Chemical pulping, led by Kraft, dominates a 195×10^6‑ton global market. The edge comes down to chemistry, cooking conditions, and a brutal tradeoff between fiber yield, strength, and energy.
In 2023, about 195×10^6 t of pulp-for-paper was produced worldwide, with ~158×10^6 t (≈81%) coming from chemical pulping and ≈25×10^6 t (13%) from mechanical/semi-chemical routes (Statista). Among chemical processes, the Kraft (sulfate) method overwhelmingly dominates (≈80–85% of world pulp) (ScienceDirect) (ScienceDirect), while sulfite pulping has slipped to a niche (~3–6%) (ScienceDirect) (ScienceDirect), down from ~60% in 1925 to ~3.7% by 2000 (ScienceDirect). Soda pulping accounts for most of the balance (~9% of chemical pulp) (ScienceDirect).
Global production and method mix
Mechanical pulping, including groundwood, thermomechanical (TMP), and chemi-thermomechanical (CTMP), constitutes ~12–15% of global pulp (≈25×10^6 t in 2023, Statista). Semi-chemical methods such as chemi-TMP, NSSC (neutral sulfite semichemical), and CMP sit in the middle on yield at 65–85% (Paperonweb).
Kraft, sulfite, mechanical: tradeoffs
Kraft is an alkaline cook using sodium hydroxide (NaOH) and sodium sulfide (Na₂S). Its advantages: broad feedstock flexibility, strong high-quality pulp, high-solids energy recovery via black liquor, and efficient chemical recovery (ScienceDirect). Kraft pulp yields are medium-strength with high bulk and typically high for chemical pulps (ScienceDirect). Disadvantages: intense odor and SO₂/SOx emissions (requiring scrubbers), expensive recovery boiler capital, and most dissolved organics (lignin/hemicellulose) are burned as fuel—low value‑add for byproducts (ScienceDirect).
Sulfite is acidic (or neutral/alkaline) and uses SO₂/bisulfite salts (Na, Mg, Ca, or NH₄ bases). It typically attains about 8–10% higher yield than a comparable Kraft cook (ScienceDirect), produces very bright pulp that bleaches easily, and can recover hemicellulose sugars or lignosulfonates. Alkaline sulfite variants also produce virtually odorless gas. The drawbacks are complex chemical recovery (especially older acid-sulfite mills) and generally higher operating costs. Kraft largely displaced sulfite because bisulfite recovery—particularly with Ca/Mg—was cumbersome. Today, sulfite mainly serves specialized grades (e.g., fine printing papers, dissolving pulp), and its market share is very small (~3–6% globally, ScienceDirect) (ScienceDirect).
Mechanical pulping delivers extremely high yields—typically 85–95% of the wood mass (Paperonweb)—with minimal chemicals and high opacity/bulk, making it a fit for newsprint and boards. The cost is high electricity usage for refining, shorter fibers and lower strength (papers yellow/age poorly), and higher rejects such as shives/knots. Figure 1 (below) summarizes typical yields and fiber properties: mechanical pulps (>85%) far exceed chemical pulps (45–65%, Paperonweb) but have lower strength.
Kraft process chemistry (NaOH/Na₂S)
The Kraft cook, at pH ~13–14, dissolves most lignin while sparing cellulose. Sodium sulfide (Na₂S) hydrolyzes to hydroxide (OH⁻) and hydrosulfide (HS⁻); HS⁻ is the active nucleophile (ScienceDirect). The dominant reactions cleave lignin’s β‑O‑4 ether bonds (about 40–60% of all lignin linkages, De Gruyter). Two mechanisms operate: a slow OH⁻‑dominated path (attacks phenolic and non‑phenolic β‑O‑4 structures) and a fast HS⁻‑dependent path that requires a phenolic end‑group and “unzips” lignin from that end; the sulfide‑independent (OH⁻) step tends to limit the overall rate (De Gruyter) (De Gruyter). The lignin polymer is depolymerized into small fragments (“Kraft lignin”) that are fully soluble in the alkaline black liquor (De Gruyter) (De Gruyter).
Carbohydrates, selectivity, and yield
Hemicelluloses are partially degraded by alkaline hydrolysis and peeling, while crystalline cellulose is largely preserved (De Gruyter) (De Gruyter). Kraft cooking is “milder” to cellulose—losses of cellulose are typically <10%—while most degradation occurs in glucomannans/pectins (De Gruyter). A typical Kraft cook dissolves ~80% of the wood’s lignin and ~50% of hemicelluloses, but only ~10% of cellulose (Pulp&PaperMill.com). In practice, chemical pulping yields are “medium” (~45–65%) of wood mass (Paperonweb), while a neutral sulfite cook of the same wood yields ~10% higher pulp (soai ~55–70%) (ScienceDirect).
