After the thermal treatment of agricultural waste, the char obtained was analysed by XRD. A diffraction peak appears at 2thetha= 22degrees. How can it be concluded if it is silica or cellulose peak?
You are right. Both are almost amorphous in nature and XRD peaks appeared in the similar region. To confirm about the peak in your sample you may measure EDX, XPS, FTIR and Raman. Further information may obtain by considering the composition and from the peak size, shape, height/intensity etc. and also can compare it with references, and XRD data of pure cellulose and pure silica.