December 14, 2006 | General

Anaerobic Digestion Plant Yields Nutrients And Power (Hungary)

BioCycle December 2006, Vol. 47, No. 12, p. 61
The Nyìrbátor facility processes about 100,000 metric tons/year of wastewater from slaughterhouses, crop residues and manure, producing about 7 GWh of electric energy and nutrients for about 1,000 hectares.
Mihaly Petis

A 2001 DIRECTIVE from the European Union (EU) established that the percentage of electric energy produced using renewable energy sources reach 22.1 percent by 2010. Following negotiations, a 3.6 percent requirement for increasing the ratio of electric energy produced from renewable sources was formulated with respect to Hungary on behalf of the EU.
Building a biogas plant can be regarded as an efficient investment project for environmental protection and renewable power production. Digestion occurs in anaerobic conditions, without groundwater and air pollution, and during this process fuel is produced by methane producing bacteria. The biogas plant in Nyìrbátor, Hungary, which processes agricultural and animal husbandry waste and wastewater from slaughterhouses, commenced operation in 2003. The plant, which has a 1.6 MW electricity production capacity, produced nearly 7 GWh of electric energy on a biogas basis.
The advantages of biogas production are acknowledged and valued differently by Member States of the EU in their budget depending on their financial resources and preferences of the society. One of the highest values is recognized by the German government (Table 1) in the prices of biogas for several kinds of deliveries. In Hungary, only the basic energy price is granted at present in the average purchase price of 23 HUF/kW but the surplus value (e.g., hot water utilization) is not acknowledged.
In order to extend the production of biogas and to improve its efficiency, the price of this product should reflect both the actual value and that acknowledged by the society. New legislation is needed to determine the magnitude of biogas production and its place in the economic and social structure. To increase the revenues of biogas production, the waste disposal charges levied to animal waste and residues of the food industry are also needed.
Conditions for the profitability of industrial-scale biogas production include: Adequate quantity and content of raw materials; Selecting an optimal location for the investment project; Reducing production costs by taking local conditions into account; Having the necessary infrastructure and complementary buildings in place; Adequate size of crop area for using the produced liquid manure; Utilization of the surplus thermal energy; Improvement of environmental conditions; and Increase of revenues from waste disposal.
The biogas plant at Nyìrbátor is unique in its layout and is the largest in the world. It is a component of a large agricultural system with crop production, animal breeding, slaughterhouses and a processing plant. The plant is designed to process 100,000 metric tons/year of mixed waste with dry matter content of 6 to 10 percent annually. It includes two mixing tanks, six mesophilic fermentors operating at 38°C, six thermophilic fermentors operating at 55°C (retention time of 25 days for each phase), four storage tanks for liquid manure, and four gas motors with the capacity to generate 2,500 kWh. There are four phases of the process – hydrolysis, acid producing, acetic acid production, and methane production.
Raising the temperature (mesophilic to thermophilic) accelerates the decomposition process. The higher the temperature is, the greater the amount of gas that evolves during a unit of time. At the same time, however, the methane content of the biogas decreases. At the Nyìrbátor plant, the homogenized raw material is fed into the fermentors every four hours. Fermentors and storage tanks are placed in such a way that the biomass is transported from the receiving tanks only once. After that, it is moved by gravitation from the mesophilic to the thermophilic units, and from the latter to final storage. With this technical solution, the running costs of the plant are much lower.
Biomass used at the Nyìrbátor plant includes: Wastewater from slaughterhouses – 78,000 metric tons(t)/year; Manure – 11,000 t/year; Crop residues – 4,500 t/year; Animal residues – 14,500 t/year; Other feedstocks – 7,000 t/year. Nutrients with the appropriate composition and proper preparations are necessary to feed as little harmful material as possible into the fermentors. Well-balanced microbiological processes ensure higher amounts of biogas and constant quality.
The internal height of the fermentors is 5 m; 4.2 m is filled by the biomass column and 0.8 m is headspace. Two-thirds of the biogas produced is methane and one-third is carbon dioxide; the gas has high water content in this phase. Biogas produced is transported through two gas collecting tubes placed on the top of the fermentors into the gas treating unit and subsequently into two gas storage tanks, each with a volume of 1,000 m3.
Biogas sensors are installed in the fermentors. If the quantity of the gas increases, it is released through an automatic device into the gas treating unit through a gas meter. The gas is dehydrated in the treatment unit via cooling. Daily volume of the condensed water amounts to 40 to 60 liters. Composition and volume of the various gaseous products are analyzed and measured. It is very important to maintain proper sulfur content in the biogas. Separation of the sulfur is achieved by adding oxygen and iron hydroxide. The precipitated sulfur goes into the substrate and is dissolved.
Composition of the biogas ranges from 50 to 70 percent methane and 30 to 50 percent carbon dioxide; hydrogen sulfide content is 50 to 100 ppm. The caloric value of the biogas is 20-22 MJ/m3. (By comparison, the caloric value of natural gas is34 MJ/m3.) Depending on its methane content, 1.5 to 2 kWh of electric power can be produced from 1 m3 of biogas. Other plant outputs include 90,000 metric tons of liquid manure (adequate nutrients for about 1,000 hectares) containing 320 tonne of N, 130 tonne of P2O5, and 340 tonne of K2O. The ratio of nutrients is 2.4:1:2.6. (Editor’s Note: Another article in this issue’s BioCycle International presents highlights of some of the sessions at ORBIT 2006, held in September in Germany. A paper titled, “Agricultural Utilization Of A Liquid Manure Originated From a Biogas Plant,” analyzed the effect of the fermented liquid manure from the Nyirbátor plant in crop production. Crops utilized in the trial were sweet corn and silage maize in sandy or meadow soil.)
The Nyìrbátor plant also is equipped with the GORE Cover system for composting residuals from the anaerobic digesters, feathers, straw and liquids from hygienization processes. These feedstocks are composted either all together or alternating, depending on availability. The system treats 25,000 to 30,000 m3 per year.
Dr. Mihaly Petis is director of the Nyìrbátor plant, located in the Nyirseg region in Northeast Hungary. This article originally appeared in Biohulladék Magazin.

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