Friends, today in this article we will learn about the transportation and collection of biogas. I am going to give you information about all this in a very good way. So let’s start.
Bio-gas Storage
Bio-gas and bio-methane can be stored for on-farm use. Most bio-gas is used as soon as it is produced. Bio-gas storage is required in some cases, when consumption exceeds production or when digester equipment is being repaired.
Some important factors for on-farm bio-gas storage are as follows-
(i) Needed volume (only small quantity of bio-gas needs to be stored at any one time)
(ii) Possible corrosion due to H₂S and water vapour
(iii) Cost (Since bio-gas is a relatively low value fuel)
Low pressure storage of biogas
For bio-gas systems, a floating gas holder mounted on a digester is useful for low-pressure storage. These systems operate at a pressure of less than 10-inch water column (2 psi). Floating gas holders may be made of steel, fiberglass or flexible fabric. Storage tanks fitted with separate floating gas holders may be used for storage of digestate and raw bio-gas.
The advantage of a digester with an integral gas storage component is lower capital cost of the system. Flexible inflatable fabric top gas holders are low cost and trouble-free, and do not react with H₁₂S that is present in the bio-gas. Flexible membrane materials used to cover these types of gas holders include high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low density polyethylene (LLDPE) and chlorosulfonated polyethylene covered polyester (such as hypalon). The thickness of these cover materials varies from 18 to 100 mils (0.5 to 2.5 millimeters). Various sizes of gas bags are also available which can be added to the system and are made of the same material and protect against puncture damage.
Medium pressure storage of clean biogas
Bio-gas can also be stored at pressures ranging from 2 to 200 psi, but this is rarely done in the USA. To prevent corrosion of tank components and for safe operation, the bio-gas must be free of H2S before storage. Cleaned bio-gas must be slightly compressed before storage in a tank. Compressing bio-gas requires about 5 kWh per 1000 ft³ at 250 psi. Assuming the bio-gas contains 60% methane and the heat rate is 13,600 Btu/kWh, compression requires 10% (approx.) of the energy content of the stored bio-gas.
Biomethane storage
Biomethane is less corrosive than bio-gas and potentially more valuable as a fuel. For these reasons it is possible and desirable to store it for on and off-farm use.
High-pressure storage of pressurized biomethane
To save space the biomethane is stored by compressing it. For high pressure gas scrubbing is important which removes impurities like H₂S and water vapour which condense and cause corrosion. The gas is stored in steel cylinders. Safety devices like pressure-relief-valves and rapture disks are used in the storage system. The cost of compressing the gas from 2000 to 5000 psi is much higher than that of medium pressure storage. For compression up to 2000 psi, 14 kWh per 1,000 ft. of biomethane is required.
Storage of Liquid Biomethane
Biomethane can also be stored in liquefied form which is called LBM. LBM has two main advantages-
- easily transportable
- Dispensed to either LNG vehicles or CNG vehicles
If LBM is to be used off-farm, it should be transported by tankers truck having 10,000 gallon capacity. LNG storage tanks are double-walled and thermally insulated vessels having 15,000 gallon storage capacity for stable conditions. For a dairy having 1,000 cows and 15,000 gallon capacity, it is sufficient for LBM production for six weeks.
Over-the-Road Transportation of Compressed Biomethane
If transportation of biomethane via natural gas pipelines and dedicated pipelines is expensive or impractical, another method is over the road transportation in which compressed biomethane is transported.
Biomethane has very low energy density at ambient pressure so it must be compressed at high pressure (around 3000 to 3600 psi) to be economically transported in an over the road vehicle. Some major requirements for over-road transportaion are following –
- Transportation in DOT- approved tanks (for example-DOT-3AAX seamless steel cylinders) that do not exceed the rated tank pressure.
- Water vapor content less than 0.5 lbs/million & cf (ie, less than 10 ppm H₂O)
- 98% minimum methane content
- Appropriate hazardous materials markings
Over the Road Transportation of Liquefied Biomethane
Most of the infrastructure issues related to biomethane distribution are resolved through over the road transportation of LBM. Bulk LNG is transported in LNG tankers. LNG is transported at low pressure (20 to 150 psi) because it is a cryogenic liquid (its nominal temperature is -260°F), requiring special handling.
What did you learn today :-
Now you must have known about transportation and collection of biogas and you must have got the answer to all these questions very well.
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