Cooking conditions and control levers
Delignification is governed by the H‑factor (a time‑temperature integration). Key control parameters are effective alkali (EA, the charge of NaOH+Na₂S), sulfidity (fraction of EA as Na₂S), temperature profile, and cook time. Higher EA or longer cooks drive a lower Kappa number (an index of residual lignin) but with more carbohydrate loss and lower yield (Pulp & Paper Canada) (Paperonweb). Mills optimize to reach just the target Kappa for a given grade.
Raising sulfidity accelerates lignin removal—more HS⁻—so the pulp can reach target Kappa in less time, indirectly increasing yield by shortening carbohydrate exposure (Pulp & Paper Canada). A detailed study notes that increasing sulfidity by itself does not change yield, but by shortening the cook it indirectly increases yield (Pulp & Paper Canada). Additives like anthraquinone (AQ) selectively protect carbohydrates and speed delignification, giving higher yield for the same Kappa (Pulp & Paper Canada).
Temperature must be tuned: higher peak T speeds all reactions (more delignification per time) but can trigger polysaccharide peeling, so the time/temperature profile is carefully controlled. Modern digester control targets constant Kappa by adjusting EA or dilution. In practice, many mills use ~16–25% EA (as Na₂O on wood), ~25–35% sulfidity, and cook at ~160–170 °C for 1–2 hours to obtain pulp in the 45–50% yield range. Overcooking drops yield rapidly—an extra 5% alkali might only improve Kappa by a few points but cut yield by several percent.
Controlling chemical charge (EA, sulfidity) hinges on accurate chemical metering; operators apply dosing discipline with equipment such as dosing pumps to keep charge on target.
Economics and the “just‑enough” cook
Economically, a 1% change in yield is huge at mill scale. Industry practice holds chemical pulp at the minimum alkali needed for quality. Unbleached kraft pulp Kappa is often stopped in the mid‑20s for bleachable grade; extending to Kappa 10–15 for fully bleached requires extra chemical and lowers yield with marginal brightness gain. Cutting multitier cooking (e.g., pre‑impregnation at moderate T, then final cook) and using additives (AQ, polysulfides) are common techniques to decouple delignification from fiber damage.
In summary, maximizing yield and minimizing chemical use means “just‑enough” cooking. Kraft pulping chemistry (NaOH/Na₂S) is highly selective, allowing most cellulose to survive (De Gruyter); the goal is to adjust EA, sulfidity, time, and temperature to remove lignin effectively but spare carbohydrates. Achieving 10 points higher Kappa would require dramatically more alkali or time. Mills often operate close to the threshold: any “overcook” (even 5–10 °C or ~15 min longer) can cost several percentage points of yield for little benefit. Real‑time Kappa/viscosity control and improved impregnation are continuously implemented to keep yields high.
Regulatory context and mill operations
Kraft mills face strict controls on odorous emissions and effluent COD; in Indonesia, the Environment Ministry’s PROPER rating scheme challenges pulp mills to minimize pollutants (Kementerian Lingkungan Hidup). In practice, large kraft mills (e.g., APP, APRIL) must hit technical targets while complying with these standards—recovering and reusing almost all NaOH/Na₂S, and investing in scrubbers/biotreatment to meet discharged water norms. For ancillary equipment that supports wastewater operations, mills turn to wastewater ancillaries suited to primary and secondary treatment trains.
Typical yield ranges by process
Table 1 (compiled from Paperonweb):
- Groundwood (GW): >95% (very high yield)
- Thermomechanical (TMP), Refiner mechanical (RMP): 85–95% (high yield)
- Chemi‑TMP (CTMP), NSSC, CMP: 65–85% (medium‑high yield)
- Kraft/Sulfite (chemical): 45–65% (medium yield) (Paperonweb)
Summary comparison
Kraft pulping yields strong, versatile pulp with efficient chemical recovery and energy generation (ScienceDirect), but at the cost of sulfur emissions and loss of lignin value. Sulfite preserves more carbohydrates (≈10% higher yield vs. Kraft, ScienceDirect) and bleaches easily, but recovery is more complex and use is niche. Mechanical maximizes yield (>85%, Paperonweb) without cooking chemicals, but demands massive energy and produces weaker fibers.
Sources and further reading
Global pulp statistics: Statista (Statista). Technical advantages/drawbacks and yields: ScienceDirect, ScienceDirect, Paperonweb. Kraft chemistry and control: De Gruyter, De Gruyter, Pulp & Paper Canada. Indonesian regulatory context: Kementerian Lingkungan Hidup. Chemical reaction overview: Pulp&PaperMill.com.
