ASHRAE Journal - September 2023 - 38
TECHNICAL FEATURE
Acidic Gas Reduction
For Residential Natural
Gas Furnace
BY ZHIMING GAO, PH.D., MEMBER ASHRAE; KASHIF NAWAZ, PH.D., MEMBER ASHRAE; KYLE GLUESENKAMP, PH.D., MEMBER ASHRAE; STEVE KOWALSKI, MEMBER ASHRAE
The United States has more than 119 million homes, and 47% of them rely on natural
gas as their primary heating fuel. However, most U.S. homes still use noncondensing
units with an annual fuel utilization efficiency (AFUE) of ~80%, rather than efficient
condensing furnaces. This is due to the high cost of condensing furnaces, which
usually use expensive corrosion resistant super-ferritic stainless steel alloy heat
exchangers (HX) to avoid corrosion and fouling acid condensation, along with the
cost and difficulty of retrofitting the venting used by the replaced furnace. This article
discusses a low-cost acidic gas reduction (AGR) catalyst technology1 that enables a
novel condensing natural gas furnace with an ultraclean (i.e., nearly zero) flue gas
and neutral condensate that is environmentally friendly.
Background
One effective approach for improving noncondensing
furnace efficiency is to extract further heat from the flue
gas in a secondary heat exchanger in which the flue gas
temperature goes below its dew point, which results in
the flue gas condensing in the heat exchanger. However,
sulfur, nitrogen and carbon, which typically exist in
both the ambient air and the natural gas or propane
used to fuel the furnace, form key acidic gases (e.g., SOx
[sulphur oxides], NOx [nitrogen oxides], formic gases)
in the flue gas. When the flue gas cools below its dew
point, the acidic gases combine with water vapor to
generate an acidic condensate of pH ~3.42-3.76,2 which
is more acidic than typical acid rain at pH ~4.2-4.4.
The acidic condensate can cause serious corrosion and
fouling problems.
In high-efficiency, condensing natural gas furnaces,
a typical strategy to avoid corrosion and fouling from
acidic condensation is to use corrosion-resistant
super-ferritic stainless steel alloy HXs.3 Super-ferritic
stainless steel costs significantly more than regular
aluminized steel, which contributes to limiting the
market penetration of efficient condensing furnaces.
Meanwhile, current furnaces generate nearly 40 ng/J
Zhiming Gao, Ph.D., Kashif Nawaz, Ph.D., Kyle Gluesenkamp, Ph.D., and Steve Kowalski are senior R&D staff at Oak Ridge National Laboratory, Oak Ridge, Tenn.
38
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ASHRAE Journal - September 2023
Table of Contents for the Digital Edition of ASHRAE Journal - September 2023
Table of contents
ASHRAE Journal - September 2023 - Intro
ASHRAE Journal - September 2023 - Cover1
ASHRAE Journal - September 2023 - Cover2
ASHRAE Journal - September 2023 - 1
ASHRAE Journal - September 2023 - Table of contents
ASHRAE Journal - September 2023 - 3
ASHRAE Journal - September 2023 - 4
ASHRAE Journal - September 2023 - 5
ASHRAE Journal - September 2023 - 6
ASHRAE Journal - September 2023 - 7
ASHRAE Journal - September 2023 - 8
ASHRAE Journal - September 2023 - 9
ASHRAE Journal - September 2023 - 10
ASHRAE Journal - September 2023 - 11
ASHRAE Journal - September 2023 - 12
ASHRAE Journal - September 2023 - 13
ASHRAE Journal - September 2023 - 14
ASHRAE Journal - September 2023 - 15
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ASHRAE Journal - September 2023 - 17
ASHRAE Journal - September 2023 - 18
ASHRAE Journal - September 2023 - 19
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ASHRAE Journal - September 2023 - Cover3
ASHRAE Journal - September 2023 - Cover4
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