Classification based on the atomic ratio helps us to understand the heating value of a fuel, among other things. For example, the higher heating value (HHV) of a biomass correlates well with the oxygen-to-carbon (O/C) ratio, reducing from 38 to about 15 MJ/kg while the O/C ratio increases from 0.1 to 0.7. When the hydrogen-to-carbon (H/C) ratio increases, the effective heating value of the fuel reduces.

The atomic ratio is based on the hydrogen, oxygen, and carbon content of the fuel. Figure 2.10 plots the atomic ratios (H/C) against (O/C) on a dry ash­free basis for all fuels, from carbon-rich anthracite to carbon-deficient woody biomass. This plot, known as van Krevelen diagram, shows that biomass has much higher ratios of H/C and O/C than fossil fuel. For a large range of biomass, the H/C ratio might be expressed as a linear function of the (O/C) ratio (Jones et al., 2006).

(H/C) = 1.4125 (O/C) + 0.5004 (2.3)

Fresh plant biomass like leaves has very low heating values because of its high H/C and O/C ratios. The atomic ratio of a fuel decreases as its geological age increases, which means that the older the fuel, the higher its energy content. Anthracite, for example, a fossil fuel geologically formed over many thousands of years, has a very high heating value. Its lower H/C ratio gives higher heat, but the carbon intensity or the CO2 emission from its combustion is high.

Among all hydrocarbon fuels biomass is highest in oxygen content. Oxygen, unfortunately, does not make any useful contribution to heating value and

makes it difficult to transform the biomass into liquid fuels. The high oxygen and hydrogen content of biomass results in high volatile and liquid yields, respectively. High oxygen consumes a part of the hydrogen in the biomass, producing less beneficial water, and thus the high H/C content does not translate into high gas yield.