Selecting a Biogas Desulfurisation Process: The Limits in GB/T 51063-2014 and What Each Route Can Achieve

1. Which Standard Applies, and at What Scale

GB/T 51063-2014, Technical code for large and medium-scale biogas engineering, was approved and issued by the Ministry of Housing and Urban-Rural Development under Announcement No. 672 on 2 December 2014 and entered into force on 1 August 2015. Clause 1.0.2 defines its scope as the design, installation, acceptance, operation and maintenance of biogas projects treating agricultural organic waste, high-strength industrial organic wastewater, industrial organic residues and sludge by anaerobic digestion, with biogas production of not less than 500 m3/d and gas supply as the primary purpose.

Clause 3.0.2 adds thresholds by end use: not less than 500 m3/d for domestic supply, not less than 1200 m3/d for power generation, and not less than 10000 m3/d for upgrading and compression. A project producing a few thousand cubic metres of biogas per day therefore falls under this code and should not be fitted with a generic off-the-shelf purifier by rule of thumb.

2. Limits: The end use changes everything

Clause 3.0.3 requires biogas quality for domestic use, power generation and upgrading to comply with Table 3.0.3. That table sets the lower heating value at not less than 17 MJ/m3 and hydrogen sulphide at not more than 20 mg/m3 for centralised domestic supply and not more than 200 mg/m3 for power generation, with the water dew point for upgrading left to agreement between the parties. (Table values were taken from public compilations of the standard text; verify them against the current official edition before use in a project.)

This single limit drives the whole selection. Both cases are described as meeting the hydrogen sulphide limit, but centralised domestic supply has to reach a 20 mg/m3 polishing threshold while generation accepts 200 mg/m3 - one order of magnitude apart in both capital and operating cost.

3. What the Clauses Demand

Clause 4.4.1 is unambiguous: biogas from the anaerobic digester shall be desulfurised and dehydrated, with the process chosen by technical and economic comparison according to end use, flow rate, quality targets and local ambient temperature. Clause 6.5.4 of HJ 2013-2012 gives the same mandatory direction - biogas shall be dehydrated and desulfurised before entering utilisation equipment - and clause 6.5.3 requires cleaning and utilisation design to comply with NY/T 1220.1, NY/T 1220.2 and GB 50016.

Clause 4.4.2 offers three routes: biological desulfurisation, dry-bed desulfurisation or wet scrubbing. Clause 4.4.3 covers staging: where one stage cannot meet the quality requirement, two stages shall be used, and the second stage should preferably be dry-bed desulfurisation.

Position in the process train

• 4.4.4-1: biological desulfurisation shall be placed ahead of the dehydration unit, because inside the biological tower the gas must contact the nutrient solution intensively and leaves saturated, so drying has to follow.
• 4.4.4-2: dry-bed desulfurisation shall be placed after dehydration, because excess moisture makes the bed damp, caked or muddy, degrading contact and efficiency; the iron oxide reaction is exothermic and produces water, which condenses downstream, so a second drying step is advisable after the bed.
• 4.4.4-3 to 6: a standby unit shall be provided so towers can be switched while the medium is replaced; valves shall be fitted upstream and downstream; sampling ports shall be reserved; spent medium and sulphur sludge shall be disposed of in line with environmental requirements.

4. What Each Route Can Achieve

The clause explanation to 4.4.2 grades the processes, and it is the most useful passage for selection:

• Biological (rough removal): colourless sulphur bacteria such as Thiobacillus thiooxidans and Thiobacillus ferrooxidans oxidise hydrogen sulphide to elemental sulphur under micro-aerobic conditions; with suitable temperature, contact time and air rate hydrogen sulphide can be reduced to 75 mg/m3, removing most of it, which the code calls rough removal.
• Dry bed (fine removal): at ambient temperature the gas passes through the bed, hydrogen sulphide reacts with active iron oxide to form iron sulphide, and exposure to air regenerates the medium; the cycle can be repeated two to three times until the pore surface is blocked. After dry-bed treatment hydrogen sulphide can be below 20 mg/m3, which the code calls fine removal.
• Wet scrubbing: a wet oxidation process using alkaline solution with a desulphurisation catalyst absorbs hydrogen sulphide, regenerates with self-aspirated air and floats elemental sulphur off as froth. Treated gas can be below 50 mg/m3, and the code notes from survey data that wet scrubbing shows clear technical and economic advantages on plants producing more than 10000 m3 per day.

Matched against the limits in Table 3.0.3, the selection logic becomes direct:

1. Power generation, up to 200 mg/m3: often met by a single biological stage, which also reduces load on any downstream step.
2. Centralised domestic supply, up to 20 mg/m3: biological treatment alone is insufficient; stage a dry-bed unit downstream, which is exactly what clause 4.4.3 anticipates.
3. High hydrogen sulphide and large flows: let wet scrubbing carry the bulk load to roughly 50 mg/m3, then decide whether dry polishing is needed.

The clause explanation also contains an important negative constraint: biogas destined for upgrading and compression shall not be treated by direct air injection. Air brings nitrogen into the gas, and upgrading requires methane above 90 percent, so the nitrogen would then have to be removed - clearly uneconomic. Both biological desulfurisation and wet scrubbing with air regeneration involve air injection, so any project feeding an upgrading line must check this point.

