Increasing the value of bark as fuel

HQ-Press, the new-generation bark press delivers higher dry solids and is much more efficient to operate and maintain.

Just because a bark press is not critical to the operation of a pulp mill does not mean that it should be overlooked.

The HQ-Press is a robust, reliable solution for bark handling in any woodyard or biomass handling operation.

“To the ANDRITZ wood processing team, the pursuit of these small and large gains is what fuels product development” according to Silventoinen and Ari Hannimäki, Director of R&D and Process, Fiber Technologies ANDRITZ.

Mechanical erection has already begun for the four HQ-Press units sold to Metsä Group’s bioproducts mill at Äänekoski.  While being a dynamic and progressive producer of bioproducts and bioenergy, Metsä Fibre, part of Metsä Group, is not exactly known for taking huge technical risks.  So, there must be something solid behind this new product.

An important side stream

“It is a cornerstone of the Äänekoski bioproducts mill project to be carbon-free,” says Timo Merikallio, Project Director for Metsä Fibre. The mill will be built alongside the existing Äänekoski mill, sharing some of the infrastructure.  Design capacity is 1.3 million tonnes of bleached pulp.

“Our plan is not only to produce high-quality pulp, but also to create a platform for using all the production side streams: biomaterials, bioenergy, and biochemicals. We anticipate that 100% of the raw material will be converted to saleable products or energy.”


"We anticipate that 100% of the raw material will be converted to saleable products or energy."

Timo Merikallio, Project Director for Metsä Fibre

Even though it is a side stream, bark plays an important role. About 60% of the bark processed on-site will be consumed as fuel in the lime kiln (gasified) as a substitute for fossil fuels.  According to Merikallio, about 40% will be sold. “Obviously, the drier we can make the bark without incurring too high energy cost, the better the sales will be.”

“Create happy customers”

The HQ-Press does several things extremely well – the most important being that it dries bark better than anything on the market using mechanical pressing techniques to conserve thermal energy.

But, dryness is only one issue. “We had one of the older EcoPress units on our existing line at Äänekoski,” Merikallio says. “Quite frankly it was a disappointment.  It was not robust enough and needed too much maintenance attention. As a result of that, it was replaced after a fairly short period of time.”

The EcoPress was a bark press design that evolved from Enso to Ahlstrom to Kone Wood to ANDRITZ. Silventoinen estimates there are about 100 units sold, which gave ANDRITZ the basis of experience in this segment of the industry. “Many of them operate quite well,” he says. “But there are also installations, such as at Äänekoski, where the maintenance is way too intensive and customers are not thrilled.”

Unlike competitive units with enclosed structures, the HQ-Press has an open, simple design. The  components are easily accessible for maintenance. As a side benefit, the open design helps with housekeeping activities, which reduces odors and mold spores in the operating environment.

Because of this, ANDRITZ withdrew from the bark press market and began offering the equipment from the leading bark press manufacturer, also in Finland. Customers still were not happy with the maintenance intervals (frequent replacement of the screen drum at a very high cost) and the difficulty in keeping the press in continuous operation.

“We saw this as a market opportunity to create happy customers,” Silventoinen says. “The quest for better performance, higher reliability, lower maintenance costs are all sound reasons for building a better bark press.”


"The quest for better performance, higher reliability, lower maintenance costs are all sound reasons for building a better bark press."

Hannu Silventoinen, Director Marketing and Sales, Fiber Technologies, ANDRITZ

Sound starting point

“We made solid technical gains in all the main technologies in a wood processing plant: debarking, chipping, screening, rejects handling, stacking/reclaiming”, says Hannimäki. “Our goal is to systematically go through every process to make it better. Bark pressing, while not critical to the mainstream processes, has taken on higher importance as mills replace their outside energy purchases and fossil fuels with their own in-house biofuels. That is one of the primary reasons our design team set out to develop a new-generation bark press.”

“We took a look at our past to see what we should retain and what we had to change,” Hannimäki says. “Our past experience re-affirmed that there are several Principles of Pressing which are still sound. Only the execution of these principles needed to change.”

The principles of pressing:

  • The bark should be pressed during one rotation of a drum – not going round and round.
  • The pressing time (retention time on the drum) should be as long as possible.
  • Higher pressure is more effective in achieving higher dry solids.
  • Water removal is more effective when pressed out in multiple directions.

While conventional presses typically achieve an annual average of 40% dry solids, the HQ-Press with its unique 2-stage pressing is able to deliver an annual average of 45% dry solids in normal operation.

Development work started in 2014. From this came some important design decisions: two-stage pressing; a large diameter drum (approx. 3 m) and a very slow speed (to encourage adequate pressing time; higher nip pressure to extract the most water; and removal of water through both the screen drum and the press drum.  

After proof-of-concept and prototyping was completed, ANDRITZ began talking about the new product with interested customers in Nordic countries. The ideas resonated quickly with pulp mills experiencing frustration with their existing bark press operations.

“Good upside potential”

ANDRITZ approached Metsä Fibre about being the first to try the new bark press for the Äänekoski project.

Merikallio recalls: “ANDRITZ has a good track record in developing products with us and we have worked together for years. The presented design concept look like ANDRITZ eliminated the problem areas. To be honest, this is not a high-risk area for us. We will have four units, which gives us one in reserve. If there are problems, ANDRITZ will make it right. So, to my mind, we have very good upside potential.”


"If there are problems, ANDRITZ will make it right. So, to my mind, we have very good upside potential.

Timo Merikallio, Project Director for Metsä Fibre

Tested and re-tested

In August and September of 2016, the Metsä Fibre project team witnessed testing of the new HQ-Press at the manufacturing workshop in Finland. Merikallio was there. “This is an impressive unit, a very substantial heavy-duty design. The dry solids output was higher than we typically achieve. If we can get a couple of percentage points higher dryness for the same energy input – without the maintenance headaches – we will be very happy indeed.”

The design of the HQ-Press features a self-orienting drum with two-stage pressing to achieve the higher dryness. In practice, this two-stage operation allows a mill to maximize for either capacity or for dryness – or optimize for both – depending on current needs.

ANDRITZ’s own extensive testing shows the HQ-Press can deliver about 5%-unit higher dry solids content than any competitive press. The annual average (summer and winter bark combined) is about 45% dry solids, while conventional presses reach about 40% annual average. This can increase the net green energy value of the bark up to 15%, which equates to a barrel-of-oil savings of € 3,400 per day.

Benefits at a glance

The HQ-Press has an open, simple design.  The components are easily accessible for maintenance.  The open design also reduces odors and mold spores in the operating environment. It runs an automated washing sequence.

The slow-speed drum rotation and the applied materials extend the drum life significantly. With the slower rotational speed, larger diameter drums, and larger diameter infeed and outfeed screws, the HQ-Press is a noticeably more robust piece of equipment for bark handling in any woodyard or biomass handling operation.