pressure H₂ equipment. It is controlled through the selection of materials with controlled hardness.
- PED Group 1 classification: hydrogen is flammable and is classified as a Group 1 fluid under the PED. Heat exchangers with pressurised H₂ typically fall into high PED categories with involvement of a Notified Body. The requirements for non-destructive examination of welds are also stricter than in conventional services.
The API 941 standard establishes, for each type of steel, the maximum combination of service temperature and H₂ partial pressure above which the material is exposed to the risk of HTHA. Operating above these limits is one of the documented causes of catastrophic failures in process installations. In heat exchangers in high-temperature H₂ service, verification against the Nelson curves is a non-negotiable design requirement, and demands knowledge of the equipment's maximum wall temperature —not only the mean fluid temperature— under the most unfavourable operating conditions.
4. Typical equipment configurations and materials
| Application | Equipment configuration | Tube material | Key regulatory consideration |
|---|---|---|---|
| Flue gas recuperator (conventional) | Finned tubes, gas-gas or gas-liquid | 316L / 321 stainless steel in the cold zone; carbon steel in the hot zone outside the condensation risk area | Minimum wall temperature above the acid dew point. PED according to pressure and fluid. |
| Condensing economiser | Finned tubes, gas-liquid | 316L stainless steel throughout the condensation zone | Materials for contact with acid condensate. Drainage geometry. PED applicable. |
| Process gas cooling (shift, purification) | Finned tubes, gas-liquid | 316L stainless steel; Duplex 2205 if H₂S present | Verify Nelson curves (API 941) if high T + H₂. NACE MR0175 if H₂S. PED Group 1. |
| H₂ compression intercooler | Finned tubes, gas-liquid | 316L stainless steel | High-pressure H₂: PED Group 1, high category, strict NDE on welds. |
| Gas dryer / PSA feed cooler | Finned tubes, gas-liquid | 316L stainless steel | Management of H₂O condensation. PED applicable according to pressure and volume. |
| Electrolyser BOP cooling | Finned tubes, liquid-liquid or gas-liquid | 316L stainless steel | H₂ in contact with cooling fluid: verify tightness. PED if thresholds are exceeded. |
5. Regulatory framework: PED and hydrogen-specific requirements
Heat exchangers in hydrogen service are subject to PED 2014/68/EU when they exceed the pressure and volume thresholds of Annex II. Since hydrogen is flammable and classified as a Group 1 fluid, the resulting PED categories are usually high, with mandatory involvement of a Notified Body in categories III and IV.
- PED Group 1 and H₂: the classification of hydrogen as a Group 1 fluid activates the most demanding categorisation tables of Annex II of the PED. A heat exchanger with H₂ at 30 bar can reach category III or IV even with moderate volumes, requiring the involvement of a Notified Body in the conformity assessment.
- NDE on welds: equipment in H₂ service typically requires a higher level of non-destructive inspection on welds than equipment in conventional services, given the risk of weld defects as an entry path for hydrogen and a potential initiation point for brittle fracture.
- Purge and vent management: heat exchangers in H₂ service must be fitted with purge and vent points conceived for the safe elimination of hydrogen, given its wide flammability range (4–75% vol. in air) and its tendency to accumulate in high areas.
- Technical documentation: BOIXAC provides the technical documentation of the supplied equipment —including calculations, material certificates and inspection records— required to complete the certification and approval process in hydrogen production installations.
6. Hydrogen in the European energy context
Hydrogen plays a growing role in the European Union's industrial decarbonisation roadmap. The European Hydrogen Strategy and the REPowerEU package set ambitious targets for renewable hydrogen production by 2030, which translates into an increase in industrial-scale electrolysis projects across Europe. At the same time, the tightening of the emissions trading system (ETS) makes hydrogen production by reforming without CO₂ capture more costly, driving investment in the energy efficiency of these plants.
In reforming plants, heat recovery from the furnace flue gases is the most direct efficiency lever available. A well-conceived recuperator —or a condensing economiser where conditions allow— can significantly reduce the specific natural gas consumption per unit of H₂ produced. In electrolysis installations, recovery of the stack heat and thermal optimisation of the balance of plant are the equivalent vectors for improving electrical efficiency.