Mercer Celgar pulp mill's new chip craker (2020) near Castlegar, British Columbia, Canada

Cracking the Chip: Mercer Celgar’s Recipe for Success

Mercer is a dedicated global team of 2,300+ people who create bioproducts for a changing world. While our processes and global marketplace may be complex, our approach to operating our company is not. 

We believe in Mercer’s unified core purpose to align ourselves with what we believe is important in sustainability – a piece of that being the continuous improvement in resource efficiency.

Improving Fibre Yield – A 2020 Sustainability Goal

At Mercer Celgar, one of our sustainability goals for 2020 was to improve our fibre yield. Part of what it means for us to operate sustainably as a business involves the sustainable production and use of materials, energy, fuels, and chemicals. Improving our fibre yield means finding ways to efficiently and productively utilize the maximum possible amount of the wood chips (fibre) that come into our mill during our pulping process by improving our processing operations – allowing us to reduce operating costs and save valuable raw materials and energy. 

*Wood chips are created from using small to medium-sized pieces of wood that are formed by slicing or chipping from larger pieces of wood – from trees, branches, stumps, roots, and wood waste

As you may know, pulp is made using a continuous manufacturing kraft process. There are a lot of steps in this process, but one of the most important steps is making sure we have the right ingredients prior to making our pulp.

Once we have the wood chips onsite at Celgar, we make sure they are of the right quality and ready for processing. We ‘screen’ them which allows for the separation and removal of all the things that cannot be introduced to the pulping process. Like all cooking recipes, you need quality ingredients to make a quality product. So how do we remove unwanted items from our wood chips? 

Chip Screening

The chip screening process selectively separates contaminants from the chips. Contaminants include oversized or overthick chips which take too long to cook; chips that are too small or thin (pins) as they can cook too fast and plug the flow of the pulp through the continuous digesting process; we also remove the ‘dust’ we call fines as well as sand, stones, or scrap metal. These contaminants are hard on our equipment so we make sure they are removed prior to coming into the mill. 

After we’ve screened the chips, we then take the ‘unacceptable’ chips and process them to be of a size that is acceptable in thickness, known as “accept chips.” To be sustainable, we ensure that as much fibre or wood chips as possible are utilized in our kraft pulp making process. 

Why Does Chip Size Matter?

You may wonder, “Why does a wood chip that is a little larger or a little thicker need to be removed?” The size of the wood chips is very important during the pulping process due to the effects on end-pulp quality, pulp yield (amount of pulp we produce), and digester operation as well as post-digester handling. We cook our chips in a big kettle we call a ‘digester’. 

The chips go into the digester after being screened and while in this digester, we add white liquor and heat to soften the chips and turn them into a brown, porridge type substance (pulp). It’s a very specific recipe, so the perfect ingredients matter – and that includes the right size of chip to be cooked for the right amount of time at the right temperature.

It’s Like Cooking a Turkey

When you want to cook a large turkey,  you may decide that a longer cooking time at a high temperature is going to be needed to ensure the centre of the turkey is cooked perfectly. You turn up the temperature, set the timer, and what happens? The outside of the turkey will be overcooked and inedible. And then you think about the cost of not having the right recipe for your turkey.

When you analyze the cost of not following a recipe, you may consider the cost of transportation – driving to get the turkey, time to select it, buying it, and transporting it home. You also had to purchase a large number of ingredients to add to the cooking process. You took the time to prepare it, with additional equipment and helpers; and then with the time it took to get the turkey into the oven, you had to use power and energy to cook it. 

But because you didn’t follow instructions, the turkey was overdone and inedible and you had to do it all over again – higher costs and your guests are upset because they wanted a perfect turkey dinner. This is very costly – you’re frustrated, out more money, and your guests might never come back to your house to have your turkey again. It’s the same in our pulping process.

Perfect Chips: Perfect Pulp

As wood chips are the main ingredient in the making of pulp, if Mercer were to use overthick chips in the pulping process, the cooking white liquor would not fully penetrate the centre of the chip and the result would be an uncooked centre.  To make sure the centre of the chip was cooked, we would then have to recook the chips (wasting both time, energy, and we would have to use more chemicals). 

If we were to compensate by using longer cook times in our digester, the result would be a fully cooked centre of the chips but then, like the turkey, we would have overcooked the outer parts. This would lead to a lower pulp yield and lower pulp strength. As the pulp is actually just a combination of short or long fibres, if we cook it too long or with too many chemicals, the fibres will break or become weak. 

Our recipe for pulp must be perfect. The key to the best possible cook of pulp in our digester that will consistently produce the best possible yield is to use the proper size of wood chips. 

Cracking the Chip: Improving Our Pulp Recipe One Chip at a Time

In order to efficiently utilize as much of our valuable fibre resources as possible, we undertook the installation of a Chip Cracker in 2020. 

This piece of equipment is appropriately named. The Chip Cracker will gently crack each overthick chip using rolls that the chips go through. The stainless steel rolls then perforate or crack them, allowing the white cooking liquor to penetrate into the chip more easily in the digester. With better penetration of the white liquor, the chips will cook at a rate very similar to chips created using the traditional slicing method. An additional advantage of the chip cracker process is that the overthick chips create very few pins, fines, or damage the wood chip fibres

The installation of the chip cracker allows us to realize annual savings in reduced maintenance and increased yield, all while saving valuable raw materials and energy – making our process at Mercer Celgar more Sustainable. By design.