The ash content of a lignin sample would largely depend on the biomass type and the process where the lignin is derived from. For example, if the lignin is recovered from Kraft pulping, then it can contain cooking chemical residues that form ash when the sample is combusted.

The following paper reported lignin ash contents ranging from about 0.5 to 36 wt-%.

https://bioresources.cnr.ncsu.edu/BioRes_09/BioRes_09_1_725_Sameni_KRLS_Therm_Charact_Lignin_Residue_Ind_Processes_4717.pdf

Thank you sir Ville Paasikallio

I agree with Dr. Paasikallio, but you considered that lignin is very resistant to thermal degradation and depending of the temperature used in analysis may not be enough to burn it completely

Dear Ahsanullah Soomro,

I believe it has a connection with the formation of the cell wall. The first layer to be formed is the middle lamella, with deposition of calcium and magnesium pectins. In the sequence the primary and secondary walls are formed. In the end, the largest amount of lignin is in the secondary wall, but the highest concentration in the middle lamella, where the minerals are initially deposited.

I hope it helped you.

We must be very attentive to the methodologies used before comparing the results. Temperature ranges, biomass origin, chemical and / or physical modifications of samples, are some items that must be observed to conclude about the presented question. We should not forget that the inorganic components can be removed during extraction steps with water and if this occurs, the results are affected. losses of some types of metals and salts during the analyzes, as well as insufficient oxidation of some types of carbonates, can also influence the final result.

The below mentioned researcher has many articles on ash and silica of several types of lignocellulosic biomass, specially the older articles. Happy new year.

https://www.researchgate.net/profile/Yehia_Fahmy