Representing woody biomass concentration for kinetic reactions
First step is finding the molar masses of each compound. Since cellulose, hemicellulose and lignin are collections of similar molecules, I picked the following as a crude estimate:


cellulose: $\ce{C6H10O5}$ with $M_c = 162~\mathrm{g/mol}$
hemicellulose (L-Arabinose): $\ce{C5H10O5}$ with $M_h = 150~\mathrm{g/mol}$
lignin: $\ce{C31H34O11}$ with $M_l = 582~\mathrm{g/mol}$


Using a density of $700~\mathrm{g/dm^3}$ you will find the relative masses of the three compounds to be:


$0.48 \cdot 700~\mathrm{g/dm^3} = 336~\mathrm{g/dm^3}$ (cellulose)
$0.28 \cdot 700~\mathrm{g/dm^3} = 196~\mathrm{g/dm^3}$ (hemicellulose)
$0.24 \cdot 700~\mathrm{g/dm^3} = 168~\mathrm{g/dm^3}$ (lignin)


With $n = m/M$ we now find the number of moles of each compound in the sample ($1~\mathrm{dm^3}$):


$2.074~\mathrm{mol/dm^3}$ cellulose
$1.307~\mathrm{mol/dm^3}$ hemicellulose
$0.289~\mathrm{mol/dm^3}$ lignin


Keep in mind that this is a very rough estimate. You'll probably get a better result by taking the average of all species of hemicellulose, etc. in your sample and cross-checking it to make sure it fits the ratios of the elements (weight percent).