Environmentally Friendly Waste System

Meet up with Mr Desmond Lim.

We wanted to create a methane gas capture system in order to harvest the methane gas from the biogas produced from the digestion of the waste feed. With the help and guidance that he has given us, we have taken a look into several factors.

In order for us to safely and effectively collect Methane, we needed to research on its properties which is what we have been doing over the last two weeks.



We recall the biogas composition:

Compound Vol %

• Methane 50-75
• Carbon dioxide 25-50
• Nitrogen < 7
• Oxygen < 2
• Hydrogen sulfide < 1
• Ammonia < 1

As observed: the amount of H2S as compared to the Methane produced is insignificant


Properties of Methane:
-Colourless
-Odourless
-Explosive in Nature
 -light gas (less dense than air and CO2) Methane< Air < CO2
 -not very soluble in water

We were given two methods to collect the methane gas. The first, being the scrubber system (inverted cup) and the second method, being the balloon system.

SCRUBBER SYSTEM (using water to scrub of CO2 and H2S)

Advantages:

Obtain methane of higher purity which increases the calorific value.


Disadvantage:
 •For Safety reasons, the scrubber system is not advised.
 •Methane is odourless and colourless. Due to the careless usage, failure to turn off the methane supply can lead to direful consequences
 •Hence H2S is sometimes added in to give the “rotten egg” smell as a pre-warning that methane gas is leaking. Early detection before explosion occurs. (The problem with adding H2S is that it is very toxic and just a small leak could increase the concentration enough to kill)


Essential Piping:
 1) Pilot Line to burner
 - Safety feature to burn off CH4 to become CO2 if pressure in vessel too high.
 - Burn off the Air initially during startup since air provides oxygen for combustion.
 - Only begin tapping when burner can combust (indication that CH4 is mostly present for tapping)

 2) Additional piping to allow digester gas to mix with outlet gas of storage tank to induce H2S into the outlet gas.



Gas valves ONLY



- should be domed shape/ circular/ spherical in shape
- Such that pressure exerted on the container will be uniform due to equal force exerted

Most efficient in storing high Pressure gases.



Weight:
For clear indication on how much the drum has risen due to the increasing volume of methane gas entering. If not at the beginning, drum would have risen uncontrollably, not allowing use to identify how much methane has already entered.

Helps to also immerse the drum totally in water.



BALLON STORAGE SYSTEM

Disadvantage:
Does not scrub off the CO2 from the digester gas.



Properties:
-container made of polyurethane material
-should also have pressure gauge, Pressure relief valve, and pilot line to burner to burn methane off.
-Must have counter weight as a form of safety indication
 -Without weight, container is already bloated when gas enters, unable to see whether material can sustain. If weight is raised to a certain height, we know there is too much pressure and tension of material is about to be reached.