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National Pollutant Release Inventory (NPRI) Emission Estimate Guide for Primary Aluminum Producers

7. Estimating air emissions:Part1

This section outlines the appropriate methodology for estimating emissions from some of the sources listed above.

The data table below, produced by Environment Canada, contains the codes used by the NPRI to describe the various methods for estimating releases.

Estimation codes for the method used to calculate emissions
CodeMethod
M1Continuous emission monitoring
M2Predictive emission monitoring
M3Source testing
CMass balance
E1Site-specific emission factors
E2Published emission factors
OEngineering estimates
NINo information
NANot applicable

Source: Environment Canada, 2011

Emission factors found in the spreadsheet were calculated using a specific method as indicated in the spreadsheet. Also, these emission factors can vary depending on the physical properties of the substance, the effectiveness of the controls and the estimation method used. In view of these limitations, please use them judiciously. Furthermore, the same pollutant can come from many different sources. Therefore, the user must use the appropriate emission factor to calculate the emissions of this pollutant in all processes from which it is released and add them up to estimate the total emissions produced annually.

When reporting emissions to the NPRI, the user must report all releases from primary sources, such as those from stacks, as “point releases.” Secondary emissions such as those released from roof vents will be classified as “fugitive emissions.”

Note for calculating polycyclic aromatic hydrocarbon (PAH) emissions:

Facilities use direct measurement methods and, according to the aluminium smelter industry spreadsheet, these source tests are used to calculate the quantity of PAHs released. Subsequently, the facility must add the PAHs to determine whether or not the overall reporting threshold set by the NPRI has been exceeded. There are also individual thresholds for PAHs.

Note for calculating hydrogen fluoride (HF) emissions:

In general, the sum of emissions measured monthly at the pot room roof vents and emissions measured annually at the scrubber stack is used to estimate hydrogen fluoride emissions. The information under the “TF and HF” tab in the spreadsheet indicates that the hydrogen fluoride emission rate is calculated using sampling results.

Note for calculating emissions from fuel combustion:

In general, the appropriate emission factors are applied to the total amount of the same type of fuel used throughout the facility.

Note for calculating particulate matter (TPM, PM10 and PM2,5) emissions:

The same emission factor can be used to calculate emissions from several similar combined sources such as fans and dust collectors.

Note for calculating volatile organic compound (VOC) emissions:

If the total amount of VOCs emitted exceeds the NPRI reporting threshold, the facility will have to identify the amount of each VOC it releases. All substances in the VOC category proposed by the NPRI are listed under the “List Part 5 Speciated COV” tab in the spreadsheet.

Note regarding calculation examples:

The examples in this guide refer to the data contained in the May 6, 2009, version of the spreadsheet.


7.1 Emissions from raw material storage and handling

7.1.1 Process description

Raw materials (alumina, pitch, petroleum coke, bath constituents, fuel, etc.) are received and transferred by truck, train or boat to the plant storage site (silos, tanks and warehouses). Conveyors are generally used for loading and unloading.

Key operations: receipt of alumina and coke and storage in silos; alumina and coke transfer via conveyor.

7.1.2 Emissions

Reportable NPRI substances emitted during raw material storage and handling, which are listed in the spreadsheet, are total particulate matter (TPM), PM10 and PM2.5particulate matter. Other potential emissions such as PAHs and volatile organic compounds (VOCs) may also be released during these activities.

7.1.3 Emission calculations

Sample calculation for total particulate matter (TPM) emissions:

A facility wants to estimate the amount of TPM released annually by a dust collector on an alumina silo with a 63,000 m3/h flow rate. The dust collector operates 5,000 hours per year.

The first step is to retrieve the emission factor under the “Total Particulate” tab of the spreadsheet in the NPRI toolbox.

The emission factor is shown below:

EF(TPM) = 5 mg/m3 for dust collectors with flow rates greater than 17,000 m3/h and 15 mg/m3 for those with flow rates less than 17,000 m3/h

Then, the user uses the following equation:

ETPM = FE * Ut * Ca * 1kg/106mg * 1t/1,000kg

where:

  • ETPM = Emission of total particulate matter (tonne/yr)
  • FE = Emission factor for the dust collector (mg/m3)
  • t = Number of hours the dust collector is used (h/yr)
  • a = Dust collector cleaning capacity (m3/h)

Therefore, the annual TPM emissions from the dust collector are:
ETPM=5mg/m3 * 500h/yr * 63,000m3/h * 1kg/106mg * 1t/1,000kg =1.575tonnes/y


7.2 Petroleum coke calcining emissions

7.2.1 Process description

In the aluminium industry, petroleum coke is typically calcined in rotary kilns at high temperatures. After it has been crushed and screened, the calcined coke is usually stored until it is fed into mixers to produce anodes.

