Biogas technology at municipal level: Pioneering innovations

Mr. Schulz, of the Weihenstephan Agricultural Engineering Association in Freising, near Munich, presented a paper emphasizing how biogas technology can benefit municipalities, given current environmental problems. Calculations relating to the avoidance of pollution, e.g. by reducing emissions of methane and ammonia, the use of digested sludge as a substitute for chemical fertilizers, and on reduction of ground water pollution are becoming increasingly important. For the individual farmer, biogas meant less work, said Mr. Schulz. For example, digested sludge reduced the capability of weed seeds to germinate, at the same time causing plants to be far healthier than conventional fertilizers.

Co-fermentation concept

Professor A. Gosch, of Friedberg Agricultural College in Giessen, presented a pioneering co-fermentation concept. Nutrients are added to substrates to be fermented to improve the yields of biogas plants. Gosch recommended an extremely economical method which is also environmentally compatible: lawn cuttings, maize silage, used cooking fat, sour whey, brewery waste, household waste, can be used as additives. In one plant, even fat from a deep fryer was processed - a waste product that poses a problem for restaurants in particular. Professor Gosch explained that the bacteria had to be “weaned“ to the new substances gradually.

Instead of composting household waste, which involves heavy costs for municipalities, this alternative waste disposal plan proposes that farmers should be paid to dispose of them. As a result, the energy yield of biogas plants would be increased and the nutrients would enrich the (biological) fertilizer. Farmers in Baden-Wurttemberg have put forward a proposal for small, decentralized plants with capacities of 1, 000 tonnes p. a. to process the biowaste of communities with 700 to 15, 000 inhabitants. The financial benefits are said to be appreciable, both for the municipality and for the plant operator. (Professor Gosch is offering two publications on co-fermentation at cost price. Address: Wiesenstraáe 14, D-6000 GieBen, FRG)

The remarks on plant engineering in connection with solid manure technology reflected recent findings concerning the profitability and energy yield of plants with steel tanks and concrete slurry pits. The use of solid manure and bedding straw is the most far-reaching recent chance in the technology for agriculture.

Solid manure technology

Due to the higher percentage of organic dry matter in the substrate the gas yield can be tripled as compared to the yield from the more liquid dung urine substrate. To achieve this, however, the solid components must be thoroughly pulverized with a powerful stirrer, first in the influent collecting tank and then in the digester.

Today, there are two types of plant which optimize gas production using a stirrer - the concrete pit plant (biogas storage plant) and the steel tank plant (through-flow type). With the concrete pit plant, a concrete liquid manure tank with a concrete cover is expanded to convert it into a biogas plant. Storage and digestion take place in the tank. The gas produced in the digesting chamber is collected in the chamber itself, in a bag made of plastic sheet.

As a variant of this, open manure pits can be covered with double plastic sheet. The outer, fabric-reinforced sheet keeps its shape stable, while the lower sheet rises and falls depending on gas production and consumption. By combining the liquid manure with the digestion chamber the plant is more compact than a steel tank plant, and can be built with less labour and materials. The onIy d is advantages are those resulting from mixing fresh substrate and fermented material from the digesting chamber. Thus, the quality of the biofertilizer and compatibility with (vegetable) plants are reduced, while the odour is stronger.

In addition, the storage plant method leads to considerable fluctuations in gas production over the year. In practice, this problem is counteracted by using co-generative motors, excess power being fed into the grid. Since 1991, the utility companies have bought this power at 75% of their resale price.

On the other hand, the steel tank plant has proved to be a reliable type of through-flow biogas plant for 30 years now, even with problematic types of liquid manure. The heart of the plant is a horizontal steel tank- normally a used heating oil tank - with a paddle type stirrer. The gas is stored in the tank. The only disadvantages of this type of plant are the weather protection and the space requirement.

Fuel for block-type thermal power stations

The south-west German division of the Technical Surveillance Association (TUV) has so far laid down only minimum technical requirements. These include in particular safety devices to control blowback, overpressure and vacuum and automatic gas flow interruption during motor operation.

However, regulations for lowpressure operation are much more urgently needed, because according to the TUV expert unjustified or excessive demands are already pushing investment costs up.

Fig. 1 Storage-type biogas plant with double-skin pit cover and swivel-mounted gaslight stirrer.

Biogas as a fuel for block-type thermal power stations represents the ther best possible way of utilizing the energy thus produced. The outlook for co-generation has become all the more interesting since legislation was passed in Germany ensuring that electricity fed into the grid from private producers is paid for.

Internal combustion engines normally generate far more heat than electrical power, but if combined with a gas storage tank it would be possible to conclude agreements with power station operators to provide them with electricity in particular to help cover peak loads. The heat given off by the engine can be used to heat the plant and residential accomodation.

Many different technical solutions have been evolved for co-generation. Various models were presented by different speakers. While the Fiat Totem (Total Energy Module) is commonly used in digester gas technology, one could just as easily imagine using a Chevrolet engine for other applications.
In tests, a converted turbo diesel engine attained a level of efficiency in electricity generation 40% higher than that of conventional block-type thermal power stations. A low-cost variant of a co-generation “kit“ might also be suitable for developing countries, as was shown during the presentation.

Model calcurations presented at the meeting indicated that the technology would be profitable only with a 25% government subsidy - at today's prices! On the other hand, the calculations did not take into account the future benefit of the plant if it is used for disposal of biowaste, with a higher energy yield and production of higherquality fertilizer.

The heart of the plant is a horizontal steel tank with a paddle-type stirrer. Plants of this type have proved highly reliable, one having been in use for 30 years without giving any problems.

In particular, it copes well with floating scum, sediment layers and congestions, even with problematic manures such as liquefied solid manure with a high straw content and pig manure, which tends to form sediment layers. Nor are there any problems with gas-tightness or thermal insulation.

Energy consumption for mixing is very low. When the liquid manure is pumped in daily, the same quantity of fermented material overflows into the manure storage tank at the other end. The elongated shape of the tank helps to prevent loss of unfermented manure.

Used heating oil tanks with a capacity of 50 to 100 m3 are normally used.

They are between 2.5 m and 2.9 m in diameter end from 12 m to 14.5 m long. If necessary several tanks can be combined.

Through-Flow-Type Biogas Plant

The plant is insulated on the outside with 20 cm thick Rockwool A digester volume of 16 - 20 m3 is required per cubic metre of liquid pumped in daily The plant takes up more space than a storage-type plant and requires weather protection, but the gas is stored in the same way.