ANDRITZ offers the Wet gas Sulfuric Acid (WSA) process for kraft mills

The Wet gas Sulfuric Acid (WSA) process, developed by Haldor Topsoe and offered by ANDRITZ, creates an internal sulfuric acid production cycle in a kraft mill, which decreases the costs of make-up chemicals and reduces sulfur effluents. This enhances both environmental and economic performance of the kraft pulping process. The benefits of the WSA process are best achieved in combination with ANDRITZ Lignin Recovery technology, but there are many other instances where in-mill production of commercial-grade sulfuric acid is desirable.

ANDRITZ adaptation of WSA technology for kraft mills

The WSA process is not new in industry, but its application within a kraft pulp mill is. There are over 130 references for WSA technology around the world handling a variety of sulfurous gas streams as well as spent sulfuric acid. The technology itself is proven, is optimized, and is readily available for direct application in the pulping industry.

WSA benefits

The product of the WSA process is commercial-grade concentrated sulfuric acid. The WSA process complies with even the strictest air emissions requirements, and it does not produce any solid or liquid waste. Depending on the technical circumstances, a significant amount of energy in the forms of superheated steam may be available from the WSA process.

The on-site production of sulfuric acid means that a kraft mill can significantly reduce – or eliminate -- the need for fly ash dumping in order to rid the mill of excess sulfur. This would also avoid large sodium losses in the kraft cycle, decreasing the amount of purchased make-up NaOH required. The cost savings are more significant in situations where the internal production of sulfuric acid opens up the possibility of also utilizing the salt cake from the ClO₂ generator as a make-up chemical. In addition to economic benefits, the environmental benefits are very real as the total volume of effluent and solid waste is reduced to large extent.

The basic WSA process

In the WSA process, commercial-grade sulfuric acid is produced on-site from a mill’s Concentrated Non-Condensable Gases (CNCGs) by combustion, catalytic conversion and condensation. More than 99.9% of the sulfur in CNCGs can be converted to concentrated sulfuric acid, if required.

Sulfuric acid is generated in a mill by first oxidizing the CNCGs into sulfur dioxide in an incinerator, then further oxidizing the sulfur dioxide into sulfur trioxide over a catalyst, and finally condensing the sulfur trioxide together with the water vapor that comes from the kraft process with the CNCGs.

Since the process is operated above the acid dew point from incineration all the way to the condenser, the hardware can be constructed out of standard materials, lowering the capital cost, without the risk of corrosion within the system. Due to optimized conditions, the process is not sensitive to the composition of a particular CNCG stream from a particular mill, or to its moisture content.

Combination with Lignin Recovery

When producing high-quality lignin, the amount of sulfur in the chemical recovery cycle is considerably increased due to the large amount of sulfuric acid consumed in the acid washing sub-process. When excess sulfur is dumped, the amount of NaOH required as make-up chemical is high. For this reason, it makes sense to combine a Lignin Recovery system with the WSA process, which is an economic and compatible way to remove the excess sulfur from the recovery cycle and process sulfur compounds to recyclable concentrated sulfuric acid.