Liquor gun: A dangerous cost driver for recovery boiler maintenance. But it’s the key to a sustainable and cost-effective pulp mill

You have to keep costs for your daily operation down? Pay more attention to your liquor gun: it is the key tool to keep the reduction degree and therefore the costly makeup chemicals as well as emissions levels in the required range.

Special attention must be kept on recovery boiler and its operation, which is the heart of each pulp mill. The liquor gun is pacemaker of the boiler and triggers the main KPI of the boiler operation. Facing liquor gun errors may cause massive problems on the boiler and even a boiler stoppage. To prevent liquor gun errors in future we are researching and trailing with prototype liquor guns.

What are the errors and how they are affecting the boiler?

We are faced with several problems concerning liquor guns. During our site visits and research activities we observed that errors arise from dissipated material and the loss of spraying pattern.

The temperature as well as the pressure of the liquor and the liquor properties itself are causing high wear on the surface of the bends and other parts of the liquor gun.

Also the plugging of the guns is a critical topic. The scaling inside the liquor guns is also an uncontrollable variable and must be checked constantly. All these problems can even cause a boiler stoppage event and lead to production losses.

Furthermore “spoon burning” has to be mentioned as a serious liquor gun problem. It causes the total loss of spraying pattern and a massive grow of the char bed.

All these stated errors are able to cause a boiler stoppage event, which can lead to massive production losses.

 

Are there solutions?

There are several methods to avoid these issues.

For example proper inside and outside cleaning, by selection of materials with higher wear resistance or the design of the liquor gun.

All these proposals will lead to lower maintenance and operating costs.

What is a recovery boiler?

A recovery boiler – often called soda recovery boiler or chemical recovery unit – is the heart of a pulp mill. It’s responsibility is to make the Kraft process economical by recovering the process chemicals and the production of energy from process waste. The gained energy is able to cover the whole energy consumption in a modern pulp mill.

A recovery boiler system has high operating costs and causes high asset costs as well. Furthermore, it effects significant personal risks in a mill. Due to these, a careful handling is required.

How the boiler is fired?

The boiler is fired with black liquor, which contains the process chemicals and the glue of the tree – the so called lignin. Black liquor is a product of the digester which separates fibre from the wood. The process chemicals are mainly inorganic and therefore not combustible whereas the dissolved wood is organic and combustible.

The weak black liquor is evaporated from a low consistency to a concentration higher than 75% dry solids. The resulting strong black liquor is pumped to the as fired liquor tank and than to the liquor gun. The liquor gun is an equipment which serves as a burner for the black liquor and is one the most important parts for the combustion in the boiler.

What is the liquor gun and how it is working?

The liquor gun is a device which is mainly out of high alloy stainless steel to resist high temperatures and the high pH value of black liquor.

There are several types of liquor guns, for example the splash plate gun or the beer can gun.

Let’s take the splash plate gun for explanation. This special type consists of a flange which connects the gun with a clamping device. The pipe is guiding the liquor to the spoon or the so called splash plate, which creates the spraying pattern . The structure of the pattern, formed by the splash plate, depends on the liquor properties, e.g. viscosity, temperature or pressure.

  1. Splash plate guns
  2. Beer can guns

The spaying pattern, as well as the resulting droplet size,  is one of the main factors how the combustion takes place in the recovery boiler. A reliable combustion is also a  significant factor for the economy of the mill, due to the resulting recovery degree of the chemicals.

What is influencing the spaying pattern when pressure and temperature of liquor are chosen right?

If pressure and temperature of the liquor are chosen right, the spaying pattern is mainly influenced by the cleanliness of the guns. Therefore, a manual cleaning of the gun is quite often required. Modern boilers are equipped with liquor gun rodders, for a automatically gun cleaning. Often this automatic rodding systems are only removing the coarse deposits on the liquor gun and doesn’t remove the scaling which is growing on the splash plate. These deposits often looks like icicles, which are growing on the splash plate.

What do we want to achieve?

We want to prevent known errors in the future and give the clear possibility to lower the maintenance as well as the operational costs. Further a clearer indication of boiler operation can be given to prevent future errors and stretch the boiler life time. This service can give huge possibilities to improve production volumes, lower the shuts or shut down times and therefore increase the turnover of the entire mill.

If you are facing one of these problems, you are cordially invited to reach out and discuss your case with one of our experts to increase the productivity of your mill and your production output.

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