28 TAPPSA JOURNAL | VOLUME 3 2013

GSA INSTALLATION

FLSmidth installs gas suspension absorber at Mondi Richards Bay

The GSA, which will be in operation by October 2013, is being supplied via an engineering and supply contract and follows in the wake of a previous order placed with FLSmidth for electrostatic precipitator upgrades on Mondi’s recovery and power boilers.

FLSmidth’s regional manager – Sub-Saharan Africa, Christoffer Dyre says this is the company’s first GSA reference site in Africa, although the technology is widely used in other parts of the world.

“This technology is capable of reducing sulphur dioxide (SO2) emissions by as much as 97% and is superior to dry scrubber technology that is associated with far higher operating expenditure. The GSA effectively prepares Mondi for the imminent, more stringent environmental regulations associated with the National Environmental Management Air Act,” Dyre says. “The Act legislates and standardises the quality of air in South Africa in line with international best practice. In a stepped approach, existing plants must comply with certain plant emission standards by April 1 2015.

“With its ability to comply with extremely low emission

FLSmidth, a leading One Source supplier of complete plants, equipment and services to the global minerals and cement industries, is installing a gas suspension absorber (GSA) at Mondi’s Richards Bay mill that produces two of the company’s key products — Baycel, a premier grade bleached hardwood pulp made from 100 % eucalyptus fibre, and Baywhite, a white top kraft linerboard.

levels, our GSA technology is built for the future. One of the reasons for this high efficiency is that the reactor is based on gas suspension technology. This means that a very large concentration of fly ash, dust particles and lime builds up inside the reactor — normally 50 to 100 times higher than in a conventional reactor. The effect is high removal degree and low operating cost of the GSA system.”

A modular design allows the GSA installation to be erected in a short time and offers a great deal of flexibility, while maintenance costs are low compared to more conventional systems. This is primarily owing to the minimal wear and tear achievable from having fewer moving parts and a simple stationary nozzle.

THE SYSTEM

The GSA’s flue gas duct system incorporates all ducts required for transporting process flue gas to the reactor and to the stack. The GSA reactor system comprises an inlet bend, a venturi and a riser section. Flue gas passes through the venturi via the inlet bend and into

GSA INSTALLATION

▲ FLSmidth’s gas suspension absorber, FLS-GSA™

the reactor. In the reactor pollutants are removed by chemical reactions with the injected lime. The inlet bend serves to ensure proper distribution of the flue gas in the venturi via guide vanes located in the bend. The cross section of the duct is narrowed in the venturi to increase the linear flue gas velocity and ensure that solid material is kept in a fluidised bed in the riser section.

A dual fluid nozzle is installed in the venturi to allow dosing of fresh lime slurry and water into the riser section. The nozzle was developed by FLSmidth and is

a pressurised-air atomising nozzle that has proved very sturdy, hard-wearing and non-choking.

The main part of the flue gas treatment takes place in the riser section through close contact between the lime and the flue gas. In this section the lime reacts with the acid constituents in the flue gases, capturing and neutralising them. As a result of the very large reaction surface formed by the fluidised bed, contact between the lime and the acid constituents in the flue gas is very efficient and the degree of acid removal is correspondingly high compared to other systems.

Water is injected into the riser section through the nozzle to cool down the flue gas. The main part of the cooling process takes place through evaporation of water from the wet, recirculated material. The amount of water is adjusted to maintain the required temperature in the purified flue gas. This temperature must be as low as possible, owing to the slurry’s increased ability to absorb acid components in the flue gas at decreasing temperatures. However, a temperature that is too low increases the caking tendency of the re-circulated material in the reactor and it is only when striking a temperature compromise between these two properties that optimal process temperature is achieved. The control system of the GSA ensures the optimum balance is kept at all times.

In the riser section the flue gas velocity is relatively high and some of the solid particles are transported by the flue gas to the top of the riser section and into the cyclone, where most of the particles are separated

This technology is capable of reducing sulphur dioxide (SO2) emissions by as

much as 97% and is superior to dry scrubber technology that is associated

with far higher operating expenditure. The GSA effectively prepares Mondi for the

imminent, more stringent environmental regulations associated with the National

Environmental Management Air Act.

TAPPSA JOURNAL | VOLUME 3 2013 29

30 TAPPSA JOURNAL | VOLUME 3 2013

from the flue gas. Approximately 99% is captured. The captured particles are returned to the reactor via a re-circulation box that creates a buffer to maintain absorption capacity.

The re-circulation box comprises a metal box with two screw conveyors at the bottom of the box to transport solid material back into the riser section. A screw conveyor at the top bleeds out the excess fly ash and material formed by the chemical reactions. The bottom screw conveyors are controlled by a frequency converter controlled by the flow to the GSA, while the top one operates at constant speed.

An existing ID-fan between the dust filter and the stack maintains the desired pressure level in the boiler and overcomes the pressure drop generated in the ducts and the dust filter. A booster fan will overcome the additional pressure drop generated by the GSA.

The GSA process works with burned lime as the absorbent. Burned lime is initially made to react with water to form a hydrated lime slurry in a specially designed slaking unit.

From the slaker, the lime slurry is transferred to a slurry tank and from there lime slurry is dosed into the reactor with the dosing pumps.

CONTROL SYSTEM

The GSA is equipped with an automatic control and monitoring system that ensures a safe, stable and economically optimal operation by continually controlling essential process parameters. All essential process parameters are provided with alarm levels and are continuously monitored by the control system.

“During the commissioning period the FLSmidth team will implement a training programme to train Mondi personnel to operate and maintain the equipment for optimum performance,” says Dyre. “We will also back up the installation by making troubleshooting and general services available 24/7.” ■

▲ The dual fluid nozzle allows dosing of fresh lime slurry and water into the riser section

GSA INSTALLATION