Objective
To provide an example of integration of a biogas plant within a set of activities organized in a complex zoo-agricultural exploitation for total energy utilization.
Up to now, technologies for biomethanation have been studied for themselves. As a result, when implemented on farms, although technically satisfactory, biogas plants often do not fit neither, upstream, the available raw materials (wastes and crops) for biomethanation nor downstream, the actual demands of the farm. The present project aims at demonstrating that such an integration is technically feasible. Furthermore, this project constitutes a step towards "first generation" integrated farming, a more general tendency in agronomy to make farming again economically attractive or profitable.
1. The two-stage methane digester has been designed, constructed and is operational since May 1984. The feedstock is mixed with a submerged pump and diluted to a final concentration of 5.5% total solids in a 75 m3 tank, before being continuously loaded to the first-stage methane digester.
The first stage methane digester is a vertical cylindrical steel tank of 900 m3 capacity. Mixing is provided by a central gas-lift and by peripherical gas injectors. A heat exchanger serves to maintain the temperature at 37 deg. C.
The stage 2 methane digester is an un-mixed un-heated vertical cylindrical steel tank of 200 m3 capacity with, on top, a floating hard-cover gasholder of 140 m3 capacity.
2. After digestion, the mixed liquor which has decanted in stage 2 methane reactor, is split into a liquid phase which is used as irrigating fertilizer and a solid phase, part of which is recycled upstream of stage 1 and part of it dried to be used as solid fertilizer.
3. The produced biogas is washedwith NaClO by countercurrent. Condensates are trapped at a low point and H2S removed by absorption.
4. The cogenerator is now a single 150 Kw unit using 80 Nm3 biogas per hour.
5. During the start-up period, problems of mixing of feedstock have been encountered. The submerged pump had to be modified.
6. During a 1 year run, no major technical problems have been encountered. The daily gas production is higher than expected : 1 600 Nm3 per day. The yield can be estimated at 0.58 Nm3 biogas per Kg dry matter or 0.74 Nm3 biogas per Kg volatile solids. As a result more electricity than predicted is produced :
3 000 Kwh per day.
7. Problems are encountered in selling the excess produced electricity to the public utility ENEL. These problems will be solved by improving the automatic control device of injection of electricity in the network.
8. 2 tons of organic fertilizer are daily produced. A full analysis is given : 40% suspended solids, 17% volatile solids, 1.7% N, 3.5% P2O5, 1% K2Oand trace metals. Field trials indicate the good quality of the compost-like material obtained as solid material after dehydration.
9. The production price of LIT 0.31 per Kcal produced has been calculated.
The exploitation "Il Prato" is located in Sassari, Northern Sardinia. It consists of a complex group of activities : a facility for 150 000 laying hens, the corresponding chicken nurseries, the appropriate refrigerated food-type storage facilities and 20 ha of land for intensive agriculture (greenhouses or hydroponic cultures).
Potential raw material for biomethanation is constituted by 13.5 tons poultry manure per day (representing 2.9 tons dry matter). A two stage methane digestion is foreseen with partial recycling of the sludge of stage 2 upstream of stage 1.
The design data are the following :
Stage 1 will include a completely-mixed methane digester of 700 m3 working volume. The poultry manure, diluted 1 : 2.5 with water will be loaded at a rate of 1.8 Kg organic matter per m3 of digester per day. The mean residence time will be 15 d. The working temperature will be 35 deg. C. Stage 2 will include a completely-mixed methane digester of 500 m3 working volume. The mixed liquorfrom stage 1 will be loaded at 3.1 Kg organic matter per m3 of digester per day. The mean residence time will be 11 days. The temperature, uncontroled, is expected to drop to 30 deg. C. On the average, a conversion of 50% on dry matter of 40% on volatile solids and of 30% on COD is foreseen. 880 m3 of biogas with a methane content of 65% is expected to be produced per day. 180 m3 biogas will be used daily in a heating unit for the daily production of 1 Gcal (4.2 GJ) thermal energy and 700 m3 biogas will be used daily in a co-generation unit of 48 Kw for the expected daily production of 1 152 Kwh, that is 2.7 Gcal (11.2 GJ) per day.
Of this expected energy production, the biogas plant itself will require 5.5 GJ thermal energy and 322 Kwh per day. This will represent a recycle of 28% of the produced energy while 72% will become available for use in the exploitation. This available energy will be used for the operation of the various facilities and processes namely heating, cooling and ventilation of poultry houses, conditionning of greenhouses, thermal assistance of hydroponic cultures, and refrigeration of storage chambers. The digested sludge, a compost-like product, will be assessed by field trials for its fertilizing properties.
The project aims at elucidating the actual conditions for complete, quantitative, qualitative, time and space integration of a biogas plant in a farming exploitation. Besides technical considerations, sociological problems will be elementarily considered. Auxiliary intensive agriculture can be set up, if necessary, to match the integration efficiency. A closed-energy system through a biogas loop should be achieved.
Programme(s)
Topic(s)
Call for proposal
Data not availableFunding Scheme
DEM - Demonstration contractsCoordinator
07100 Sassari
Italy