Chemical Engineering February 2023 - 42

Applied Technologies
Designing a Multi-Stage Thermal
Oxidizer System
Thermal oxidizers can be used for the abatement of a wide range of air pollutants and
volatile organic compounds. This article describes the custom design of a thermal
oxidizer to treat three different waste streams from a biogas plant
Anu D. Vij
Ship & Shore Environmental, Inc.
A
biogas company needed a
treatment system for emissions
abatement of volatile
organic compounds (VOCs).
The system would be installed downstream
of a pyrolysis furnace and multiple
condensers in order to thermally
oxidize three waste streams high in
sulfur and nitrogen concentration,
while also limiting the formation of
oxides of nitrogen (NOx). A custom
proprietary system was designed,
engineered and fabricated by Ship &
Shore Environmental (S&SE; Signal
Hill, Calif.; www.shipandshore.com) to
meet this plant's unique specifications.
The system
The main distinguishing characteristic
of thermal oxidizers is that they do not
rely on direct ignition of the process
gas for oxidation. Rather, they sustain
the correct combustion conditions by
maintaining an operating temperature
sufficiently above the auto-ignition
point by providing enough time for
combustion and by the presence of
excess oxygen to complete the oxidation
reactions.
The multi-stage thermal oxidizer
system designed by S&SE (Figure
1) needed to operate under vacuum
in order to pull a vacuum on the pyrolysis
reactor and provide the motive
force for the flow of the produced
gas from the pyrolizer (Stream 1) to
the thermal oxidizer. Streams 2 and
3 are supplied to the thermal oxidizer
through positive-displacement
pumps. The induced draft/vacuum
was designed to sufficiently pull all
of Stream 1 into the thermal oxidizer
to maintain a proper vacuum at the
Stream 1 connection point.
The
plant's waste streams contain sufficient
nitrogen content that if directly
oxidized in a standard thermal oxidizer,
an excessive amount of NOx
42
process is split into two outlets, with
one outlet going through the quench
phase, while the second outlet is
going through a heat exchanger. The
second outlet can be used to increase
the inlet temperatures (Streams 2 and
3), or to recover heat that can be used
anywhere else in the plant.
FIGURE 1. A customized thermal-oxidizer system
was designed to handle the contents of three distinct
waste streams from a biogas pyrolysis unit
would form. Thus, in the new thermal
oxidizer unit, the streams are burned
in a staged operation where the initial
stage is a high-temperature reducing
(oxygen-starved) atmosphere. The
high temperature and more than 1-s
residence time assure dissociation of
organic compounds. Since combustion
of the waste streams is exothermic,
the process uses virtually no natural
gas, except during startup, which
greatly reduces costs. The exhaust
gas from this initial stage contains an
excess of combustible compounds,
primarily carbon monoxide, and hydrogen.
This reducing atmosphere
provides proactive control of NOx, as
bound nitrogen compounds are converted
to diatomic nitrogen (molecular
N2) instead of NOx.
There are typically two primary processes
for the subsequent stages in a
multi-stage thermal oxidizer. One process
uses two additional stages (for a
total of three stages). The initial stage
is the reducing stage. A second reducing
stage occurs in the thermal oxidizer
(second chamber), which is still
in a low-oxygen environment with a
temperature of about 1,650°F to prevent
excessive NOx formation. Following
this stage, the fluegas enters the
cool-down process. The cool-down
Such a system, defined as a lowNOx
thermal oxidizer, is not designed
for 100% NOx destruction,
but combining it with a selective
catalytic reduction (SCR) system
can eliminate the production of NOx.
Limited space and potential excess
production of NOx due to waste
streams high in nitrogen provided
unique challenges for the engineering
team at the biogas plant.
S&SE designed the system vertically
to fit into the limited space
available at the site. One of the
advantages of the vertical-system
configuration and its down-fired
burner is that any possible effect of
ash or particles on the burner was
eliminated. Simultaneously, the plant
does not need to use any natural gas
for combustion, which significantly
saves on costs. Finally, the heat exchanger
allows the plant to utilize a
significant amount of heat elsewhere
in the plant at no additional cost. ■
Edited by Mary Page Bailey
Author
Anu D. Vij is chief operating officer
(COO) of Ship & Shore Environmental,
Inc. (2474 N. Palm Drive,
Signal
Hill, CA 90755;
(562) 997-0233; Email:
chemical,
shipandshore.com; Website: www.
shipandshore.com). Vij has over 20
years of experience in the environmental,
petrochemical
and
air-pollution control industries,
and has specific expertise in thermal oxidation technologies.
As COO of Ship & Shore, he oversees several business
units, including Sales, Finance, Engineering, Project
Management, Procurement, Production and Services.
Prior to joining Ship & Shore, Vij served as vice president,
enclosed combustion systems, at Aereon, and was director
of engineering at OnQuest, Inc. Vij holds a M.S.Ch.E
from University of Southern California, and a B.S.Ch.E
from Panjab University in India.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
FEBRUARY 2023
Phone:
avij@
http://www.shipandshore.com http://www.shipandshore.com http://www.shipandshore.com http://WWW.CHEMENGONLINE.COM
Chemical Engineering February 2023

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