Stack emission monitoring in India: CPCB methods and reporting

What is measured in stack emission monitoring?

A survey measures the pollutants named in the consent plus the flue gas conditions needed to calculate them. Without velocity, temperature, moisture and oxygen, the concentration cannot be normalised or turned into an emission rate.

The CO and O2 methods above are the ones listed in the CPCB notification for DG sets above 800 kW, which also lists gravimetric PM, barium perchlorate with thorin indicator for SO2, and chemiluminescence, NDIR, NDUV or phenol disulphonic acid methods for NOx.

What are the CPCB guidelines for stack emission monitoring?

Three documents make up the CPCB guidelines for stack emission monitoring that labs follow in the field:

• CPCB's "Emission Regulations Part III" (COINDS/20/1984-85). CPCB's 2018 guidelines on continuous emission monitoring say the manual sampling location must follow it, and consent orders often require ports, ladders and platforms "as per Emission Regulation Part III".
• CPCB's "Methods and Standard Operating Procedures of Emission Testing in Hazardous Waste Incinerator" (September 2007). Its first chapter sets out isokinetic stack sampling: equipment, velocity, moisture, port location and platforms. Labs use it well beyond incinerators.
• CPCB's "Guidelines for Continuous Emission Monitoring Systems" (Revision 1, August 2018), which ties CEMS calibration to manual isokinetic sampling by IS 11255 (Part 1) or equivalent methods.

The State Pollution Control Board's consent order then decides which stacks are monitored, for which parameters and how often. Read it before you plan the visit.

What are the stack emission monitoring requirements for ports and platforms?

The port must sit where the flow is steady. CPCB's 2007 SOP says sampling ports shall be at least 8 stack diameters downstream and 2 diameters upstream of any flow disturbance, such as a bend, expansion, contraction, visible flame or the stack exit. IS 11255 (Part 3):2008 gives the same 8D and 2D rule. For a rectangular duct, use the equivalent diameter De = 2LW / (L + W).

The same CPCB SOP sets these port and platform details:

• Two ports at 90° for stacks under 2 m diameter; four ports for stacks of 2 to 4 m.
• Each port a flanged pipe of 0.10 m inside diameter with a 0.15 m bolt circle and a removable blind flange.
• Ports projecting 50 to 200 mm from the outer wall, and 0.9 to 1.2 m above the platform floor.
• A platform at least 1.2 m wide, extending at least 1.2 m beyond each port, able to carry three people and 91 kg of equipment, with guardrails and safe ladder or stair access.
• A 220 V, 15 A single-phase supply at the platform and at the stack base.

The number of traverse points depends on the inside diameter when the 8D and 2D rule is met:

If the port is closer to a disturbance, the SOP increases the number of points. No sampling point may be within 3 cm of the stack wall.

What is isokinetic sampling, and why does it matter?

Isokinetic sampling means the gas enters the sampling nozzle at the same velocity as the gas in the stack at that point. IS 11255 (Part 1) explains why: particles larger than about 3.5 µm do not follow the gas streamlines. Sampling faster than isokinetic gives a falsely low particulate concentration; sampling slower gives a falsely high one.

In practice the team measures velocity at each traverse point with an S-type pitot tube, sets the sampling rate for that point, and keeps adjusting it as the velocity changes. Record the isokinetic ratio achieved, because a reviewer will ask for it.

How do you normalise concentrations and calculate the emission rate?

Standards compare concentrations at stated reference conditions, so measured values must be converted before any pass or fail statement. The notification that set the current boiler standards, G.S.R. 373(E) of 16 May 2023, gives the oxygen correction:

Normalised value = Measured value × (21 − reference O2) / (21 − measured O2)

The reference is 6 % O2 for solid fuels and 3 % O2 for liquid fuels, on a dry basis. For DG sets above 800 kW, the reference is 15 % O2, and PM, NMHC and CO are normalised to 25 °C, 760 mm of mercury and dry gas.

The emission rate uses the measured concentration and the dry flow at normal conditions:

Emission rate (kg/h) = concentration (mg/Nm³) × flow (Nm³/h) / 1,000,000

Worked example: a coal-fired boiler of 6 t/h

A lab samples a 6 t/h coal-fired boiler. The measured PM concentration is 120 mg/Nm³ (dry, normal conditions), flue gas O2 is 10.5 % dry, and the dry flow is 9,000 Nm³/h.

1. Normalise to 6 % O2: 120 × (21 − 6) / (21 − 10.5) = 120 × 15 / 10.5 = 171.4 mg/Nm³.
2. Limit for a 2 to under 10 t/h boiler on fuels other than agro-based fuels: 150 mg/Nm³.
3. Result: 171.4 mg/Nm³ does not comply, although the raw 120 mg/Nm³ looks well inside the limit.
4. Emission rate from the measured value: 120 × 9,000 / 1,000,000 = 1.08 kg/h.

Reporting only the raw value would have given the customer a false "complies". It is an easy error to make, so show both values on the report.

What are the stack emission standards for boilers and DG sets?

Stack emission standards for boilers are in Schedule I of the Environment (Protection) Rules, 1986. Entry 70 was substituted by G.S.R. 373(E) dated 16 May 2023, which came into force two years after publication. Its PM limits, normalised as above, are:

The same entry covers thermic fluid heaters and hot air generators on equivalent fuel consumption, and sets minimum stack heights. Boilers of coal or lignite based power plants follow the thermal power plant standards instead. For SO2 and NOx, G.S.R. 96(E) of 29 January 2018 (entry 106B) sets 600 mg/Nm³ SO2 and 300 mg/Nm³ NOx for industrial boilers on "other fuels", at 6 % O2 for solid and 3 % O2 for liquid fuels, dry; no values are set there for agro-based fuels or natural gas.

DG sets with engines above 800 kW follow entry 96 (G.S.R. 489(E), 9 July 2002). It sets, at 15 % O2:

• NOx in ppmv, depending on plant capacity, area category and commissioning date.
• PM of 75 mg/Nm³ for HSD and LDO; 150 or 100 mg/Nm³ for furnace oil and LSHS, depending on commissioning date.
• NMHC of 150 or 100 mg/Nm³ depending on commissioning date, and CO of 150 mg/Nm³.

It also requires measurement at steady load above 85 % of rated load. Plants with total engine capacity above 50 MW must monitor NOx continuously, and plants must monitor the other parameters at least once in six months. Engines of 800 kW or less are not covered by this notification.

A State Board can set stricter limits in the consent. Where the consent is stricter, report against it.

What should a stack emission monitoring report contain?

A stack emission monitoring report should let a reviewer repeat every calculation. Include:

• Industry name, address and the consent reference.
• Stack ID, what it serves, fuel type and consumption, and the pollution control device.
• Stack height, inside diameter at the port, and the port's distances from disturbances.
• Process load during sampling, with date and start and end times.
• Barometric pressure, flue gas temperature, velocity, moisture, O2, CO2 and CO.
• Flow rate at actual and at normal dry conditions.
• Number of traverse points, sample volume and the isokinetic ratio achieved.
• Each pollutant as measured, as normalised with the reference O2 stated, and as an emission rate in kg/h.
• The standard or consent limit, the comparison, and the method for each parameter.

See NABL test report format for general report rules, and our guide to ambient air quality monitoring for the ambient side of an environmental monitoring contract.