Mercer Stendal cuts emissions and boosts energy efficiency with innovative gas combustion project

At Mercer Stendal, innovation is driving cleaner pulp production. A new project co-combusting rich gas in the caustic (leaching) boiler has reduced emissions, improved energy efficiency, and strengthened operational stability—all without major infrastructure changes.

Challenges of traditional rich gas combustion

In pulp production, rich gas is a byproduct generated during chemical recovery. It contains energy but is difficult to handle. Traditionally, this gas has been burned in dedicated boilers that are sensitive to process fluctuations. When these systems fail or become unstable, the gas is either flared or released through the stack—causing odour issues and environmental impacts.

Co-combustion of rich gas in the leaching boiler

The project “Lean Gas Combustion at the Lye Boiler”, led by process engineer Stephanie Stein, focuses on a more sustainable technology that effectively solves these problems and ensures stable and continuous combustion.

To implement this technology, the lye boiler was equipped with a new burner port during the general overhaul in November 2022. The necessary peripherals, such as piping, valve stations and other plant technology, were then installed step by step. The final integration of the system was completed during the general overhaul in June 2024. After the necessary tests and inspections with the manufacturer Andritz and TÜV Nord, the plant was put into operation in October 2024.

Stable combustion, lower emissions
The new burner delivers up to 10.5 megawatts of thermal energy and now feeds directly into the caustic boiler, which operates at an average of 460 MW. Because the boiler’s main combustion process is independent, rich gas fluctuations no longer impact stability—ensuring continuous, controlled combustion.

This stability significantly reduces the need for emergency flaring or venting through the stack.

Even when the boiler runs at partial load (50–60%), the system can combust rich gas with minimal support. If the load is above 60%, no auxiliary fuel is needed at all, further boosting efficiency and cutting emissions.

The existing rich gas boilers remain operational and serve as a backup system. In addition to improving environmental performance, this solution offers the following advantages:

Efficiency gains across the board
Co-combusting rich gas in the caustic boiler brings multiple operational advantages:

• Higher-value steam: The process produces higher-quality steam (DH1), improving overall energy efficiency.
• Chemical recovery: Chemicals in the rich gas are reused in the leaching cycle, reducing the need for fresh inputs.
• Fewer emissions treatments: Unlike rich gas boilers, the new setup doesn’t require caustic soda to clean flue gas—sulphur stays in the process, keeping SO₂ emissions low.
• Improved reliability: The system is more robust, with fewer malfunctions and a more stable operation overall.

The technology used is now state-of-the-art and has already proven itself in other companies. “With the lean gas co-combustion project, we have not only improved our environmental balance, but also increased our energy and resource efficiency. This is a clear commitment to sustainable business practices,” says project manager Stephanie Stein.

A step forward in sustainable operations

The successful implementation of lean gas co-combustion marks a meaningful step in Mercer Stendal’s efforts to improve environmental performance and energy use. It also demonstrates how targeted innovation—led by experts like Stephanie Stein—can deliver measurable results. For the Energy team and across Mercer, this project sets a strong example for future upgrades that align efficiency with sustainability.