Raw Materials: Wood, Fibers and Pulp Selection

Introduction: the wood pile in the wood yard

In the previous episode we established why paper is paper — hydrogen bonds weave fibers into a web. This episode takes one step further back: where do these fibers come from? For a paper machine, the raw-material yard is where everything begins. Wood, bamboo, bagasse, wheat straw, and even recovered waste paper differ enormously in composition, but all ultimately answer the same question: of this raw material, how much cellulose fiber can become paper, how easily can it be separated out, and what does it look like once separated.

In practice, the most useful handle on raw materials is the "ratio of the three major components" — cellulose, hemicellulose and lignin, plus a small amount of extractives. Cellulose is the "skeletal fiber" we want; hemicellulose is an associated short-chain sugar that affects refining and bonding; lignin is the "natural glue" holding fibers together, and one of pulping's core jobs is to remove or soften it. How much lignin there is directly determines whether the raw material is easy to bleach, easy to refine, and whether it needs chemical recovery.

This episode first covers the chemical composition of wood and the structural differences between the two wood classes (softwood, hardwood), then the peculiarities of non-wood fibers, and finally lands on the most practical question: faced with a pile of raw material, should you choose softwood kraft, eucalyptus pulp, BCTMP, DIP or CTMP? And how many cubic meters of wood does it take to produce 1 tonne of pulp?

1. Wood's chemical composition: the ratio of the three "siblings"

Take an oven-dry piece of wood apart and its main constituents are three polymers: cellulose, hemicellulose and lignin, plus a small amount of low-molecular substances extractable by solvents (resins, terpenes, fatty acids, etc., collectively called extractives). The biggest difference between softwood and hardwood is hidden in these three numbers.

The typical engineering ranges are: softwood with about 42% cellulose, about 27% hemicellulose (mainly xylan), about 26–28% lignin, and 2–5% extractives; hardwood has markedly less lignin, only about 18–24% — less lignin means it is easier to remove by cooking, easier to bleach, and relatively easier to refine. This also explains chemically why hardwood pulp is generally "easier to bleach" than softwood pulp and consumes less energy.

One more point: although hemicellulose is not the "main fiber," it is crucial — shorter than cellulose, branched and strongly hydrophilic, it promotes hydration and fibrillation during refining and also affects sheet stiffness and absorbency. Extractives (resins, waxes) are the "troublemakers" in papermaking: they readily precipitate and foul on the dryer cylinder and press felts, and also affect sizing and printing. So both stock preparation and cooking must aim to remove them as much as possible.

Specifically for hemicellulose types, softwood and hardwood also differ: softwood hemicellulose is mainly galactoglucomannan, while hardwood is mainly xylan — which is why, when briefs lump softwood hemicellulose broadly as "xylan ~27%," it should be understood as an engineering approximation "of total hemicellulose." Lignin, on the other hand, is a highly cross-linked aromatic polymer of phenylpropane units; its original function is to "cast" the cellulose microfibrils into a rigid wood skeleton. What pulping does is precisely to break this cross-linking and dissolve the lignin out of the fiber wall. The higher the lignin content (softwood), the more alkali and heat are consumed and the heavier the cooking and bleaching burden — a chain of logic that runs through the chemical pulping of EP03.

2. Softwood: the tracheid and the "microfibril angle" that decides modulus

Softwood's wood structure is relatively simple; its main body is a slender cell called the "tracheid" — it is responsible both for conducting water and for supporting the trunk. During the growing season tracheids differentiate into earlywood and latewood: earlywood forms early in the season, with thin walls and large lumens, so the material is light and soft; latewood forms later, with thick walls and small lumens, so it is dense and hard. The alternation of earlywood and latewood within a single log forms the visible growth-ring bands.

An individual fiber's strength depends heavily on the orientation angle of the microfibrils in its S2 wall layer — the "microfibril angle" (MFA). The S2-layer microfibril angle of softwood tracheids is typically between 10° and 30°: the smaller the MFA, the more "straight" the fiber is along its axis and the higher its single-fiber modulus (stiffness); a larger MFA makes the fiber softer and more compressible, changing the sheet's bulk and stiffness behavior. This is also why, for the same species, faster-grown raw material with a higher latewood ratio and smaller MFA yields pulp of better strength.

From a pulping perspective, softwood's value lies precisely in these long tracheids: they are long, and their wall structure is suited to fibrillation during refining, contributing tensile and tear strength to the finished sheet. That is why softwood kraft has long been the "skeletal" raw material for kraft paper, sack paper, board top layers and specialty strong papers.

The ratio of earlywood to latewood (i.e., the width ratio of earlywood/latewood bands in the growth ring) also affects the final pulp behavior: wood with a high latewood ratio has high density, thick fiber walls and high refining energy demand, but gives good sheet stiffness and strength; wood with a high earlywood ratio is softer, easier to refine and bulkier. For the same species, the earlywood/latewood ratio varies with site and tree age, which means that behind the three words "softwood" there is still considerable room for real pulping behavior to fluctuate — at raw-material acceptance you cannot rely only on the species label, but must combine density and slice observations.

