Wood Chips

Question 1

What are the 2 types of debarking process?

Answer 1

Drum debarking (multiple logs) and ring debarking (single logs)

Question 2

What is a slasher unit? (Wood handling)

Answer 2

When the logs are too long for the process.

Question 3

What is the purpose of debarking?

Answer 3

Its dark
Contains little fibre
High extractives content
consumes cooking chemicals
lowers digestor yield
Carriers dirt and grit

Question 4

What is the action in a debarking drum?

Answer 4

Bark is removed through friction of the logs rubbing against each other

Question 5

What are 2 types of drum debarking?

Answer 5

Wet and dry
Wet adds water to help process but adds time as bark needs to be dried and pressed before entering bark burner.

Question 6

What are positives and negatives of drum debarking

Answer 6

• 4-5% wood waste
  -inferior wood chips
  + relatively low cost
• high power consumption

Question 7

What 2 conditions take a long time to debark

Answer 7

Dry wood during summer and frozen wood fr4om the winter.
Fresh wood and de-iced water much quicker

Question 8

Principle of ring debarking

Answer 8

Logs fed individually
Motors to cut and strip bark

Question 9

What is the reason for chipping?

Answer 9

Most pulping processes need small pieces of wood so that water, chemistry and heat can penetrate quickly and uniformly

Question 10

What is the most critical chip parameter in kraft pulping?

Answer 10

/thickness of chips. Delignification strongly depends on this

Question 11

What is ideal chip size?

Answer 11

20mm long (grain direction)
4mm thick
Off size negatively affects pulp quality

Question 12

How can you ensure optimal chipping?

Answer 12

Proper speed control and maintanence of knife sharpness

Question 13

How does overly thick chips affect the process?

Answer 13

Non uniform penetration of chemicals and high level of rejects

Question 14

What is the importance of chip quality

Answer 14

Thickness - penetration - quality/rejects
creation of fines can cause blockages and resistance to flow
affects digestor

Question 15

What happens during chip screening

Answer 15

large chips are seperated out to be further processed at a re-chipper which crushes them into smaller pieces
finals are usually burned with the bark

Question 16

How are chips commonly stored?

Answer 16

In chip piles outside, has drawbacks including wind and airborne contamination. Fungal diseases

Question 17

Why should you store chips that will be pulped using sulphite process?

Answer 17

During first 2 months of storage loss of extractives is high, this is beneficial
Whereas in kraft process fresh chips are better to recover more byproduct

Question 18

Why is it best to use fresh chips for the kraft pulping process

Answer 18

bewcause byproduct recovery is important. When chips are stored there is a high loss of extractives, may be beneficial in some cases eg sulphite pulping

Question 19

What is the influence of temperature on debarking? What and how can you improve debarking?

Answer 19

Cold temperatures negatively effect debarking as the bark adhesion is much higher. To improve this we can add a deicing stage. As well as decreasing retention time de-icing frozen wood also majorly improves the cleanliness.

Question 20

Explain 2 different types of chip screening

Answer 20

Can do by length and more recently developed screening based on thickness

Question 21

How can you classify wood chips?

Answer 21

Using SCAN MC40.88 method
Sample of ~10L chips taken fractionated out in 10mins by different size slots and holes.
Can also classify using a scanner

Question 22

Why do you remove the bark?

Answer 22

Bark contains little fibre but a high extractive content, this means it would have a low yield after cooking and instead just consume the cooking chemicals. The bark is also very dark and carries a lot of dirt and grit.

Question 23

What are the major debarking processes?

Answer 23

Drum: mechanical debarking when logs rub against each other inside a rotating drum – the friction removes bark.
Ring: Logs are debarked individually rather than all together, motor scraping the bark off.

Question 24

Explain the difference between wet and dry debarking drums?

Answer 24

Wet debarking: Water added to early solid portion of drum to help loosen the bark. Rest of the the drum has slots for bark to fall out, the bark has to be dried before it can be fired.
Dry debarking: Entire drum has slots for bark to fall out. These are longer and rotate faster. Bark can then be directly fired.

Question 25

What is the relationship between residual bark in the chips and original stem diameter?

Answer 25

Larger stem diameter often means lower bark content.

Question 26

What is an ideal chip? What are the problems with too big and too small chips?

Answer 26

20mm long in grain direction and 4mm thick (thickness is most important for kraft!). Offsize chips adversely affect processing and pulp quality. Can cause non-uniform penetration of cooking chemicals and high screen rejects. Too small chops can cause resistance to liquor flow and blockage of screens.

Question 27

Why do you screen chips

Answer 27

Need to screen chips to gain uniformity for cooking chemical impregnation as well as reducing the reject amount. Overly thick chips have a much higher reject % and small chips can block the screens.

Question 28

Why should chip storage be managed and how?

Answer 28

Chips should be stored well in order to reduce wood losses and degradation. It depends on the final use the optimal storage because loss of extractives is high when stored outside for the first 2 months, this reduces resin problems in the sulphite process. But is byproduct recovery is important the chips should bypass storage wherever possible.
Consider a first in first out approach usually.