ArtsAutosBooksBusinessEducationEntertainmentFamilyFashionFoodGamesGenderHealthHolidaysHomeHubPagesPersonal FinancePetsPoliticsReligionSportsTechnologyTravel

BENEFICIAL USES BIO GAS

Updated on June 22, 2015

In examining the assumption a number of issue come forward.

  1. T he first is the assumption that the 50days are required at 270C to produce 36 1 gas. This crude gas production assumption does not take into account either the feed that the cattle have been fed or the age of the manure. Cattle manure from cows fed a high protein is much more digestible than from those fed cop residues.
  2. The second is that fresh manure is also much more digestible than dried manure.
  3. The third , as stuckey noted based on the kinetic of anaerobic digestion, that approximately 80% of the organics are degraded in 15 days and 90% in 30 days. Therefore, there would not seem to be a justification for a 50 day detention time if maximum gas is the objective of the system. Extra detention time means a larger digester and more cost.
  4. The fourth issue is the assumption that 1 1 of water needs to be added to each kg of manure. The addition of the water gives a solids content from 3 to 12% depending upon the age of manure. Marcheim has operated digesters at 16 to 18% solids. To keep the digesters to a minimal size, one would like to have the smallest amount of water in the digester.

The incosistencies of the approach of ESCAP become more obvious when its guide book is examined further . These do not seem to be any ratoinal for the detention period for chicken manure, the assumption is that 44 1 of gas will be produced per kg of feed and that the detention period should be 30 days; for pig manure, the assumption is 45 1 gas by 1kg when digested 40 days; and for human wastes, the yields is 24 1 of gas when digested for 60 days.

Some of the more recent publications have indicated that in India it may have been recognized that temperature does have an impact. The detention period for design has been changed to 30 days for south India, 40 days for central India, and 55 days for the cooler mountain arears.

The approach suggested in the National Academy of Sciences (NAS) example in determining size of the digester is to select a height-to-diameter ratio 1:1 then, determine the dimensions of the digester an add a gas holder which will store one half of one days's production. The second approach is to use a truncated conical (hopper) bottom tank for the digester. In the example, NAS essentially keeps the diameter the dame and reduces the wall length and adds the hopper bottom.

However, from an operations' point of view there appears to be no justification for consideration of the hopper bottom. The hopper bottom is an difficulty to construction because a deeper pit would be needed.

CONCEPTS USED FOR A CHINESE DIGESTER

Translation are available of the Chinese Bio gas manual in many languages. This manual, in its English translation, describes the objectives of the initial development of the Chinese digester; i.e. dispose of human excreta for the prevention of disease, produce a soil conditioner (fertilizer), and provide energy which could be used for cooking.

The Chinese water pressure system ideally was designed to use a gravity feed system to collect the fresh manure and human wastes and place them in the digester. In addition, crop residues were often added to the digester to produce additional soil conditioners. Significant features include the incorporation of gas and slurry storage within the inexpensive digester vessel, which used a fixed cover and could be built in the field. While the digester met the objectives, two major disadvantages are common. The first is operational problem, two major disadvantages are common. The first is operational problem and the second is the labor required to remove the slurry and transport it to the field. Both of these actors are frequently overlooked.

In any arears China, manure and night soil are applied to the fields as a soil conditioner. By making the digester larger, storage is provided to minimize the losses of ammonia nitrogen from the slurry. The slurry is removed twice a year abd applied to the fields. Therefore the primary criteria used in designing the chinese digester in China was the quantity of soil conditioner that the farmer had the raw materials to produce and store between application to the field.

The system ws widely applied and chen reported in 1983 that 5 to 6 million units had been built. Larger number have been reported but these numbers appear to be in error. Because of the successful introduction of the digester in China, other countries rushed to copy the system often without examining their own countries conditions.

