3.1. Drawbacks of the conventional commercial pulping technique

The conventional commercial pulping procedures of annual plants include both kraft and soda pulping. The major drawback of these pulping protocols is that in the case of alkaline cooking liquors, apart from the delignification process, significant decomposition of carbohydrates portion of the lignocellulose biomass is observed by peeling-off reactions and alkaline hydrolysis. Optimum alkaline cooking condition is pertinent as excess alkaline loading could result in lowering of the ISO brightness due to the concept of alkaline darkening. Furthermore, sodium hydroxide easily dissolves phenolic lignin structures; besides, lignin undergoes condensation reactions under strong alkaline conditions which reduce the reactivity of residual lignin. In the like manner, strong alkaline cooking liquors dissolve high silica content of annual plants and precipitates evaporation problems in soda recovery boilers and in the causticizing plant [34].

In contrast, conventional kraft pulping has been used commercially for paper packaging from annual plants such as wood and some bamboo species, due to good paper properties. Kraft pulping has been found suitable for all ranges of fiber sources. It gives paper of high tensile strength and also high efficiency of the recovery of cooking chemicals [35]. In non-phenolic lignin moieties only β-ethers are cleaved. This has also an impact on bleaching. Among the disadvantages of kraft pulping process, low yield pulp, high consumption of bleaching chemicals, emission of obnoxious odor from the pulping process and large mill size require a tremendous capital investment.

Compared to soda pulps, soda/anthraquinone (soda/AQ) pulps have slightly better bleaching ability, higher yield and sometimes better strength. Similarly, alkali sulfite pulping had been known to give good strength properties and significantly higher yields than kraft process [36]. The addition of anthraquinone helped to reduce Kappa number from the unbleached pulp.

In order to overcome the abovementioned challenges, Patt and Kordsachia [37] discovered that the addition of methanol or ethanol extended delignification to levels below kraft or sulfite pulps. They named their process as alkali-sulfite-anthraquinone methanol (ASAM). Unlike the conventional pulping protocols, ASAM pulping process has dual advantages of paper properties and higher pulp brightness. In addition, ASAM pulping results in pulp with high pulp yield, low Kappa number and high paper strength. Furthermore, the obnoxious odor from methyl mercaptan that is generated during kraft pulping is completely absent in ASAM pulping.

#### 3.2. Mechanism of alkaline sulfite anthraquinone and methanol (ASAM)

Alkaline sulfite anthraquinone with methanol pulping known as the ASAM process was developed by Patt and Kordsachia in 1986 as an alternative cooking process option for soda and kraft pulping processes. According to the authors, the chemical materials that make up the ASAM process are sodium sulfite (Na2SO3), sodium hydroxide (NaOH), anthraquinone (AQ) and methanol (CH3OH). These materials play a unique role in the pulping milieu because, sodium sulfite and sodium hydroxide are the major ingredients of the alkaline sulfite process while the anthraquinone and methanol act as catalysts to enhance the chemical penetration delignification of the lignocellulose biomass. Table 3 shows the characteristics of cooking parameters in the ASAM pulping process, while Table 4 gives a summary of the key role of each chemical constituent in ASAM blend.

The mechanism of ASAM pulping process can be viewed with respect to the cooking parameters.