5. Hard Parameters for Dry-Bed Towers

Clause 4.4.6 gives values that can be checked directly:

• 4.4.6-1: iron oxide medium is preferred, with a space velocity of 200 to 400 per hour; the explanation notes that higher space velocity shortens contact time and that the choice must weigh hydrogen sulphide concentration, operating temperature and working zone height.
• 4.4.6-2: pressure drop across each metre of fresh medium shall be less than 100 Pa, controllable through the inlet velocity.
• 4.4.6-3: each layer of granular medium should be packed 1.0 to 1.4 m deep; the explanation adds that beds deeper than 1.5 m should be split into layers to avoid channelling, with a tower height-to-diameter ratio generally of 4:1 to 3:1.
• 4.4.6-4: linear velocity through the granular medium should be 0.020 to 0.025 m/s; too low a velocity leaves the gas stagnant, while increasing it thins the gas film and improves removal.
• 4.4.6-5: operating temperature 25 to 35 degrees Celsius, with insulation or heating in cold regions.
• 4.4.6-6 to 8: a drain valve at the lowest point of the tower; an independent vent line for every unit; an air inlet for in-tower regeneration, with online oxygen monitoring where regeneration is performed in service.

A practical warning is warranted. Space velocity, linear velocity and layer depth are mutually constraining: the medium volume follows from V = Q / space velocity, the cross-section from A = Q / (3600 x linear velocity), and the layer depth from h = V / A. All three must be checked together. The published clause explanation does not offer a worked combination satisfying all three, so the equipment specification should demand a calculation showing all three, not just a quoted media volume.

To show the arithmetic without assuming consistency: for Q = 500 m3/h and a space velocity of 250 per hour, the required medium volume is V = 500 / 250 = 2.0 m3. That figure satisfies only the space-velocity check; the cross-section and layer depth still have to be verified separately against linear velocity and the 1.0 to 1.4 m per layer rule.

6. Biological Route: Control Points and Red Lines

Clause 4.4.5 sets out:

• 4.4.5-1: system generally comprises the biological tower, circulating water tank, circulation pump, blower, sludge discharge pump and dosing pump.
• 4.4.5-2: the tower shall be easy to clean, maintain and service, with inspection windows and manholes, because elemental sulphur tends to clog the packing.
• 4.4.5-3: the circulation tank shall have a temperature sensor and heating.
• 4.4.5-4: the downstream gas line should carry online oxygen monitoring linked to the blower, with residual oxygen below 1 percent. The explanation notes that the explosive range depends on methane content, making below 1 percent oxygen a basic requirement.
• 4.4.5-5 and 6: the nutrient solution shall sustain the desulfurising bacteria, and performance shall meet the process requirement.

Regeneration and operating safety are covered too: the explanation to 4.4.6-8 requires the air admission rate during online regeneration to be controlled so the tower does not heat rapidly and deactivate the medium. Related operational requirements include respiratory protection and ventilation when replacing medium, and wetting of spent medium stored outdoors to prevent self-ignition.

7. Dehydration Must Not Be Overlooked

Clause 4.4.7 prefers cold drying for dehydration, allowing gravity separation or solid adsorption, and requires: a separator or condensate trap before cold drying or adsorption units; a trap at the lowest point of the gas line before drying; and a dew point measurement port on the outlet line. The explanation points out that condensate not removed ahead of the dryer will block pipes in winter and can affect valve operation, and that dew point monitoring is necessary to verify that the delivered gas meets the quality requirement.

8. A Selection Sequence You Can Copy into a Design Report

1. Fix the end use and flow: check the scale thresholds in clause 3.0.2 of GB/T 51063-2014 (500, 1200 and 10000 m3/d) to confirm the project falls inside the code.
2. Fix the outlet target from Table 3.0.3: 20 mg/m3 for centralised domestic supply, 200 mg/m3 for generation, subject to confirmation against the official text.
3. Choose the main process from inlet concentration and flow: wet scrubbing for large flows and high concentrations, biological rough removal for most conventional duties.
4. Where the target is 20 mg/m3, stage two units under clause 4.4.3 with the dry bed second.
5. Set the position relative to dehydration under clause 4.4.4: biological ahead of it, dry bed after it.
6. Check the dry tower against clause 4.4.6: space velocity 200 to 400 per hour, initial pressure drop below 100 Pa per metre, layer depth 1.0 to 1.4 m, linear velocity 0.020 to 0.025 m/s, temperature 25 to 35 degrees Celsius, insulation in cold regions.
7. Provide biological monitoring under clause 4.4.5: residual oxygen below 1 percent interlocked with the blower, plus temperature sensing and heating on the circulation tank.
8. Do not bypass clause 6.5.4 of HJ 2013-2012: biogas shall not enter utilisation equipment until dehydration and desulfurisation are complete.

Sources: clause numbering corresponds to chapter 4.4 and clause 3.0.2 of GB/T 51063-2014 issued under Ministry of Housing and Urban-Rural Development Announcement No. 672, and the mandatory dehydration and desulfurisation requirements in clauses 6.5.3 and 6.5.4 of HJ 2013-2012. The specific limits in Table 3.0.3 were compiled from multiple public renderings and should be confirmed against the current official edition before use.