Economic evaluation study for a full-scale village-community plant

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The products of the plant
Feasibility estimate for a turn-key farm waste utilization system

The main components of a turn-key system include:

Infrastructure

The village or farm must make available a reasonably flat area of 200m² with 30 cm depth of gravel over the area. An electrical supply of 100-200 amps and a water-pipe of 2-3 inches is needed next to the plant.

Construction

Foundations and columns according to soil structure, with 50-100 m² of concrete floor. A second small concrete floor (50-100 m) for the Peatrum, with a wall around, for the separation system. A larger concreted area will be needed, in addition, if the Peatrum is to be composted (for greenhouses) on site.

Charging system

Chopping machine, pump, mixer, stone-trap, piping and accessories.

Heating system

Boiler, heat-exchangers, circulation pump, regulation and control system, insulation and accessories. There is the possibility of using hot water from an electricity generator supplied separately.

Digestion system

Appropriate volume of digester(s), biogas circulation system for mixing, insulation with a thickness of 5.0-7.5 cm. polyurethane, outlets for digested slurry and biogas, piping, control and regulation of temperature and pressure, and accessories.

Biogas system

Blower, gas traps, pas meter, piping, compressor (10 atm.) and a gas storage of 15 m³ (depending on needs), gas regulators, pressostats and accessories under suitable cover.

Slurry separation system

Slurry container, separation system with elevated stage, controls and accessories.

A total energy generator system

There is the possibility of using the biogas to generate electricity and hot water (to be used for the digestion system), based on current biogas co-generation.

Control and monitoring system

An electricity panel, a control container, electrical accessories in the area, computerized control system, including digital temperature meter and temperature-controlled heating system, controls for mixing system, gas pressure and biogas storage, gas meters, alarm system, etc.

The products of the plant

1. Biogas.

One end-product of anaerobic digestion is a gas that consists of 62-65% methane (CH₄) and under 40% carbon dioxide (CO₂). This gas can be used as an energy source and has, without scrubbing, a caloric value of 5,800 Kcal/m . Scrubbing is only feasible for engine use. It has been used in burners for hot water or steam generation and in systems that produce electricity and hot water or steam (a co-generation system). In some cases, the separation of methane from carbon dioxide is feasible, and then not only the methane is used as an energy source, but also the carbon dioxide, to enrich the atmosphere of greenhouses or for cooling purposes. This separation process is only worthwhile if on- site use for carbon dioxide is essential.

2. Separated Slurry (Peatrum or Cabutz).

The digested slurry is sieved on a vibrating sieve, and the fibrous fraction of the digested cow manure, which consists of 25-40% solids, is collected. A special treatment of washing and adding limestone is used to make a very good substitute for peat moss, used as a casing soil in the champignon mushroom industry. It can also be used as an organic fertilizer and soil conditioner for gardening, though it has a higher value for nurseries and greenhouses, where it is used as a substitute for sphagnum peat moss. In this case, a secondary treatment of composting is essential, which can be done inside or outside the plant. Peatrum or Cabutz are already commercially known product in Israel, and therefore its price is estimated here at 25% lower than the price in Israel for sphagnum peat-moss.

3. Liquid Fraction.

The liquid fraction of the sieved digested slurry can be used as an excellent fertilizer of balanced N.P.K. content. Its value has already been established in experimental stations, but transportation is expensive and therefore no economic value is as yet attributed to this product in the context of the feedlot-farm operation.

4. Slurry

It has been established that the digested slurry, before separation, has a high value in aquiculture as a substitute for pellet feed, replacing up to 50% of the commercial feed. Only in special cases can the slurry be used for this purpose: it must be used in the near vicinity of the plant.

Feasibility estimate for a turn-key farm waste utilization system

The following tables give the basis information on several proposed turn-key plants, based on Israeli estimations. The estimated quantities of wastes which will be processed in the different plants are based on data presented in Table. 11.6.

Table 11.6: Estimated quantities of wastes from 100 cows and 50 calves in a feedlot with shelters and with bedding

For each digestion system described below, quantities are derived from the above table by multiplying the figures by the number of head of cattle. Each system is based on charging the digesters 5 days a week with a volume which is one eighth of the digester volume per day. The average retention time will therefore be 11.2 day

Table 11. 7: Several digestion systems for farm wastes

Table 11.8: Plant sizes and estimated investment for each of the a-d farms

Table 11.9: Expected quantities of produce from a-d farms. Based on the schematic description of a plant for agricultural waste utilization (11.1)

* Energy is expressed in Diesel fuel equivalents for generating hot water.

Fig. 11.2: Schematic description of a plant for agricultural waste utilization (with 1,000 cows and 500 calves)

Table 11.10: Annual expenses for several anaerobic digestion systems

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