Energy consumption and production
Pulp and paper production is among the most energy intensive segments of all industries. However, it is useful to distinguish between pulp and paper manufacture. Pulp and paper mills are not always integrated, and while pulp mills produce more energy than they need, paper mills require large quanitites of steam to run their dryiers. Kraft and soda pulp mills are energy self-sufficient, and often generate excess steam and electricity which can be used by an associated paper mill or sold to neighboring industries or communities. Black liquor is an integral part of the standard pulping process. The primary reason the black liquor is "burnt" is to recover the chemicals present in the liquor. Hence the common names of the boiler in which this is done are the process recovery (PR) boiler, or recovery boiler. Without chemicals recovery, the process would be uneconomical. The chemical recovery process is an exothermic chemical reaction which produces enough heat to gasify lignin and other volatile constituants of the black liqor. The reacted chemical salts are removed from the smelt bed into a smelt tank where they form the green liquor system in a pupl mill. The energy derived from the process produces enough steam (and in some cases electricity) to supply about 50 percent of the typical US pulp and paper mills energy needs. Additional energy is often recovered from the combustion of bark and other wood residuals. This value is generally higher in some of the North European countries where higher energy prices drive them to spend more capital in order to obtain increased efficientcies.
The organic materials in kraft black liquors consist mainly of
dissolved lignin degradation products along with degradation products
of hemicellulosic and cellulosic hexose and pentose sugars. The
latter are present as saccharinic acids which are anhydrides of
the various sugars.
Since black liquor composition varies with the process and its concentration is manipulated as it passes through the pulp mill from the washers through the evaporators to the recovery boiler, the ratio of black liquor to wood charged does not define its energy content. However, a simple approximation applies. Assuming an approximate raw pulp yield of 50%, the dissolved organics in the black liquor contain somewhat more than half the original energy content of the wood. Most of the higher energy content lignin ends up in the black liquor, while the pulp is mainly higher oxygen content carbohydrates. In addition oxygen-poor [and energy-rich] extractives such as fatty acids and resin acids are original black liquor components until they are skimmed off as by-product tall oil.
The yield from kraft pulping varies with the wood type, the extent of lignin removal, and the cooking conditions. Typical "high-yield" values range up to 50 to 55% (dry pulp out/dry wood chips in), but these are high kappa number pulps that have low brightness and that require significant bleaching in order to make them suitable for writing or printing grades of paper. Extended cooking can reduce yields to 40 to 45%, but these pulps may require less bleaching in order to attain high brightness.
Yield losses also occur during the bleaching process. Bleaching
with elemental chlorine (chlorine gas dissolved in water) is on
its way out. Elemental chlorine produces relatively large amounts
of Absorbable Organic Halides (AOX), and these are strongly suspected
of causing wide-scale environmental ahd health problems. Bleaching
with chlorine dioxide still apparently has a role to play in the
North American pulp and paper industry, but it too is under pressure
as consumers gain awareness about environmental effects of dioxins
and other chlorinated aromatic compounds.
Elemental chlorine has strong proponents because it is inexpensive
and a highly effective bleaching agent. Chlorine dioxide is more
expensive (by a factor of about 2 to 3) and it is somewhat less
Elemental Chlorine Free (ECF) pulp generally uses one or more
stages of chlorine dioxide as a bleaching agent. Total Chlorine
Free (TCF) bleaching uses hydrogen peroxide, ozone or oxygen as
Additional information about pulping can be found on the web site of the Technical Association of the Pulp and Paper Industry (TAPPI) under publications/pulping