Meeting the challenges of one pass processing

December 12, 2018

One-pass debarking trial in Zululand with the new SP 661E head.

Mondi Forests has been on a mission to reduce the number of passes that harvesting heads are required to de-limb, de-bark and cross-cut the stems of Eucalyptus trees, while still meeting the log delivery specs demanded by the Richards Bay mill.

This initiative, which is just one component in Mondi’s on-going efforts to continuously improve their forestry operations, has seen the number of passes reduced over the past few years from five to three. Now the focus has shifted to one-pass processing.

The big benefit of reducing the number of passes that the harvesting head is required to make to produce a log acceptable to the mill is the productivity improvement. Fewer passes means reduced costs, reduced fuel burn, less wear and tear on equipment and more tons processed per shift. It also means reduced stem damage, which can account for up to 2% fibre loss and impacts negatively on the quality of chips and the pulping process.

Mondi has benchmarked its forestry operations on the Brazilians where one-pass mechanical harvesting is the norm. However conditions in Brazil are well suited to one-pass processing as they grow a single Eucalyptus hybrid with uniform stem form in conditions that do not vary greatly across the landscape in terms of rainfall, temperature and altitude.

South Africa, as every forester knows only too well, is a different story. We grow several different Eucalyptus species and hybrids in conditions that vary considerable from flat, wet coastal areas to higher altitude mountain slopes and valleys with wet summers and dry, cold winters.

As a consequence there is great variation in the bark stripability from species to species, from location to location and from month to month.

Foresters examine harvested timber at the Focus on Forestry field day in the KZN Midlands last year, after processing by the Log Max E6.

The improvement that has already been achieved in Mondi’s harvesting operations is the result of a closer collaboration between their technical foresters, contractors and equipment suppliers. It has required a change in mindset as well as an ability to get more performance out of harvesters and harvester heads in particular.

“We started playing around with the head technology, trying different feed rollers and different pressure settings in different conditions,” explained Mondi’s harvesting technical manager, John Eggers.

“Two years ago we started three passes, now we’re ready to reduce it further to one pass, but in order to do that we need to change the current head design.”

This, too, is now happening, and all of the leading head manufacturers, including Waratah, Log Max, SP and Ponsse, are coming out with new generation heads capable of processing Eucalyptus in one pass.

“We know we’re not going to be able to do it everywhere all the time, but we’re looking to do it where conditions are right,” said John.

The new heads typically have two feed rollers instead of three, and depend more on the knives to do the de-barking work rather than the rollers. This means less pressure on the rollers and therefore less damage to the log. Reducing the number of passes that the head is required to make is key in reducing stem damage and fibre loss.

A number of trials and field days have been taking place as Mondi foresters, contractors and equipment suppliers and manufacturers have been working together to make one-pass harvesting a reality.

The results, according to John, are encouraging. He said that all new Mondi harvesting contracts coming up would be phasing in the new heads. Meanwhile the existing heads are being modified and retro-fitted with bottom knives to optimize them for reduced passes.

“We’ve reached a point where we know it works – now we are implementing it,” said John.

He said that harvesting contracts in future would be more site specific, either working on a one-pass rate or a three-pass rate. He expects that 50-60% of total volumes harvested on Mondi plantations would be processed in one-pass.

Upgrades that have taken place at Mondi’s Richards Bay mill over the past few years have also played a role in the reduced passes initiative. In the days of manual harvesting 0% bark on the incoming logs was the standard. Thus the first mechanical harvesters would feed the logs through the heads repeatedly until all the bark was off. This could mean up to 11 passes on E. macarthurii in winter, for example, leading to excessive stem damage and fibre loss, and an unacceptable drop in productivity. New log handling equipment installed at the mill has increased the bark tolerance marginally to accommodate the dynamics of mechanical harvesting, but the goal is still 0% bark. The international benchmark is .5% bark.

John emphasized that Mondi Forests would not dictate to contractors which brand of head they should use, and would leave it to the individual contractors to make their own choices. He said his team has engaged with all the major manufacturers that have come up with new generation heads capable of one-pass harvesting, and are busy doing trials to test their performance in different conditions.

The Waratah H215E in action during the trial at Dukuduku, Zululand.

Waratah H215E study
A recent study conducted at SiyaQhubeka’s plantation at Dukuduku in Zululand compared one pass with the new Waratah H215E head vs three passes with the Waratah H616C head which is currently used in the Kwambo operation.

Various methods of processing were tested, and the felled timber was laid out for direct comparison.
Productivity increases recorded by the H215E mounted on a Kobelco excavator ranged from as high as 29% in 0,3 m3 trees to negligible on smaller 0.05 m3 trees. Increased stem damage on the three pass treatments was also noticeable.

