Total Reduced Sulphur

Total Reduced Sulphur (TRS) is a combined measure of reduced sulphur compounds, typically including hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide. These compounds are strongly odorous even at low concentrations and are especially important in kraft pulp mills.

TRS is an emissions and odour-control term rather than a cleaning technology term, but it matters because the same process areas that generate TRS also create sticky and corrosive deposits.

Sources of TRS

In kraft mills, TRS can arise from digesters, evaporators, condensate systems, recovery boilers, smelt dissolving tanks, lime kilns, non-condensable gas systems and black-liquor handling. Collection and incineration of non-condensable gases are common controls.

Combustion conditions matter. Poor mixing, reducing zones, upset liquor firing or inadequate oxidation can increase TRS emissions. Lime kilns and recovery boilers can both be involved, depending on how gases are collected and burned.

Measurement and regulation

TRS is often measured after oxidation to sulphur dioxide or by specific analysers, with results reported as sulphur. Permit limits and test methods vary by jurisdiction and source type. Because odour complaints can occur at concentrations far below health-based limits, mills treat TRS as both an environmental and community-relations issue.

Sampling location, moisture, gas conditioning and calibration are important because reduced sulphur compounds are reactive and can be lost or transformed before measurement.

Cleaning relevance

Sonic horns do not remove TRS from gas. Their relevance is indirect: cleaner heat-transfer surfaces, ducts and hoppers help recovery boilers and lime kilns maintain stable combustion and gas flow. In Sylio-style pulp-and-paper work, TRS history is useful context because it points to sulphur chemistry, black-liquor carryover, non-condensable gas handling and deposit behaviour that may also affect fouling.

Plant sources and measurement context

TRS is most familiar in kraft pulp mills, where reduced sulphur compounds can come from digesters, evaporators, recovery boilers, smelt dissolving tanks, lime kilns, condensate systems and non-condensable gas handling. It can also matter in some chemical, refining and waste-treatment services. The compounds are odorous at very low concentrations, so a small leak or upset may be noticed off site before it looks significant in a process trend.

Measurement depends on the permit and plant design. Some sites use continuous monitoring on controlled vents or stacks, while others rely on periodic testing, odour investigation and combustion-system checks. Sampling is sensitive to moisture, condensation, line losses and oxidation, so TRS data should be read with the sampling method and averaging period in mind. The same total value can represent different mixtures of hydrogen sulphide, methyl mercaptan, dimethyl sulphide and dimethyl disulphide, each with different odour and combustion behaviour.

Control and cleaning relevance

Control usually means collecting non-condensable gases, keeping systems sealed, burning the gases in a recovery boiler, lime kiln, thermal oxidiser or dedicated incinerator, and maintaining scrubbers or condensate treatment. Safety reviews must consider toxicity, flammability, corrosion and confined-space entry where reduced sulphur gases can accumulate.

Acoustic cleaning does not treat TRS chemistry directly. It can be relevant around recovery boilers, evaporators, scrubbers or ducts if sulphur-bearing deposits contribute to fouling, poor gas flow or unstable combustion of collected gases. In those cases, the cleaning target is the deposit or flow restriction, while TRS control remains a gas-collection and destruction duty.

Related terms

• Recovery boiler
• Lime kiln
• Non-condensable gas