There are a number of formula by various authors in literature that can be employed to calculate Higher Heating Value (HHV) from Volatile Matter (VM).
The most recent of these can be found in these papers:
1. Nhuchhen, D.R., 2016. Prediction of carbon, hydrogen, and oxygen compositions of raw and torrefied biomass using proximate analysis. Fuel, 180, pp.348-356.
2. Nhuchhen and Afzal, 2017. HHV predicting correlations for torrefied biomass using proximate and ultimate analyses. Bioengineering, 4(1), p.7.