Hydrocarbon Processing - June 2022 - GP-37

ENVIRONMENT & SAFETY
Safe design and operation of
renewable natural gas facilities
M. YOUNG, Baker Engineering and Risk Consultants Inc., Chicago, Illinois
of waste1
The U.S. produces almost 70 MMtpy
; over time, this waste generates
biogas with the help of bacteria. Biogas
is a mixture of methane (CH4
dioxide (CO2
), carbon
) and other trace gases and
is normally released into the atmosphere.
However, waste collection plants are now
exploring ways to collect the biogas and
refine it into pure CH4
, also known as renewable
natural gas (RNG). RNG can be
used for injections into existing natural
gas pipelines or used as fuel for RNGdependent
transportation vehicles. In recent
years, the number of RNG plants has
grown significantly within the U.S.
Despite the many benefits and solutions
that RNG provides, hazards remain;
because RNG is a relatively new technology,
these hazards are not as commonly
known. Most RNG plants are small and
not covered under Occupational Safety
and Health Administration (OSHA)
PSM 1910.119, which is required if a facility
contains > 5 t of flammable gas or
liquid. However, when dealing with flammable
and toxic materials, it is important
to recognize the potential hazards and/or
risks associated with the process regardless
of formal requirements.
Examples of previous incidents and
studies conducted by the author's company
illustrate the significant hazards that
are present in RNG plants:
1. An explosion in the anaerobic
digester (AD)
2. A rupture of the animal waste AD
from vacuum or pressure build-up
inside the AD
3. A leak of toxic and flammable
material into the atmosphere
4. An explosion in an enclosed
compressor building.
This article discusses each of the
events described above, highlights the
potential hazards and provides examples
Food waste
FIG. 1. Process flow for anaerobic digestion (AD).1
Gas Processing & LNG | MAY/JUNE 2022 37
The RNG process. The process to produce
RNG starts as waste (e.g., animal,
landfill or water waste). The feedstock
undergoes anaerobic digestion (FIG. 1),
where organic material is decomposed
into biogas.1,2
Depending on the feed, the
digestion can take place in different storage
containers called an AD-usually,
landfill waste underground, water waste
in covered lagoons, and animal waste
in non-pressurized tanks. After several
weeks, the generated biogas can be collected.
Water, hydrogen sulfide (H2
CO2 and other trace chemicals are reS),
moved
from the biogas through various
separation steps to produce the highquality
CH4
product stream that can be
compressed and sent to end users.
Livestock waste
Fuel
Crops
Biogas
Anaerobic
digester
Waste water
Digestate
* Fertilizer
* Soil amendments
* Livestock bedding
Biomethane
of potential safeguards and/or mitigations
that can be employed to reduce injury
or fatality.
AD explosion. The AD's sole purpose
is to produce and collect the flammable
biogas, and operating an AD for waste
poses flammable, explosive and toxic
hazards. By design, air is normally not
present in the AD. However, with poor
maintenance or operation, a leak in the
equipment could allow air to enter the
AD equipment-with ignition, an explosion
can occur, similar to the 2018
in-ground AD tank explosion3,4
at the
Calumet Water Reclamation Plant in
Chicago, Illinois, which caused 10 injuries.
An investigation concluded that the
manhole sealing the in-ground tanks was
leaking flammable gas while contractors
were conducting hot work nearby. Measures
to eliminate nearby ignition sources
and to detect and fix the leak could have
prevented the explosion in the AD.
H2
Heat
Electricity
S is also produced as a byproduct in
the AD and poses toxic hazards that are
Gas grid
Hydrocarbon Processing - June 2022

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