Batch and continuous biogas production arising from feed varying in rice straw volumes following pre-treatment with extrusion.

This paper studies the synergistic effects on biogas production obtained when different feedstocks are co-digested with varying proportions of rice straw and explores their behavior at the laboratory scale in continuously stirred digesters. Evaluative measures included methane production, volatile solids degradation, ash accumulation, and extrusion effectiveness. The effect of extrusion on the production of energy was also investigated. Results indicated that continuous stirred digesters fed with substrates composed of 10% or 30% of ensiled rice straw (on total FM) produced 146.1 and 140.0lNCH4kgDM(-1)day(-1), respectively. When extrusion was employed, organic matter degradation was promoted and methane production was significantly raised-by as much as 16%. For the feeds containing 10% rice straw, the increase in obtained energy was higher than the energy needed for the extrusion, but the energy balance was close to zero when the percentage of rice straw was the 30% of the feed.

The effect of particle size and thermal pre-treatment on the methane yield of four agricultural by-products.

One way to optimize methane production in anaerobic digestion plants is to substitute ligno-cellulosic by-products for crops traditionally used as energy sources. However, using these by-products requires introduction of a pre-treatment system to minimize energy input and maximize energy output for an improved net energy equation. In this study, four agricultural byproducts (wheat, barley, rice straw and maize stalks) underwent various mechanical and thermal treatments prior to anaerobic digestion including particle size reduction to 5.0, 2.0, 0.5, and 0.2 cm and heat application to 90 °C and 120 °C. Mechanical pre-treatment increased byproduct methane yields more than 80%; thermal pre-treatment improved yields more than 60% for wheat and barley straw. Pre-treating wheat straw improved methane yields most, regardless of whether the method was thermal or mechanical. An electric net energy balance was also completed to analyze the feasibility of the pre-treatments according to input and output of energy.

Thermal pre-treatment of solid fraction from mechanically-separated raw and digested slurry to increase methane yield.

Anaerobic digestion plants rely on large-capacity storage tanks to manage the agronomic utilisation of the digestate. As a consequence, many Italian A.D. plants have introduced mechanical separation of the digested slurry to simplify process requirements. This study evaluated the possibility of reusing mechanically-separated solid fraction as a further biomass input anaerobic digestion plants. The effects of storage and thermal pre-treatment on digested solid fraction were assessed through biogas and methane yield measures, and then compared to the yields associated with undigested solid fraction of raw pig slurry. The specific CH4 yields of digested solid fractions ranged between 71.4 and 156.9 lN/kg VS, whereas the biogas yield from undigested solid fractions was 78.7 lN/kg VS. Solid fraction storage showed no significant effect on specific CH4 yields in any of the examined samples. However, in the case of the undigested solid fraction, thermal pre-treatment proved to be an effective method to increase CH4.

The methane yield of digestate: effect of organic loading rate, hydraulic retention time, and plant feeding.

In biogas plants, huge volumes of digestate are produced daily and stored in uncovered tanks, which leak methane into the atmosphere and cause negative environmental impacts. To better understand the effect that different operating parameters of anaerobic digestion plants have on digestate residual methane yield, four digestate samples collected from plants with very different operations were analysed in batch reactors. Their methane yields were very heterogeneous and varied between 2.88 and 37.63 NL/kgVS. The methane yield was shown to be highly influenced by the A.D. plant Organic Loading Rate and by feedstock quality; hydraulic retention time had only limited effects.

Evaluation of the biogas productivity potential of some Italian agro-industrial biomasses.

Batch trials were carried out to asses the biogas productivity potential of rice and barley straw, grape stalks, grape marcs, maize drying up residues, tomato skins and seeds, and whey. Trials were carried out in 2l glass digesters kept in a thermostate controlled room at 40 degrees C for 40days. The most productive biomasses, in terms of specific methane yield, were the whey and the maize drying up residues. Their specific methane yields were 501 and 317l(N) CH(4) *kgSV(-1), respectively. Barley and rice straw gave a specific methane yield of 229 and 195l(N) *kgVS(-1). Similar result was also obtained from tomato skins and seeds. Grape stalks and grape marcs produced lowest amounts of specific methane, respectively, 98 and 116l(N) CH(4) *kgSV(-1). According to trial results and considering the availability of examined biomasses in Italy, it is possible to estimate their total energetic potential close to a value of 21,900TJ *year(-1). This energetic potential value is equal to that obtainable from the anaerobic digestion of about 6.5 million tons of maize silage.