One important aspect of biomass is its potential for carbon offsetting. The energy conversion of biomass does not contribute to CO2 emission or global warming. CO2 released during biomass conversion is equivalent to the quantity it absorbed during its growth. In other words, energy production from biomass is almost carbon neutral. The veracity of this statement is, however, debatable as the calculation of the carbon balance associated with biomass conversion is complex: Many parameters must be considered, such as growth, harvest, transportation, and pretreatments. Yet the use of biomass fuels can result in the displacement of carbon dioxide emissions that are ordinarily released when using fossil fuels. This displacement will depend entirely on the efficiency with which the biomass energy can be produced and used.

Most biomass materials contain very low amounts of sulfur, chlorine, in compar­ison to most fossil fuels. Biomass conversion or its use as energy has the potential to lower pollutant emissions.

Biomass chemical composition is obtained through ultimate (also called elemen­tal) and proximate analysis. Ultimate analysis yields the composition of biomass in terms of carbon, hydrogen, nitrogen, oxygen, and sulfur content (mass %). On the other hand, proximate analysis reports the composition in terms of volatile matter, ash, moisture, and fixed carbon. There are separate ASTM standards for measure­ments of individual components of biomass, such as volatile matter, ash, moisture, and fixed carbon.

• Volatile matter refers to the condensable and non-condensable vapor released during the early stages of biomass heating. It depends strongly on the heating rate and final temperature. There are different ASTM standards for the measurement of volatile matter; each standard is specific to different types of biomass.

• Ash is the inorganic residue composed mainly of silica, iron, calcium, magnesium, sodium, and sometimes potassium. It is the remaining material once biomass has been completely consumed in the reactor.

• Fixed carbon is an important parameter in biomass gasification, because its conversion is a limiting reaction, and it is used to size the gasifier.

• The heating value of the biomass is the energy chemically bound in the biomass with respect to a reference state. The best common ones are the lower heating value (LHV) where the reference state is water in its gas state and the higher heating value (HHV) where the reference state is water in its liquid state.

The biomass properties detailed above have significant effects on gasification condi­tions and product compositions: The basis on which they are measured and reported (wet, dry, or dry-ash-free basis) is very important. For example, low ash content improves thermal balance and reduces operating problems due to slagging and sin­tering. Water vapor is also an essential key component in gasification reactions; at high levels, it may negatively affect the process thermal balance. Biomass-containing sulfur, chlorine (may be present in low amounts), and alkali metals can also lead to the formation of corrosive components. Most biomass also contains nitrogen that can form gas-phase ammonia, which can oxidize and form NOX.