HP March 2022 eBook—Energy Transition - 142

SPECIAL FOCUS: GREEN TECHNOLOGIES
is produced near the ground. Ground-level ozone has an adverse
effect on the respiratory system, causing lung cancer and
affecting agricultural production. NOX
also reacts to form nitrate
particles and acidic aerosols, which can cause respiratory
problems. When NOX
reacts with water to form nitric acid, it
causes acid rain and deterioration of water quality. Additionally,
acid gases and airborne particles can cause reduced visibility
and reduced air quality.
Many standards, both at home and abroad, restrict NOX
emissions and outline specific regulations-e.g., Beijing boiler
air pollutant emissions standard DB11/139-2015,6
gas heater standard CJT113-2015,7
Chinese
for infrared heaters BSEN 416-1.8
and European CE standard
In all of these standards, the
emissions reporting is given in mg/kWh. Many customers require
manufacturers to provide NOX
emissions from a newly built boiler
emissions data in mg/
kWh when purchasing combustion-related appliances. For example,
in Beijing, the NOX
cannot be greater than 100 mg/kWh.
However, almost all instruments that measure the tail flame
.9
Us)
to calculate
the NOX produced by the combustion system (in mg/kWh)
is an important calculation, but many relevant documents and
standards offer complicated conversions or, conversely, very
simple tables8,9
that do not explain the theoretical basis behind
the conversions. Many standards simply do not mention the
conversion method, causing confusion and difficulty for the
engineering community. This article describes a simple, accurate
calculation method that was derived in the process of
developing low-NOX
heaters.
Background on conversion of ppm to mg/m3
.10
. The engineering
and academic communities have held recent discussions
on the conversion of ppm to mg/m3
A brief overview of
the conversion history follows for those readers seeking background
information.
Ppm is often used to express concentration, such as mass
ppm concentration or volume ppm concentration (i.e., ppmv).
Ppmv is a common way of expressing gas phase concentration.
Gas is miscible and, generally speaking, once equilibrium is
reached the gas is homogeneous, meaning its components are
evenly mixed together. In SI units, the volume for this gas is cubic
meters (m3
). If the gas mixture is divided into components
m3
. This results in 1 m3
, then the gas
of a cerm3
for
a mixture = 1 ppm.
and the concentration of one component in the gas mixture is
assumed to be 1 ppm, and if the mixture is 106
of this component should be 1 m3
tain component, or 106
Eqs. 1 and 2 are derived from the ideal gas law (for gases at
lower pressures):11
PV = nRT
PVi = ni
RT
(1)
(2)
Here, P is the pressure (in pascal); V is the volume (m3); n
is the number of moles (proportional to the number of molecules)
of all components; R is a constant, called the universal
gas constant, with a value of 8.3143 joules/K mole; T is the Kelvin
(K) temperature; Vi
nent i; and ni
is the partial volume of the gas compois
the molar number of gas component i.
Eqs. 1 and 2 can be rewritten, as shown in Eqs. 3 and 4:
106 xi =
niRT
PVT
of a burner offer a reading of NOX only in ppm or mg/m3
ing the measured NOX data (in ppm or mg/m3
V = nRT / P
Vi = niRT / P
Assuming that Vi
(3)
(4)
Next, a certain trace gas in the mixed gas can be considered.
is the volume of the trace gas and VT
is the
total volume of the mixed gas, both sides of the second equation
are divided by VT
that can be expressed as a volume ratio (Eq. 5):
=
Vi
VT
Then, xi
Vi
VT
niRT
PVT
ponent i in ppm (Eq. 6):
= 106 xi
, which gives the concentration of the trace gas
(5)
can be used as the concentration of the trace gas com(6)
Using
Xi as the mass concentration of component i in mg/m3
and assuming the molar weight of component i is Mi (g/mole)
gives Eq. 7:
= ( niMi
VT
)
X = x(MP ÷ RT) × 103
x = X(RT ÷ MP) × 10-3
RT
MiP
= 103Xi ⎦⎤
⎣⎡
RT
MiP
(7)
When the low index i in these equations is neglected, Eqs. 8 and
9 result:
mg/m3
ppm
(8)
(9)
Note that the concentration x is in ppm, the unit of concentration
X is in mg/m3
describes them at constant temperature.
, and M is the molecular weight of the trace
gas concerned (g/mole). Eqs. 8 and 9 have been generally accepted
by the academic and engineering communities, and literature10
Simple
calculation procedures are also provided on relevant
websites in America and Europe. For example, on some sites,12,13
the only required inputs are the value of the concentration x or
X of a certain gas at atmospheric pressure and 25°C, along with
the molecular weight of the gas. The program will immediately
give the value of X or x under the corresponding conditions. In
addition to the calculation,12,13
perature and pressure inputs.14
users can change the gas temThe
molecular weight of NO2
, and 1 ppm NO = 1.25 mg/m3
.
In the general combustion system, NO2
counts for approximately 5%-10% of NOX
When NO2
for about 90%-95%.15-18
the average molecular weight of NOX
is
46.01 g, and the molecular weight of NO is 30.01 g; therefore,
at a temperature of 20°C (293.15 K) and 1 atm, 1 ppm NO2
1.91 mg/m3
=
in tail flame acand
NO accounts
accounts for 10% of NOX
can be calculated as M =
, 1 ppm = 1.29 mg/m3
.
,
31.61. Under the same temperature and pressure conditions as
the previous calculation, for NOX
Calculation of NOX in mg/kWh for a combustion system.
For a combustion system, the NOX
flame can be measured in ppm or mg/m3
emission in mg/kWh dictates how many mg of NOX
in mg/kWh is not only related to the NOX
Gas Processing & LNG | MARCH/APRIL 2021 | GasProcessingNews.com
are produced
in a combustion system when it releases 1 kW in 1 hr. This
means that NOX
concentration
in the tail flame, but also to the volume flowrate of the
tail flame and the power produced by the combustion system.
concentration in the tail
. However, the NOX
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