Opsis blog

The smell associated with pulp production originates from different sulfur compounds. The most well-known is probably hydrogen sulfide, H₂S (H-S-H) but there are also compounds like methyl mercaptan (CH₃-S-H), dimethyl sulfide (CH₃-S-CH₃), and dimethyl disulfide (CH₃-S-S-CH₃). These four substances are collectively referred to as “total reduced sulfur”, TRS. Emissions of TRS together with the usual combustion gases SO₂ and NO_X (NO and NO₂) are regulated and often monitored continuously at pulp production facilities.

Pulp and Paper Production

The production process starts with pulp wood. This is turned into wood chips, mixed with chemicals, and boiled. This separates the cellulose fibres from the lignin and other materials. The cellulose is caught and aggregated to pulp. The pulp is then forwarded to the actual paper production where it can be further treated to give the paper the desired properties. Subsequent pressing and drying forms the final paper products.

The dominating method for the process stage where the cellulose is extracted is the sulfate process, also known as the kraft process. The active chemicals are essentially a mixture of sodium sulfide (Na₂S) and sodium hydroxide (NaOH), known as white liquor. The chemicals are consumed in high volumes and must therefore be recycled both for cost and environmental reasons. Several of the process steps at a pulp mill is about this recycling.

What remains after extraction of the cellulose is a mixture of water, sodium compounds, and other chemicals known as black liquor. Most of the sulfur from the sodium sulfide has been oxidised to sodium sulfate (Na₂SO₄) and the remaining sodium mostly appear as sodium carbonate (Na₂CO₃).

Chemical Recovery

In a recovery boiler, there is a reaction between sodium sulfate (Na₂SO₄) and coal which restores the original sodium sulfide (Na₂S). By further adding calcium hydroxide (Ca(OH)₂) the sodium carbonate is converted back to sodium hydroxide (NaOH). The white liquor is thereby restored and can again be used to extract the cellulose from the wood chips.

The calcium is also kept in a closed loop where calcium carbonate resulting from the NaOH regeneration is fed into a rotating lime kiln of the same type used at cement production. This generates burnt lime, CaO, which by reaction with water then is converted to the sought-after calcium hydroxide.

TRS can occur as a side product in several process steps, but as far as possible, the gases are typically collected and led into the recovery boiler where they are dissociated by incineration. The odours from TRS can thereby be prevented or at least notably reduced. A separate TRS burner can be standby to take care of the TRS gases if the recovery boiler temporarily goes off-line.

Regulations on Emissions Monitoring

Within the European Union, the emissions from most industries are controlled by the Industrial Emissions Directive, the IED. This applies also to the pulp and paper industry. Here, the specific requirements on monitoring and emission limits are found in the “PP-BATC”, which reads out as “best available technique conclusions for the production of pulp, paper, and board”.

When it comes to the recovery boiler, there are requirements on continuous emissions monitoring (CEM) of at least NO_X, SO₂, and TRS, and in some cases also of dust. This may also apply to the lime kiln and to any separate TRS burner.

The OPSIS TRS Monitor

As already noted, TRS is the name of a group of molecules whereof some are relatively complex. TRS can therefore not be measured directly (“in situ”) using the proven OPSIS open-path DOAS technique. Instead, the OPSIS TRS monitoring system is based on extraction of a relatively small flow of flue gas from the duct. In a first step, all SO₂ is removed from the gas mixture using a selective scrubber. In a second step, all remaining sulfur compounds, effectively TRS, are transformed into new SO₂ by means of a high-temperature converter. By monitoring the resulting SO₂ concentration, which is straight-forward using the DOAS technique, the TRS concentration expressed as sulfur can be established.

In the basic configuration, a TRS monitoring system from OPSIS has a single measurement cell and an AR600 gas analyser to measure the SO₂ concentration. However, in many cases, also other types of pollutants should be monitored. The basic system is then supplemented with another measurement cell, resulting in a cost-efficient monitoring solution with low maintenance needs. OPSIS also offers CEM systems monitoring H₂S only, using a slightly different converter configuration.

The OPSIS TRS systems delivered so far have all been of the type also monitoring other gases and therefore having dual measurement cells. With our cost-effective, low-maintenance, multi-component gas monitoring system, we are confident that we offer the best CEM solution on the market for the pulp industry.