A green ammonia plant is three plants in one drawing set: an electrolysis plant producing hydrogen, an air separation or nitrogen unit, and an ammonia synthesis loop with refrigeration and storage that would be familiar to anyone who has worked a conventional plant. The synthesis end is tagged and drawn as ammonia plants have been for decades. The front end is new, and it brings instrumentation a process engineer from the reformer side has not seen: stack-level electrical and gas-quality measurements, a hydrogen buffer sized to the power supply's variability, and a turndown regime that a plant fed by a reformer never needed.
This is a field guide to reading that set into the instrument index and I/O list, for the engineer handed a package assembled from an electrolyzer supplier, a nitrogen supplier, a synthesis licensor and an EPC, each in its own convention.
Key takeaways
- The front end replaces the reformer: electrolyzer stacks and hydrogen purification, a nitrogen unit, and a hydrogen buffer that follows the power.
- Oxygen-in-hydrogen and hydrogen-in-oxygen are the safety analyzers on an electrolyzer and are usually trip functions.
- The synthesis loop, the refrigeration and the storage tank read like a conventional plant, with the high-pressure hydrogen trip layer that implies.
- Ammonia toxicity drives the gas detection; hydrogen's gas group drives the hazardous-area callouts.
- The package arrives in three or four conventions and the registers hold each as printed.
The train, section by section
| Section | What it does | Characteristic instrumentation |
|---|---|---|
| Power and rectification | Takes renewable or grid power to the DC the stacks need | Transformer and rectifier measurements on the one-line; stack current and voltage reported from the power supply; the plant controller's power set point |
| Electrolyzer stacks | Split water into hydrogen and oxygen | Per stack: electrolyte or water inlet and outlet TT, hydrogen-to-oxygen side PDT, oxygen-in-hydrogen and hydrogen-in-oxygen analyzers (AT-H2-0210 style), circulation FT; separator LTs; water conductivity AT |
| Hydrogen purification | Removes oxygen catalytically and dries the gas | Deoxo reactor inlet and outlet TT, oxygen analyzer after the deoxo, dryer switching valves with position, dew point analyzer at the dryer outlet, purity analyzer to the buffer |
| Hydrogen buffer storage | Decouples the stacks from the synthesis loop across the power's variability | Buffer PT (PT-HPS-1302), TT, inventory calculation, isolation and depressurization valves, hydrogen detection around the buffer |
| Nitrogen unit | Air separation or PSA nitrogen | Product purity analyzer (oxygen in nitrogen), product PT and FT, the ASU's own cryogenic instrumentation where one is used |
| Syngas compression | Compresses the 3:1 hydrogen-nitrogen mix to loop pressure | Suction and discharge PT and TT per stage, anti-surge FT and ZT, vibration and axial displacement, discharge PT with 2oo3 voting (PT-SGC-1105-2oo3), hydrogen-to-nitrogen ratio analyzer on the makeup |
| Synthesis converter and loop | Ammonia synthesis over iron catalyst at high pressure | Bed temperature profile per bed (TT-CNV-1302-B2 for bed 2), quench FTs and control valves, loop PT with voting, converter inlet and outlet TT, waste-heat boiler drum level and steam FT |
| Separation and refrigeration | Condenses and separates ammonia; refrigeration recovers the rest | Ammonia separator LT (LT-SEP-1101), refrigeration compressor PT and TT per stage, chiller levels and TTs, purge FT with inerts and hydrogen analysis |
| Refrigerated storage and loading | Atmospheric tank at about minus 33 C, boil-off reliquefaction, loading | Tank LT with an independent LSHH (LSHH-TK-2101), TT at several elevations, PT with pressure and vacuum protection, boil-off compressor, ammonia gas detection (AT-NH3-2305) at the tank and loading point, loading arm FT and ESD |
The front end
Stacks
Each electrolyzer stack is a repeated unit and the index is built by multiplying. A plant of several hundred megawatts has dozens or hundreds of stacks, and each carries the same small set: temperatures on the circulation circuit, the differential pressure between the hydrogen and oxygen sides that keeps the membrane or diaphragm balanced, and the two gas-quality analyzers. The electrical measurements come from the power supplies over a network rather than as hardwired loops, and the I/O list should say so on the row.