One such application recently observed was in Turkey where three groups have been experimenting and conducting demonstrations using the anaerobic digester. One group had been to China, and hey decided to apply the Chinese experience to Turkey. They determined that since the major cost is that of the digester, the size of the vessel should be kept t minimum. Still they needed to provide storage for the effluent solids so they collected the effluent from the digester in open concrete pits where it was held until it could be applied to the fields. If there were excess effluent, the pit would act like a clarifiction basin where the solids would settle to the bottom and the supernatant could be removed. Overlooked was the fact that the nitrogen that enters the digester is converted into the ammonium from which is volatile. Thus a portion of the nitrogen could be lost to the atmosphere.

CONCEPTS USED FOR A BAG DIGESTER

The bag diagester operates like a plug flow reactor where the materials added each day theoretically will move as a mass through the digester until the day when the theoretical detention time has been reached. Then the digested mass will flow from the digester as a unit. A portion of the effluent is re-introduced into the inlet so that it will act as seed to re-inoculate the digester. The primary advantage of the bag digester are that it is purchased as a unit, it is inexpensive compared to the Indian or Chinese style digester, and can be easily installed and operated. The disadvantage is that the unit is harder to maintain t a uniform temperature.

To size a bag digester, similar approaches to that quoted for Indian digesters have been used.

The red mud used successfully to build the bag digesters in Tiwan is not universally available. In the United States, a neoprene material without much success because of the tendency for the materials to degrade under the sunlight. Modification of the bag digester have been observed, which include the building of a long narrow concrete trench in which a plastic cover is laced along the side-walls. Obtaining a good seal along the walls is difficult. The plastic cover is to act only as a gas collector, and does not allow much storage because of the rigidity of the plastic.

Lengths to diameter ratio's of 3 to 14 are used for bag diagesters. The inlet is above the digester to maintain a gas pressure of approximately 0.5m. This maximum pressure depends upon the materials used to manufacture the bag.

If maximum gas production is desired, an approach similar to that discussed under the Indian style digester could be used. The gas production rate over a period of time would be the governing factor.

EXAMINING A SYSTEM

In examining the technical and economic feasibility of anaerobic digestion system, a number of question need to be asked the feedstock, the digester system, and the potential used of the effluents. The general question include.

  • Quantities and cost of the potential feed-stocks;
  • Characteristics of the feed-stock and effluents;
  • Cost of obtaining the feed-stocks and production of the effluent products;
  • Cost of digester and utilization equipment; and
  • Cost of alternative fuels being used.

The financial analyses of most digestion systems show that when used for ony a single purpose, i.e. energy, health, or to produce a soil conditioner, they are not economical. Therefore, uses must be considered for the gaseous, liquid, and soild effluents. Even when the effluents are all used, the financial viability is positive only if the energy, soil conditioners, or feeds were previously purchased with money. In these cases the cash outflow of the user is reduce and he is able to pay for the plant.

Advocates of the system will try to ague to the counter and stte what they consider are successful programs, for example in India. a closer examination of the Indian systems will usually show that it is ony through subsidies and grants that the systems are built.

OPERATIONAL FACTORS

Six operational factors have been identified as critical to the anaerobic digestion process. Each of these factors to be considered when designing a system. These are:

  • Composition of the organic substrate:
  • Retention time;
  • Concentration of the substrate;
  • Organic loading rate;
  • Degree of mixing; and
  • Heating and heat balance.

COMPOSITION OF THE ORGANIC FEED-STOCK

Any organic waste that is bio-degradable can be digested. As organic matter is bio-degraded by bacteria, a portion of the materials produces the end product of methane and carbon dioxide, while part of the material is converted into bacterial cells. The portion converted into cells has been estimated to be from 2 to 25% for anerobic digestion. There is always a refractory fraction that passes through a digester under graded.

Bio-biodegradability is usually measured in one to two ways: chemical oxygen demanded (COD) or volatile solids (VS) destruction. For most application the volatile solids are the easier to measure. It should be noted that there is no direct correlation between the two, and thus, measures of bio-biodegradability are not comparable. Gossett did note that, the CODNS ration usually decreases during digestion so that the ration in the effluent is less than in the in fluent.

Values for the bio-biodegradability are available from the literature or can be measured in the field as VS destruction data are calculated. Some of the values from the literature are shown in the Table1.

