Biogas Plasticization Coupled Anaerobic Digestion: Anaerobic Pump Calorimetry

This paper presents the quantitative bomb calorimetric high heat values (HHV) for residue samples collected from the Anaerobic Pump (®TAP) and a continuous flow stirred tank reactor (CFSTR) anaerobically digesting a 50:50 wastewater sludge substrate. TAP, an advanced anaerobic digestion (AD) process, features biogas plasticization that greatly increases gas production and leaves a mineralized residual. Measured residue HHVs are compared to HHV estimates from literature empirical relationships. Two empirical formulations, the Meraz thermodynamic formulation (with 7.4% moisture) (Meraz et al. The Chemical Educator, 7(2), 66-70, (2002) and the Channiwala Universal formulation (Channiwala et al. Fuel, 81(8), 1051-1063, (2002), compared favorably (within ± 3% mean value) with the bomb measured HHV values. A stoichiometric ICC description for Ucells is derived. The thermodynamic formation potentials of all measured residues are derived including Ucells. An empirical method was used to calculate the entropy of formation (∆_fS) for all residues and Ucells. Krevelen plots show residue molar ratios of oxygen and hydrogen to carbon (H/C, O/C) are linearly correlated with HHV and formation potentials (∆_fG', ∆_fH', ∆_fS') with strong statistical coefficients of determination (R²). Residue H/C and O/C ratios fell across the peat classification on the biomass coalification diagram. A wide AD methane fermentation zone ≤ 18.6 MJ/kg is identified. The methods and correlation relationships presented enable the computation of accurate HHV and thermodynamic formation potentials without the necessity of direct thermal measurement. These quantitative results confirm that steady state AD of a well-known heterogeneous solid substrate (WWTP sludge) is a linear thermodynamic process.