Determination of the Exergy of Four Wheat Straws

Exergy is a measurement of how far a certain material deviates from a state of equilibrium with the environment. It is a useful tool for improving the efficiency of energy-resource use. The exergy values of four wheat straws (Absolvant, Max, Monopol and Vuka) were determined in this study. The effects of physical and chemical properties (moisture content, ash content, LHV and S, C, O, H and N contents) were evaluated. The moisture related exergy varied from 281.834 kJ kg^-1 (Monopol) to 366.766 kJ kg^-1 (Absolvant), accounting for 1.311-1.734% of the total exergy of wheat straws. A negative linear relationship between the exergy value and moisture content was observed. The ash related exergy varied from 53.468 kJ kg^-1 (Absolvant) to 117.675 kJ kg^-1 (Vuka), accounting for 0.253-0.556% of the total exergy of wheat straws. A negative linear relationship between the exergy value and ash content was observed. The S related exergy ranged from 6.817 kJ kg^-1 (Max) to 11.077 kJ kg^-1 (Vuka), accounting for 0.032-0.052% of the total exergy of wheat straws. A positive linear relationship between the exergy value and S content was observed. The O/C, H/C and N/C atomic ratios and the correlation factors varied in ranges of 0.7133-0.7537, 1.3475-1.5457, 0.0063-0.0225 and 1.133-1.138, respectively. The exergy values of the four wheat straws were between 21.156 MJ kg^-1 (Absolvant) and 21.503 MJ kg^-1 (Monopol). They were mainly determined by the correlation factors and the LHVs. A positive linear relationship between the exergy value and LHV was observed. The combined contribution of ash, moisture and S related exergy to the total exergy was very small (1.694-2.212%) and can be neglected.