Single pass treatments with the H215E head resulted in stems with an average of 0.35% bark remaining vs the H616C three pass treatment resulting in 0.41% bark remaining. (Single pass treatments with the H616C left significantly more bark on the stems.)

The knives do the debarking ... Phillip Hall of Forestry Plant and Equipment uses the Waratah H215E mounted on an excavator in the Lowveld.

SiyaQhubeka harvesting forester Minenhle Mdlalose explained how the rollers and knives have a huge impact on the quality of logs. She said that the H616C has two knives and three rollers. The de-barking is done mainly by the rollers and the more passes the greater the stem damage.

By comparison, the new generation H215E has four knives and two rollers, and is similar to the heads used in Brazil. She said that the log is held by the knives which do most of the de-barking work. The bottom knife also helps to prevent the bark clogging up inside the head and ending up in the log stack.

Commented Jules Larsen of Waratah: “The H215E was designed from a worldwide team with the aim of single pass Eucalyptus debarking. It excels in the straight clonal species all around the world and we considered this to be ideal for coastal South African timber.”

The Ponsse H7 Euca doing an efficient debarking job.

Ponsse has the H7 Euca head doing trials in different locations to test its processing performance in different conditions. According to Frank Uzzell of local Ponsse distributors, Green Projects, the H7 has infinite pressure settings on specially designed knives and on its rollers giving the operator a high level of control for top de-barking performance without damaging the stem.

He said one-pass processing is possible with the H7 Euca depending on the stripability of the timber.

Frank explained that the three rollers on the H7 are more efficient when the stems are not perfectly straight. It is a shorter head that can negotiate the bends in the stem.

Ponsse does have a two roller head, the H77, which is designed for very uniform, straight stems and is used extensively in Brazil. He said that this head would be introduced in SA if necessary.

SP field day
Logmech recently launched their new SP 661E at a field day in a Mondi plantation in Kwambo, attended by Mondi foresters, contractors and the SP head engineer and technician. This head is designed specifically for one-pass de-barking of plantation grown Eucalyptus. According to the manufacturers, the debarking quality and productivity is achieved through a unique combination of specially designed debarking knives, replaceable bark deflectors and two high-speed feed rollers.

Contractors take a good look at the new SP 661 E Harvesting Head at the field day on Zululand.

Despite the fact that the SP 661E only landed in Durban harbor one day before the trial, it performed extremely well and processed the stems with one pass leaving clean stacks and minimal stem damage.

Tickey van Eeden of Logmech, local distributors of SP heads, said this was the first time the new head has been unveiled anywhere in the world.

Log Max
Log Max has come out with the E6 for effective reduced pass Eucalyptus harvesting. John Barber of local distributors AfrEquip said the shape and angle of the knives on the E6 have been optimised for de-barking.

He said the E6 has friction control sensors which allows the operator to adjust the pressure on the knives which minimizes feed friction. You can also adjust the pressure on the rollers which drive and twist the stem, and break the contact between bark and stem. This enables the knives to de-bark efficiently without damaging the stem.

The Log Max E6 in action.

The E6, which is designed specifically for Eucalyptus harvesting, is equipped with two rollers and six knives. It can be mounted on an excavator or purpose built carrier, wheeled or tracked, and is also suitable for harvesting pine.

There are currently eight or nine E6s in operation in SA.

“For one-pass you’ve got to have ideal conditions with good stripability,” said John.

Contractors at the cutting edge
But what about the contractors who have to implement reduced pass processing. They are at the cutting edge of the change from multiple to three – and now one pass processing. The cost of harvesting heads is around R1.5 million so any decision to buy a new head cannot be taken lightly. They have spent the past few years getting used to the heads that perform best at three passes – now they face some tough choices that will determine how effective their harvesting systems are in meeting the additional challenge.

For the most part Mondi’s harvesting contractors have been mechanized for a while now and they are becoming increasingly familiar with the technology and the dynamics of managing a mechanized harvesting business. One-pass processing is just another challenge along that road, and if that’s what Mondi wants, then that is what they will deliver wherever and whenever possible.

Dewald Martins of Mooiplaas Forestry who contracts for Mondi in the Melmoth area said they have successfully made the transition from five-pass to three-pass processing. He said it’s still early days with one pass, which works “in some areas”.

He said that the design of the head is crucial and has a big influence on the feasibility of one pass processing. The Mooiplaas team will be testing the Waratah and SP heads to see which performs best in the conditions.

*First published in SA Forestry magazine, November 2018

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