Home | English | 中文
Concepts Products Services Projects FAQs About Contact Us Sitemap
Topics

  • Biogas Data
  • Biogas Process
  • Biogas Construction
  • Biogas Scrubbing

    (On this page...)

    Concepts

  • Sources of Biogas Production
  • Gas Holder & Metering
  • Scrubbing
  • Hydrogen Sulphide Scrubbing
  • Regeneration of Iron Sulphide
  • Carbon Dioxide Scrubbing
  • Regeneration of Calcium Hydroxide
  • 2-Stage Scrubber
  • Biogas Process

    The biogas process is part of a biological waste treatment method used commonly in waste treatment plants anywhere in the world. The only difference is that instead of releasing the gases produced during the fermentation process, the gases are collected and utilized as a gaseous fuel product.

    In the biogas production system, liquid organic wastes, animal manure, and solid organic wastes are led into a digester where an anaerobic process using bacteria ferments the wastes and produces biogas as a gaseous bi-product. The remaining non-digestible solids in the digester are collected as sludge that can later be returned back to the ground. After the digestion process, the liquid in the waste treatment plant will be rendered sufficiently harmless to the environment and can then be discharged into the rivers.

    The biogas produced in a typical plant contains about:

    • 60%~70% methane
    • 30%~40% carbon dioxide
    • Trace amounts of hydrogen sulphide

    This biogas produced is further processed so that the carbon dioxide and hydrogen sulphide gases are removed. The result is a gas consisting of mostly methane. This is very similar to natural gas obtained from the oil & gas fields.

    By using compressors in a bottling plant, the methane and carbon dioxide gases can be stored under high pressures in cylinders. These gases can be utilized in other industrial applications. Much of the biogas can be used for fuel in vehicles, electrical power generators and for other heating purposes.

    With a purposely built or converted gas engine, electrical power generation is viable. When compressed, the methane can be carried along as fuel for CNG vehicles. It can also be cooled to liquid and be carried along to power LNG vehicles. The natural gas can also be combined with diesel oil and used to run special bi-fuel engines.


    Click to view larger image

    The Sources of Biogas Production

    As we can see from the chart below, organic wastes that contains a high percentage of fats and grease are capable of producing a higher quantity of biogas.

    Gas Holder and Metering

    The biogas produced is stored in an inverted floating vessel, and sealed at the bottom by water. As the gas quantity increases, the floating vessel rises up to make space for the the large volume.

    When a certain volume of gas has been obtained, as shown by the height of the floating vessel, the gas is led out for further processing. The gas needs to be scrubbed in order to purify it further.


    Click to view larger image

    Scrubbing

    Scrubbing is an operation that removes unwanted compounds from the biogas before it is used. The main culprit to be scrubbed is hydrogen sulphide, or 'rotten egg' gas, because this will combine with the moisture in the biogas to form sulphurous acids and these can corrode almost anything.

    Hydrogen Sulphide Scrubbing

    The way to get rid of hydrogen sulphide is to give it something to corrode that you don't want; like some steel wool, for instance.

    Because of the extremely corrosive nature of the gas at this time, the piping used must be of clear glass with the gas inlet pipe running down to the bottom of the container and an outlet pipe coming away near the top. The whole assembly needs to be gas-tight.

    As the gas pass through, the steel wool will corrode from the bottom upwards taking up the hydrogen sulphide by conversion to black iron sulphide. This iron sulphide can later be reused after being oxidized to rust (ferrous oxide) by exposure to air.

    Regeneration of Iron Sulphide

    The Regeneration process is slower than the initial scrubbing action. When the black corrosion reaches about 75% of the height of the container, it's time to change the steel wool or ferric oxide for fresh, sacrificial stuff.

    Usually it is better to run two or more similar bottles or containers connected one after the other to give some flexibility by providing some 'back-up' scrubbing capability.

    For regeneration, the iron sulphide is exposed to air to oxidize it to rust (ferrous oxide). This is then washed with flowing water or slurry water and can be used again to remove the hydrogen sulphide gas.

    Carbon Dioxide Scrubbing

    To get rid of the carbon dioxide (CO2) requires that the digester biogas be diffused through a water (or lime-water) spray tower. This action dissolves the CO2 in the water which is then collected at the bottom of that tower.

    The collected water is then sprayed down a second column to release the carbon dioxide gas and vented to the atmosphere. The water is then recycled back to pick up another load of carbon dioxide.

    It is not absolutely necessary to eliminate Carbon Dioxide from the methane, but CO2 has no intrinsic fuel value and can complicate the jet and air settings of user appliances.

    The reason for this is that CO2 percentage can vary considerably from week to week of normal operation, particularly where differing feedstock constituents are used from time to time. This can vary appliance performance from 'not at all' to 'explosive', neither of which is desirable.

    In the situation where digester output quality is fairly consistent, CO2 scrubbing may be dispensed with and the appropriate flow settings of user appliances adjusted to suit the overall lower fuel value of the combined CO2/Methane mix.

    However, care will have to be taken to maintain that exact CO2/Methane balance in future. The only exception to this is when the digester gas is used exclusively as fuel for a methane genset (electrical generator) system. These systems can be made relatively tolerant of quality-variability in fuels.

    The scrubber system needs to allow a fairly free flow of gas to minimize pressure losses in the gas system since the operating pressures are so low to start with that little reduction can be tolerated before the whole thing stops flowing. Typical system pressures are around 0.5 Kp to 2.0 Kp. Since appliances usually operate at around 0.6 to 0.7 Kp, there is not much room to maneuver.

    In a system requiring carbon dioxide scrubbing, the low-pressure route will not work well. Instead, a series of pumps or a multi-stage pump/compressor is needed to pressurize the carbon dioxide scrubbing operation and for later methane compression for storage in high-pressure steel bottles.

    This more expensive storage method is usually only needed for use with vehicles to allow sufficient useful fuel to be stored or carried conveniently.

    A cubic meter of methane is roughly equivalent to a gallon, or 4.5 liters, of petrol, so more than one large gas bottle will be needed for a vehicle to have much range, even when compressed to twelve atmospheres

    Regeneration of Calcium Hydroxide

    Spray water column of lime-water (converted to calcium carbonate by the carbon dioxide gas) is regenerated back to lime-water (calcium hydroxide) after the release of carbon dioxide gas and can be used again.
     

    2-Stage Scrubber

    The 2-stage scrubber shown below requires the gas to be pressurized sufficiently in order to be able to bubble into solutions of ferric hydroxide solutions and calcium hydroxide. This further removes the unwanted impurities from the biogas.


    Click to view larger image

    Biogas farm at Chonburi, Thailand

    Joint Development of Bio-Gas by Integrated Energy Industries & Altronic Inc

    • Development of Gas Engine for Bio-Gas
    • Adaptation of Electronics for Optimal Control of System in Class I Div II Areas.
    • Optimize gas utilization for various Energy application such as Bi-fuel System, Power
    • Generation and Gas Compression
    © 2008 Integrated Energy Industries. All Rights Reserved.