Anaerobic digestion can use a number of materials as feed-stock. These include manures, human wastes, crop residues, food processing, and other wastes, or mixture of one or more of the residues and wastes.

Manures have been probably selected for feed-stocks because they are available, have good nutrient balances, are easily slurred, and are relatively bio-degradable. The range of bio-biodegradability reported varies from 28 to 70%. This variations is partly due to what the percentage of silage contains more lignocellulosic materials. Thus, in developing countries where cattle are fed agricultural wastes, the manure would be less degradable than on cattle feedlots in United States.

A Second reason for the large variation in bio-biodegradability of manures is that, fresh manure s much more bio-degradable than dried manure. The bio-biodegradability of manure as the time period between excretion and when it is fed to the digester increases.

A diary cow that weight about 450kg will produce about 39kg of manure and urine per day which will contain about 4.8kg of dry organic solids. Cows that are larger or smaller will produce wastes in the dame proportion. The organisms in the digester require a minimum of 6 days at C to reproduce and about 20 days to use efficiently the bio-degradable organics in the manure.

The effects of the types of holding area and the frequently of cleaning or collecting the manure are shown in Table 1.

Jewell pointed out there is one difficult in using pig manure where it is collected using water flush systems. In these systems, the manure arrives at the digester very diluted. As an example, in Yugoslavia at an installed site the slurry contained less than 2% solids. In the Philippines, a digester system was built on the basis of a two-to-two dilution is required or some sort of concentration system is needed to keep the digester economical.

Poultry manure is very concentrated compared with cattle or pig manure. It contains 25 to 35% solids. For 100 kg of live weight of chickens, Jewell estimated the methane gas production to be 2.3m3. In addition, there is an abundance of volatile acids or ammonia in chickens manure which can under some conditions inhibit the methane production in the digester. Thus the poultry manure is either diluted or mixed with other feed-stocks to avoid the possible toxic conditions in the digester.

If one is to use manure as a feed-stock some of the questions that should be addressed include:

  • What are the number of animals (poultry) and their size (weight)?
  • Under what condition are the animals (poultry) held

BIO-DEGRADABILITY OF VARIOUS DIGESTER FEEDS (FROM VARIOUS SOURCES)

SUBSTRATED
DAYS
TEMP 0C
% COD %Dest.
VS Dest.
Comments
Beef Manure
 
 
 
 
 
9% Com Sllage & 88% Com
30.6
72.5
 
 
 
91.5 Com
Inf.
55
52.1
 
 
40% Com Silage
 
55
52.1
 
 
53.4 Com & 7% Com Silage
 
55
73.5
 
 
87.6% Corn
 
 
 
 
 
6-8 Week Old Feed
 
55
60.0
 
 
Lot Manure
 
 
 
 
 
Cattalls
120
35
 
59.3
Batch
Cattle Manure
80
 
 
28.1
 
Corn Leaves
120
35
 
71.8
Batch
Corn Meal
90
35
 
84.9
Batch
Corn Stalks
120
35
 
77.2
Batch
Corn Stover
40
35
64.8
 
Batch
 
40
35
59.1
 
Batch
Chicken Litter
10
60
13
20
High lignin content aged one year
Manure
15
32.5
78.1
67.8
10% TS feed
 
28
31
45.3
 
10% TS feed
 
120
35
 
75.6
Batch
Dairy Manure
25
35
30.2
28.9
 
Straw Manure
120
35
 
57.5
Batch
Manure
110
35
48.4
44.4
Batch
Elephant Manure
120
35
 
52.5
Batch
Hyacinth-Bemuda
12
35
40.3
39.2
Batch
Kelp (sea weed)
12
35
57.7
45.1
 
Kelp (treated)
120
35
 
62.0
Batch
Meat packing waste
15.1
26
44.3
 
 
Municipal Solid
 
 
 
 
 
Waste and Sludge
 
 
 
 
 
