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YPPC

AN INNOVATIVE LANDFILL

GAS PRODUCT REDESIGN

By Mario Ho Tak Cheung, assitant
engineer at the Hong Kong and FIGURE 1 LOCATION MAP OF LFG UTILISATION PROJECT AND TPP
China Gas Company (HKCG) and
YPPC 2019 runner-up

LFG IS A NATURAL by-product of the
decomposition of organic material in
landfill. It mainly consists of methane
and carbon dioxide, which can be
utilised as fuel. Flaring LFG causes
environmental impacts as it emits
carbon dioxide, the greenhouse gas, to
the atmosphere. Directly emitting LFG
to the atmosphere causes an even more
serious greenhouse effect as methane
is 21 times more damaging to the
atmosphere than carbon dioxide.
As an environmentally-friendly
company, HKCG has made LFG recovery
part of its mission. Instead of releasing
LFG into the air, it can be captured,
converted and used as a renewable
energy source.
Using LFG helps to reduce odours
and other hazards associated with
LFG emissions and prevents methane which was to redesign the LFG’s product contact with a heat source.
from migrating into the atmosphere. In gas, turning it into synthetic natural gas As we can see, SNG is outside the
Hong Kong, the NENT and SW landfill (SNG). This gas is then mixed with TG and combustible zone (red triangle area).
sites were home to two previous LFG utilised in customers’ gas appliances. However, the gas mixture follows the
utilisation projects. air line and eventually crosses the
SW produces treated LFG, which is Design combustible zone as more air is mixed
used as a fuel at TPP. No CV control is Calorific value (CV) control with LFG or SNG. An abnormal operation,
applied because the site makes use of In order to mix a gas with TG, it is such as a valve position failure, could
a short underground gas transmission necessary to identify the composition result in too much air mixing with the
pipeline (1.3km). and physical properties of the gas, gas. In other words, the oxygen content
NENT increases the CV of the LFG by which is summarised in Table 1. in air (21 per cent) may exceed the
extracting part of the carbon dioxide to TG is sold according to its calorific minimum oxygen concentration (MOC)
reduce transmission costs. The LFG is value, so the LFG has to be strictly of 13 per cent and create the potential
then converted into substitute natural controlled at 17.27MJ/Sm³ (the same risk of a fire or even an explosion.
gas and transmitted to TPP via a 19km as the TG). For any non-flammable Theoretically, mixing any gas
underground pipeline. component, the CV is 0MJ/Sm³. The with oxygen content lower than the
A new challenge presented itself in simplest way to achieve the same CV MOC point should not be inside the
the form of a landfill site called SENT. is to dilute the landfill gas with non- combustible zone. So, the LFG was
It is located far from TPP or any gas flammable gas. designed to mix with 95 per cent N₂
production plant. If HKCG was to lay an One of the options is to mix it with (five per cent O₂) to reduce CV.
underground pipeline to connect TPP air. When mixing any flammable gas
and SENT, the installation would be with oxygen, the explosion limit is an Specific gravity control
very expensive and not cost effective. important parameter to check. The Apart from the CV, specific gravity (SG)
As such, HKCG investigated an flammability plot in Figure 2 was drawn is also an important parameter for
innovative LFG utilisation solution, to identify how likely combustion is on gas safety. If the SG is larger than 1, it

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