Table 3.

Comparative account of different pulping technologies applied to the various agro-residues. Adapted from [81].

Pulping Technology	Conditions (Agricultural Residues and Pulping Conditions)	Specific Features (Removal of Lignin, and Other Parameters)	Environmental Impact *
Thermo-mechanical process	Used for all agricultural residues	Softening of biomass by steam followed by fibrilization	+++
The whole wood fiber manufacturing process	Agricultural residues can be used for this process by using different mechanical means after steaming	Conversion of biomass tissue into the fibrous state without chemical action	++
Semi-chemical pulping process	Wheat straw, rice straw, corn stalks, cotton stalks	High yield of pulp with significant removal of lignin and hemicellulose	+++
Chemical pulping	Suitable for agricultural residues	Efficient delignification, the minimum requirement of fibrillization	++++
Sulfite process	The acid sulfite process is not suitable for pulping agricultural wastes	Using a buffered acid solution of calcium bisulfite or magnesium bisulfite. Conversion of lignin into soluble lignosulfonate acid and consequently lignosulfonates production	++++
Soda and modifications (e.g., soda-anthroquinone)	Silica is easily dissolved in an alkaline medium. Recommended for agricultural residues.	NaOH solution reacts with the free –OH groups in the lignin molecules and converts it into sodium ligninate (alcoholate)	++
Kraft process	Suitable for woody biomass. It produces stronger pulp than most other chemical and mechanical processes.	NaOH is replaced by Na2S enhancing the delignification	++++
Organosolv	Delignify bagasse, cotton stalks and wheat straw.	Using a broad range of organic solvents	++++
Bio-pulping	Use of microorganisms or enzymes in the pulping process	Enzymes from wood degrading fungi for selective degradation of lignin, low temperature and atmospheric pressure but requires long processing time	+

* +: environmentally friendly approach, ++: low impact, +++: medium impact, ++++: severe impact.