3. Hardwood: fast-growing eucalyptus and the annoying "vessel picking"

Hardwood's structure is more complex than softwood's: besides wood fibers, it contains large numbers of vessels — coarse pores specialized for water conduction. The presence of vessels has two consequences: first, they occupy about 20–40% of wood volume, making hardwood overall looser and lower in density; second, when hardwood pulp is used in printing papers, the small holes left by vessels on the sheet surface take ink unevenly during printing, producing the well-known defect called "vessel picking / vessel marking."

Most worth mentioning among hardwoods is eucalyptus: a typical fast-growing species, plantation-grown and harvestable in about 7 years, with a basic density of about 450–550 kg/m³. Eucalyptus fiber is moderate, high in brightness and low in pulp cost, making it the main hardwood pulp source for global cultural/printing papers (especially coated papers such as LWC/SC). Birch is also an important northern hardwood raw material, with slightly longer fibers and better strength.

Process-wise, the remedies for "vessel picking" are: first, controlling the Z-direction distribution of vessels through refining and furnish mix (pushing them toward the core layer as much as possible); second, raising surface strength and sizing; and third, adjusting ink viscosity and printing pressure on the printing side. At the end of the day, this is an inherent limitation brought by raw-material structure, and one should be aware of it when choosing hardwood pulp.

It is worth mentioning that although eucalyptus is the absolute mainstay of the Southern Hemisphere (Brazil, Chile, Indonesia, southern China), not all eucalyptus is suitable for pulping: different provenances (e.g., E. globulus, E. grandis, E. urophylla) differ markedly in fiber morphology, density and bleachability, and the genetic selection and initial planting density at the forestry stage transmit directly to the pulp mill's yield and strength. That is why large pulp mills are often tied to forest estates, purpose-breeding from the source toward "pulpwood" targets rather than buying random logs on the market. This is also one of the logics of modern pulp-and-paper integration: designing the raw material's chemical and morphological characteristics "into" the forest in advance.

4. Non-wood fibers: bamboo, bagasse, wheat straw and cotton/linen

Besides wood, there is a large cast of "non-wood" raw materials in papermaking, indispensable in specific regions and for specific paper grades.

Bamboo: fiber length about 1.5–4 mm, between softwood and hardwood, an excellent papermaking fiber, widely used in southern China, especially suited to cultural papers and packaging papers.

Bagasse: a by-product of sugar production, with medium fibers, an important circular-economy raw material; its drawback is many pith cells and a high content of miscellaneous cells, requiring strengthened cleaning and screening.

Wheat straw (and rice straw): short fibers, high silica content. The direct trouble from high silica is silica scaling in the chemical-recovery section — silicon dioxide accumulates during evaporation and combustion, affecting the black-liquor evaporators and the recovery boiler, and is a major technical bottleneck for scaling up wheat-straw pulp.

Cotton and linen: bast/seed-hair fibers, with α-cellulose content exceeding 90% and almost no lignin, top-grade raw materials for making specialty papers (archival paper, banknote paper, filter paper, electrical insulation paper); their drawback, of course, is high cost and limited supply.

The common trait of non-wood raw materials reminds us: choosing a raw material cannot rely only on fiber length, but also on the "contaminant spectrum" — silica, pith cells, stickies, waxes — whichever of these will become the pain point of your line.

5. Fiber-length distribution: why softwood pulp is "long" on strength

Fibers in raw material are not all the same length but follow a distribution. Softwood fibers are long, with their length distribution concentrated in the longer 2–5 mm range; hardwood fibers are short, concentrated in 0.7–1.6 mm. The shape and position of the distribution directly determine the strength potential of the finished sheet: a high proportion of long fibers means more fibers "spanning multiple nodes" in the network, which most benefits tensile and tear strength.

6. Pulp selection: the "personalities" of the five major pulp grades contrasted

The same wood can be made into different pulps, and different pulps have completely different "personalities." The five major pulp classes commonly used in engineering and their typical positioning are: softwood kraft (NK, strength backbone), eucalyptus kraft (EK, smoothness, opacity, low cost), BCTMP (bleached chemi-thermomechanical pulp, good bulk and opacity), DIP (deinked recovered pulp, cost and environmental advantages), and CTMP (chemi-thermomechanical pulp, absorbency and bulk, often used in board core layers).

7. Material-consumption coefficient: how much wood to make 1 tonne of pulp

Converting raw material into "pulp," every mill cares about one hard metric: to produce 1 tonne of oven-dry pulp, how much raw material (by volume or oven-dry mass) must be put in. The common engineering ranges are: 1 tonne of bleached softwood kraft needs about 4.5–5.0 m³ of wood (estimated at a wood basic density of about 400 kg/m³); 1 tonne of eucalyptus pulp needs about 3.5–4.0 m³ of wood. Hardwood has a higher basic density, less lignin and often not-low yield, so the wood volume per unit of pulp is actually smaller.

Here the "wood volume" should be converted using basic density (oven-dry basic density, oven-dry mass/green volume, kg/m³), not the air-dry density, otherwise the feed quantity will be miscalculated. A softwood with basic density of about 400 kg/m³ means each cubic meter of green wood contains only about 400 kg of dry substance; multiplying by the cooking yield (softwood KP about 45–55%) gives the oven-dry pulp obtainable.

