Biomethanation of Minnesota reed sedge peat

Abstract

The high moisture content of peat, the occurrence in peat of large concentrations of fermentable organics of plant origin, the use of hydraulic peat mining techniques, and the possibility of recycling digested peat residue to the peat bogs are factors that support the position that biological gasification methods can be effectively utilized. Chemical characterization work indicated that peat is rich in organic matter, has all the essential elements needed for biological gasification, and should, in theory, be digestible. However, the chemical analyses also point to three possible impediments to vigorous digestion of peat: (1) The presence in peat of large concentrations of cellulose, hemicellulose, lignin, and other complex substances of low biodegradabilities; (2) The nonavailability of such essential elements as nitrogen, phosphorus, etc., that are mostly organically bound; and (3) The toxicities arising because of nitrates and other inhibitors.

The work reported here consisted of exploratory digestion runs designed to study the effects of such factors as digestion mode, inoculum source, temperature, loading, detention time, feed particle size, and chemical pretreatment on peat digester gas yield, production rate, and gas quality. Work with Minnesota Reed Sedge Peat showed that it could be digested in both batch and continuous modes with or without external nutrient additions. A thermophilic temperature of 55°C effected higher gas yields than other mesophilic or thermophilic test temperatures. A thermophilic (55°C) methane yield of 0.14 std m³/kg (2.2 SCF/lb) VS (volatile solids) added was obtained with raw as-received peat during batch digestion for about 86 days, compared with a methane yield of 0.11 std m³/kg (1.7 SCF/lb) VS added observed during semicontinuous thermophilic (55°C digestion at a 60-day detention time and 0.80 kg VS/m³-day (0.05 lb VS/ft³-day) loading. Subsequent batch digestion showed that particle size reduction and acid or alkali treatment could cause a three fold increase in methane production over that obtained with raw untreated peat. This suggests that a methane yield of up to 0.31 std m³/kg (5 SCF/lb) VS added — which is about 60% of the maximum theoretical yield — may be obtained by optimizing the digestion and feed pretreatment conditions.