The oxygen-in-hydrogen analyzer deserves its own paragraph in any handover. A membrane or diaphragm failure lets the product gases cross, and hydrogen with a few percent oxygen in it is on its way to its flammable range inside the plant's own piping. The analyzer is drawn with an alarm and a trip, often duplicated or voted, and on most designs it is a safety instrumented function with an integrity level from the project's LOPA. Hydrogen-in-oxygen on the oxygen side is its mirror image.
Purification and the buffer
The hydrogen leaving the stacks is saturated and carries traces of oxygen. A catalytic deoxo reactor consumes the oxygen, a dryer removes the water, and a dew point analyzer at the dryer outlet, with a purity analyzer to the buffer, is the last gate before the gas is stored. The buffer itself is a pressure vessel or a bank of them, and its pressure is the plant's inventory: the plant controller runs the stacks against the power available and the synthesis loop against the buffer level, and the two never quite agree. The buffer PT, the isolation and depressurization valves, and the hydrogen detection around it are the tags that hold that arrangement together.
The synthesis end
From the syngas compressor onward the plant reads like a conventional ammonia plant, and the instrument signature is the same one a reformer-fed plant carries: a multistage compressor with its anti-surge and vibration, a converter with a bed temperature profile and quench control, loop pressure with voted transmitters, an ammonia separator, refrigeration, and a purge. The trip layer is the high-pressure hydrogen one: high-high loop pressure, compressor trips, low-low separator level, converter high-high bed temperature.
What is different is turndown. A conventional loop runs at design rate for months. A green loop is asked to follow the hydrogen supply, within the limits the catalyst and the compressor allow, and the control narrative has more modes and more interlocks between the buffer, the compressor and the converter than the licensor's standard sheet carries. The instrument index does not change much for this; the cause-and-effect matrix does.
Storage
The refrigerated tank sheet is the one the safety file reads first. A large atmospheric tank of liquid ammonia at about minus 33 C carries level with an independent high-high trip on the inlet, temperature at several elevations to watch stratification, pressure with both pressure and vacuum protection, and a boil-off compressor that reliquefies the vapour. Around it, and at the loading point, is ammonia gas detection at the low levels the toxicity demands, with the ESD on the loading arms and the transfer pumps. The ammonia gas group is less severe than hydrogen's for the hazardous-area callouts, but the toxic exposure limits are what set the detection layout.
Conventions on one package
A green ammonia project assembles a package from parties in different countries:
| Package | Typical convention | Example tag form |
|---|---|---|
| Electrolyzer supplier (often European) | IEC 81346 reference designations or KKS | =E1+Q1-F1 or 10LAC01CT001 |
| Nitrogen unit supplier | ISA 5.1 with the supplier's area codes | AT-PT-0105 |
| Synthesis licensor | ISA 5.1 with the licensor's numbering | TT-CNV-1302-B2 |
| EPC balance of plant | ISA 5.1 with the project's area codes | PT-HPS-1302 |
| Owner's existing complex, where there is one | The owner's house rule | Whatever the host plant uses |
The tie-in sheets between packages carry two conventions, and the registers hold each tag as its sheet prints it with the sheet reference on the row. A stack tag in IEC form and a loop tag in ISA form are both correct; a stack tag rewritten into ISA form is a tag the electrolyzer supplier's commissioning team will not recognize.
What the registers carry
For a green ammonia plant the useful columns beyond the standard instrument index are the package or supplier the tag came from, the stack or train number for the repeated units, the transport (hardwired or serial, because the electrolyzer power supplies and the plant controller talk over a network), and the SIS flag for the oxygen-in-hydrogen, loop pressure and storage functions. The equipment list is long on the front end (stacks, rectifiers, separators, dryers) and conventional on the back. The line list carries hydrogen, oxygen, nitrogen, syngas and ammonia services and the low-temperature class on the refrigeration and storage lines, and filtering on the service column produces the hydrogen subset the hazardous-area review asks for.
Tagsight reads the electrolyzer, nitrogen, synthesis and storage packages into these registers, in IEC 81346, KKS and ISA 5.1 as drawn, with every row referenced to its sheet and confirmed by the engineer before it exports. The integrity levels, the turndown strategy and the safety file remain the project's engineering.