Question

why does wood initially burn with a flame and letter only glows without a flame

Answer 1

Flame is produced because of burning of volatile substances. Wood is made up of carbon compounds which get oxidised in presence of heat. It also release some volatile material which burns with flame initially. But as oon as all the carbon compounds is break into smaller chemicals and gas formation is ceased. then only carbon residue is left which do not get vapourized and so that do not give any flame but burns slowly without flame looks like glowing charcoal .