Importance of Feedstock in a Small-Scale Agricultural Biogas Plant

Abstract

Although no legal sustainability criteria have been formulated for electricity and heat production from biogas, the sustainability and profitability of large-scale biogas plants which use mainly energy crops is now questioned. Small (farm-size) biogas plants characterized by CHP electrical output in the range between 15 kWel and 99 kWel, operating on agricultural wastes and by-products, seem more suitable; however, the variety of feedstock may be crucial in the proper design and operation of such family biogas plants. This paper aims to present the problems that occurred in small agricultural biogas plants fed with sheep manure (SM), horse manure (HM), and grass-clover silage (GCS). This paper also focuses on analyzing the energy balance and carbon dioxide (CO2) emissions related to four technological solutions (Scenarios 1–4) based on various feedstocks, grinding and feeding systems, and wet/dry fermentation. The biogas plant was originally based on dry fermentation with an organic loading rate ~10.4 kgVS·m−3·d−1, a hydraulic retention time of 16 days, and temperature of 45 °C in the fermentation chamber. The material was shredded and mixed in a mixing device, then the mixture of manures and silage was introduced to the horizontal fermentation chamber through a system of screw feeders. The biogas and the digestate were collected in a reinforced concrete tank. The biogas was sent to the CHP unit of an installed electrical power of 37 kWel, used to produce electricity and recover the heat generated in this process. Scenario 1 is based on the design assumptions used for the biogas plant construction and start-up phase. Scenario 2 includes a new feeding and grinding system, in Scenario 3 the feedstock is limited to SM and HM and wet fermentation is introduced. In Scenario 4, a dry fermentation of SM, HM, and maize silage (MS) is assumed. Avoided CO2 emissions through electricity and heat production from biogas were the highest in the case of Scenarios 1 and 4 (262,764 kg CO2·y−1 and 240,992 kg CO2·y−1) due to high biogas production, and were the lowest in Scenario 3 (7,481,977 kg CO2·y−1) because of the low specific methane yield (SMY) of SM and HM. Nevertheless, in all scenarios, except Scenario 3, CO2 emissions from feedstock preparation and biogas plant operation are much lower than that which can be avoided by replacing the fossil fuel energy for the electricity and heat produced from biogas. Our observations show that a small agricultural biogas plant can be an effective energy source, and can contribute to reducing CO2 emissions only if the appropriate technological assumptions are adopted, and the entire installation is designed correctly.

The Impact of Manure Use for Energy Purposes on the Economic Balance of a Dairy Farm

“…However, some national studies have shown that both the lack of subsidies and the decrease in revenues (sometimes by as little as 20%) or the necessary change in substrates may make the investment unprofitable [129]. In the case of this study, such a negative factor could be the introduction of a fee for emissions and their payment, or the need to change substrates, which may contribute to significant operational (and thus financial) problems [130].…”

Comprehensive Analysis and Greenhouse Gas Reduction Assessment of the First Large-Scale Biogas Generation Plant in West Africa

“…By the end of 2019, 9527 biogas plants had been developed in Germany [33]. However, the eco-friendliness of energy crops, the second most utilized feedstock in European biogas plants, for biogas production is questioned because of the impact on soil fertility and food production [34,35].…”

Leverage of Essential Oils on Faeces-Based Methane and Biogas Production in Dairy Cows

“…There are several reasons for this situation, including low biogas efficiency with a minor degree of organic degradation, mainly due to the high water and fiber content [14,[48][49][50], the need to build large-capacity sealed tanks to store the resulting digestate [39], and lack of a sufficiently large and well-balanced (carbon to nitrogen ratio, organic or lignocellulose content, humidity, etc.) resource of substrates for year-round protection of the biogas plant feedstock [51][52][53][54]. The abovementioned aspects contribute to the insignificant profitability of investments in biogas plants.…”