Municipal Soild wastes
15
35
45.6
38.2
12.5 sewage sludge
Newsprint
90
35
 
88.1
Batch
Peat (U.S.)
60
35
 
11.1
 
 
56
35
 
16.7
 
Pig Manure
15
35
58
60
14% Protein ration
 
15
32.5
54.6
60.9
10% TS feed
 
120
35
 
72.7
Batch
Wheat Straw
120
35
 
55.4
Batch
Water Hyacinth
120
35
 
58.8
Batch
Yeast Waste
 
30
67.1
62.6
 

ESTIMATED MANURE AND GAS PRODUCTION RATES (based on 450 kg live weight)

 
DAIRY CATTLE
BEEF CATTLE
PIG
CHICKEN
Manure production kg/day
39
26
23
27
Total Solids kg/day
4.8
3.4
3.3
7.9
Volatile Solids k/gday
3.9
2.7
2.7
5.8
Dig.Efficiency
35
50
55
65
COD:VS
1.05
1.12
1.19
1.28
Gas production 1/kg VS*
219
325
381
490
1/450 kg Animal/day
860
870
1020
2860
Note: Values may vary from these values due to difference in fee ration and management practices. * Based on theoretical gas production rate of 831 1/kg of volatile solids (VS) destroyed and assumes the CH4 :CO2 ratio is 60:40 and the conversion of

Composition of Feedlot Manures.

 
Environmental Lot
Concrete Lot
Dirt Lot
Water (%)
85
65
29
Total Solids (%)
15
35
71
Volatile Solids (% of total solids)
78
67
35
Biodegradable Volatile solids (% of volatile solids)
60
31
34
Manure production - Total solids (kg/animal/day)
4.5
2.6
4.4
  • If the animal (poultry) are confined, what type of pens are used and how and when is the manure collected?
  • What quantities of manure are collected?
  • What are the existing uses and prices (if sold) of the manures?
  • What are the transportation costs of the manure from the production site to the conversion facility?

Agricultural residues have been used although usually with manure. The most desirable materials for a completely mixed system are the high moisture herbaceous materials; which are for dry digestion system almost any organic materials can be used. Specific materials that have been used as digester feedstocks include water hyacinths,algae,com stover, wheat, and rice straw.

A frequently argument against the use of crop residue is that they are already used as animal feeds or for fuel. Experience in china has shown that where crop residues are used as fuel for burning, the efficiency is less than 10%. When the same crop residues are used which produce more net energy leaving the nutrients to return to the soil. The crop residues then not burned can be used energy leving the nutrients to return to the soil. The crop residues then not burned can be used as animal feed. Thus in China, the use of the digester has allowed the animal population to increase.

In some parts of the world, cattle are bedding materials is more stable, it produces less gas in the digester and leaves more solids in the effluent. For example, Jewell reported that 100kg of manure will produce twice as much as 50kg of manure and 50kg of bedding mixed in the digester.

This stability in the agriculture residues is due to the lignocellulosic bonds. The agricultural residues have an additional disadvantage in that they may float and form a scum layer. Size reduction or chemical treatment can improve the handling and bio-degradability of the residues.

water - born plants such as water hyacinth and duck weed have been used in a limited number of cases as digester feed.

some information that should be obtained in considering crop residues includes:

  • The type of crop and arears that they are grown;
  • Quantities and the form in which they are available;
  • Characteristics of the residues (e.g., moisture, volatile soilds and size of particles);
  • Current use and price of the residues if sold;
  • period of availability;
  • Storage characteristics if known; and
  • Cost of collection and transport to the conversion facility.

Human wastes represent the major problem from a health standpoint. They must either be properly used or disposed of to reduce the transmission of disease. If the wastes are proposed to be used as a feed-stock for a digester, it is very likely that any community will have a number of collection systems. One may find household served by night soil collectors, while other sections are served by pit latrines, septic tanks, and / or water carriage systems. It is important if human wastes are considered as a feed-stock that the sources, quantities, and characteristic of the wastes be identified.

Specific question involve:

  • Type of waste collection system;
  • Number of persons served by each system;
  • Characteristics of wastes collected from each system; and
  • Current use and price (if sold) of the wastes.

Comments

    0 of 8192 characters used
    Post Comment

    No comments yet.

    Click to Rate This Article