This mass balance is used daily in stock preparation, cooking and recovery design: when wood arrives, first measure basic density and moisture, back-calculate the oven-dry feed quantity, then combine with the target yield to compute theoretical pulp output, and finally balance against recovery capacity and white-liquor supply. Change the raw material (e.g., softwood to eucalyptus) and both basic density and yield change, so the entire line's material flow must be re-balanced.

8. The "contaminant spectrum" in raw materials and stock-prep losses

Choosing raw material cannot rely only on fiber length and the three major components, but also on the "contaminant spectrum" — things that contribute neither strength nor help, only add processing cost. Bark, knots, rotten wood, metal inclusions, sand and stones brought in by logs are all removed or cause losses in the stock-prep stage. Bark contains almost no usable fiber and carries large amounts of extractives and silica; if it enters cooking it only increases alkali consumption and scaling; knots are extremely hard wood, hard to cook, and easily leave uncooked knots in the pulp; metal and sand directly threaten the chipper knives and the refiner. Proper stock prep (debarking, knot removal, dedusting, metal detection) is the prerequisite for stable cooking and equipment protection.

Chip specifications themselves affect yield and quality. Ideal chips are mostly 15–30 mm long, uniform in thickness and low in fines; chips that are too short or too fine (pin chips / sawdust) easily over-degrade during cooking and become rejects in screening, directly lowering yield. So the stock-prep stage's "chip acceptance rate" and "fines/dust rate" are two daily indicators to watch: low acceptance means burning expensive raw material as fuel; high fines rate damages both cooking uniformity and yield. This is also why, even after basic density and moisture are measured accurately, stock-prep losses can still pull the actual wood consumption away from the theoretical 4.5–5.0 m³/t range.

Non-wood raw materials have a more "individual" contaminant spectrum: wheat straw's silica, bagasse's pith cells and sugars, bamboo's nodes and gums, cotton/linen's shives and tow — each with its own trouble. They often decide what cleaning and screening equipment this line should install, rather than simply looking at fiber length. When buying raw material, a complete "composition + contaminants + moisture" dossier is far more useful than a phrase like "good fiber."

9. Why kraft became the global mainstream (preview of EP03)

This episode is about "what pulps raw materials can become," but an unavoidable question is: why does the kraft process (KP) hold an overwhelming majority among global market pulps? The answer lies in the chemical data above — softwood lignin is as high as 26–28%, and to remove it efficiently while preserving cellulose as much as possible requires the synergistic action of a sulfur-containing chemical (Na₂S) under high temperature and strong alkali; the kraft process happens to strike the best balance among delignification selectivity, strength retention and recovery economics, so it can both handle hard-to-cook softwood and, through chemical recovery, recycle the chemicals and burn the organics as energy.

In contrast, the sulfite process has a slightly higher yield (about 50–65%) and was once prosperous, but it is sensitive to raw-material quality and its liquor recovery is complex, so its share is now much smaller; mechanical and chemi-mechanical pulps (SGW/RMP/TMP/CTMP/BCTMP) take another path — almost no delignification, retaining lignin through physical fibrillation, with yields as high as 85–96%, at the cost of limited strength and brightness and high energy consumption. So the essence of "pulp selection" is finding one's optimal point within the pentagon of "delignification degree — yield — strength — cost — environment."

Next episode, EP03, we dive into the kraft digester: how exactly do NaOH and Na₂S in the white liquor chop up and dissolve the lignin? What do the jargon "three delignification stages," kappa number and H-factor actually mean? And how does chemical recovery turn black liquor back into white liquor to close the loop? The raw-material chapter ends here; the chemical-pulping chapter officially opens.

10. The blending mindset: no universal pulp, only the right combination

In reality no single pulp can simultaneously satisfy strength, bulk, opacity, smoothness and low cost — these five almost mutually constrain each other. The engineering approach is to blend 2–4 pulps (furnish mixing): softwood KP as the strength backbone (its share often floats between 10–40% depending on strength needs), euca. or hardwood KP providing the body and surface, BCTMP/CTMP boosting bulk and opacity, and DIP holding down cost. The essence of a recipe is "fill the volume with cheap pulp, and defend performance with expensive pulp."

Blending must also consider compatibility: different pulps have different refining response, Zeta potential and drainage rate, and mixing may cause layering or uneven retention. So the online blending accuracy of the furnish chest, the homogeneity of the mixed stock, and the matching of wet-end additives are all key to successful blending. A common trap is: to cut cost, pile on DIP, only to see strength drop, printing linting and ash sensitivity — at which point one often has to backfill softwood KP or re-adjust the refining curve, rather than keep adding DIP.

Finally, close the loop of "raw material — pulp grade — paper grade": the chemical composition of the raw material decides what pulp it suits, and the pulp's characteristics decide what paper it suits. Once you thoroughly understand this chain, you can quickly find an alternative blending scheme when raw-material prices rise or pulp grades are short, without crashing. This is precisely the link between this episode and the next (chemical pulping) — understand the material first, then understand the cooking.