Methane Production by the Anaerobic Digestion of Tall Fescue samples pre- and post-ensiling, prepared by Thermal (40°C) or Freeze Drying

Abstract

Herbage samples for small-scale batch digestion tests are often dried and milled to provide smaller representative sub-samples for analyses.  Thermal drying, however, can change the chemical composition of a feedstock and these changes may impact on specific CH₄ yield.  Consequently, freeze drying is often the preferred system for drying grass samples.  However, additional difficulties are encountered when evaluating silage samples as a result of the loss of volatile compounds during both thermal and freeze drying.  As grass and grass silages are readily used as feedstocks for biogas production, the impact of drying method needs to be considered for herbages pre- and post-ensiling.  This study investigated the effects of thermal (40ºC) and freeze drying on the chemical composition and specific CH₄ yield of tall fescue, pre- and post-ensiling and at three sequential harvest dates in the primary growth.

Tall fescue samples pre- and post-ensiling were either thermally (40ºC) or freeze dried prior to milling.  Dried, milled samples were used to determine herbage chemical composition and specific CH₄ yield in a small-scale (160 ml) high-throughout batch digestion test.

A relatively small increase in specific CH₄ yield (237 and 249 L CH₄/kg VS_added for herbage pre- and post-ensiling, respectively) was observed for herbages post- compared with pre-ensiling and this may reflect the presence of quantities of fermentation products in the dried silage samples.  Compared with freeze drying, thermal drying resulted in a small decrease in specific CH₄ yield in batch digestion tests (11, -22 and -17 L CH₄/kg VS_added for Harvests 1, 2 and 3, respectively) and this reflected the small changes in herbage chemical composition due to drying method.  In general however, the impact of drying method on herbage chemical composition and specific CH₄ yield was similar for herbages pre- and post-ensiling.  More importantly, drying method had little effect on the relative ranking of samples across harvest dates and for samples pre- and post-ensiling.

Keywords: grass, grass silage, thermal drying, freeze drying, anaerobic digestion, biogas

d;=� pr0���O�bottom:.0001pt;text-align:justify;text-justify:inter-ideograph; line-height:140%;layout-grid-mode:char;mso-layout-grid-align:none'>Despite the numerous solar dryers reported by researchers in Nigeria, farmers in Zaria Nigeria continue to dry tomato by traditional open sun drying method, the inherent problems associated with this method notwithstanding.  Therefore this research is aimed at investigating suitable solar collector material that is available, affordable and can easily be worked on by rural farmers.  Metal, wood, cement and mud which are commonly available local materials were used to fabricate four types of solar dryers and the dryers were mounted truly facing south.  Sliced tomato samples were used for performance test of the dryers.  Moisture content of the samples, dryer temperature, wind speed, relative humidity and solar radiation were monitored at 2 hours intervals.  From the analysis, the dryers’ savings in drying time over the open sun drying for wood, cement, mud and metal dryers were 131.25%, 131.25%, 136.17% and 192.11% respectively.  Average ambient wind speed during the drying period was 0.98 m/s, while that of relative humidity was 23.85%.  The metal dryer dried sliced tomato 18 hours ahead of mud dryer, while cement and wood dryers dried the sliced tomato 2 hours longer than the samples dried in the mud dryer. Temperature, energy generation, saving in drying time and  cost effectiveness, of mud dryer when analysed at F-test at 0.01 and 0.05 degree of freedom indicated statistical significant difference when compared with wood and cement dryers. The dryers equally indicated 7.38%, 19.56%, 20.25%, 20.91% and 27. 24% system drying efficiencies, for open sun, wood, cement, mud and metal in that order.  This work revealed that mud solar dryer is available, affordable and can easily be worked on by rural farmers.  Results from this work indicated that mud direct mode natural convection solar dryer is best suited for Zaria and Zaria related geographical locations.

Keywords: solar, energy, collector, crop, drying, drying efficiencies