Does biomass emit greenhouse gases

Yes, burning plants produce carbon dioxide, after all oxygen is flameable.

>Burning of any hydrocarbon generates carbon dioxide as a by-product. Plants are mainly composed of various glucose and cellulose polymers, which essentially are hydrocarbons, which when burned, release CO2 and leave an ash residue.

A rough example to consider the burning of hay. The black smoke released as a by-product of its incineration, is CO2 (along with other gases).