Darpan and all,
A SHORT answer to what could have books written about it. I refer to this quotation that you provided from the paper:
Batch combustion in a fixed bed follows three major stages. Ignition occurs first once the surface material of the fuel devolatilises and forms a gaseous flame.
Done. Yes, that is understood.
Once this flame is established the ‘flaming phase’ takes place characterised by the combustion of volatile products and their decomposition products (secondary pyrolysis products) as luminous diffusion flames.
Note that they are discussing the UPPER flaming of the gases that have come from the raw fuel down below.
Thirdly the smouldering phase occurs which is characterised by heterogeneous char combustion and limited visible gas phase combustion.
Note that they are now discussion "char combustion" which is occuring at the bottom of the now pyrolyzed biomass.
THEREFORE, there is confusion between that is going on and where it is happening. I do NOT like this terminology. Instead, please consider:
1. Ignition stage. no change
2. FLAMING pyrolysis. This is down INSIDE the fuel bed. It is flaming because O2 is present as the pyrolytic gases are released from the fuel particle.(actually sort of glowing because the flames are VERY small. Contrast this with NON-flaming pyrolysis which is pyrolysis as it occurs inside of a retort (where there is sufficient heat but no entry of O2). Note that this is NOT referring to the flaming of the gases at the top of the gasifier (or even further away if through a pipe, etc.)
3. CHAR-gasifification. (why call this "smouldering"?) Note that the realtively small amount of gases being created are CO and they will cause VERY SMALL FLAMES (visible in a darkened room) if there is O2 where that CO is liberated. But if no O2 or if cooled somewhat, that CO will not burn, and will become CO emissions.
Personally, I will not be referring to "smouldering" as a combustion process. It is too vague.