Key operations: calcining, crushing, screening, handling and storage

7.2.2 Emissions

Reportable NPRI substances are the following: VOCs, Nitrogen oxide (NOx), carbon monoxide (CO), sulphur dioxide (SO2), TPM, PM10 and PM2.5. Other potential emissions such as PAHs may also be released during these activities.

7.2.3 Emission calculations

Sample calculation for volatile organic compound (VOC) emissions:

A facility wants to estimate the amount of VOCs released annually by a rotary kiln that calcines 16 tonnes of green coke/hour. The kiln operates 6,000 hours per year (96,000 tonnes of green coke/year).

The following emission factor can be found under the “VOC” tab of the spreadsheet in the NPRI toolbox:

EF(VOC) = 0.007 kg/t green coke

The user can use the following equation to calculate the VOC emissions:

EVOC = EF * Qty * 1t/1,000kg

  Where:

  • EVOC= VOC emissions during coke calcining (tonne/yr)
  • EF = Emission factor for VOCs released during coke calcining (kg/tonne)
  • Qty = Quantity of green coke calcined per year (tonne/yr)

Therefore, the annual VOC emissions from the kiln are:

EVOC = 0.007kg/t * 96,000t/yr * 1t/1,000kg = 0.672tonnes/yr


7.3 Emissions from anode manufacturing

7.3.1 Process description

Anode manufacturing is a process by which calcined petroleum coke is mixed with pitch (used as a binder) and carbon from recycled anode butts (in prebaked facilities only) to produce anode paste used directly in Söderberg pots or compressed to form a green anode block (prebaked pots).

Key operations: handling, grinding, mixing and vibro-compaction

7.3.2 Emissions

Particulate matter (TPM, PM10 and PM2.5), PAHs and VOCs

7.3.3 Emission calculations

Sample calculation for fine particulate matter (PM2.5) emissions:

A stack releases a 12 mg/Nm3 concentration of TPM and a dry mass flow of 22,000 Nm3/h. The stack operates 24 hours a day, 365 days a year (8,760 h/yr). We need to calculate the PM2.5 released by the scrubber on the paste plant.

The first step is to open the spreadsheet in the NPRI toolbox and retrieve the appropriate emission factor under the “PM2.5” tab.

The emission factor is shown below:

EF (PM2.5) = 70% x Total Particulate sampling (TPM)

The following approach can be used:

Use the stack flow rate and the measured concentration to calculate the annual TPM mass emissions:

12mg/1Nm3 * 22,000 Nm3/h * 1t/109mg * 8,760h/yr = 2.313t/yr of TPM

Therefore, the annual PM2.5 emissions from the paste plant are:
emissionsPM2.5 = 0.7 * 2.313t/yr = 1.619tonnes/yr


7.4 Emissions from anode baking

7.4.1 Process description

Baking green anodes in ring furnaces to produce blocks of solid carbon that can be used for electrolysis. Fuel is used to heat the furnace.

Key operations: storage, handling and combustion

7.4.2 Emissions

Particulate matter (TPM, PM10 and PM2.5), PAHs, hydrogen fluoride, SO2, CO and VOCs

7.4.3 Emission calculations

Sample calculation for sulphur dioxide (SO2) emissions from anode baking:

A facility produces 105,000 tonnes of green anodes annually which, after baking,  yields 103,000 tonnes of baked anodes. The annual average sulphur content in the green anodes is 2.04% and 2% in the baked anodes. The annual average sulphur content in the alumina is considered negligible.
How much sulphur is released during this process?
The following equation can be found under the “SO2” tab of the spreadsheet in the NPRI toolbox:

ESO2 - ( [%Sacr/100 * PACR] - [%Sac/100 x PAC] - [%Sai/100 * AR] ) * 64/32

where:

  • ESO2 = SO2emissions (t/yr)
  • %Sacr = percentage of average annual sulphur content in green anode
  • PACR = green anode production (t/yr)
  • %Sac= percentage of sulphur content in baked anodes
  • AC = production of baked anodes (t/yr)
  • %Sai = percentage of annual average sulphur content in the alumina recovered from the Fume Treatment Centre of the Anode baking furnace
  • AR = quantity of recovered alumina in t/yr

NB: The sulphur content in the recovered alumina is equal to zero (0) if not significant.

Therefore, SO2released by the anode baking oven is:

ESO2=([2.04/100*105,000t/yr]-[2/100*103,000t/yr]) * 64/32 = 164tonnes/yr

Sample calculation for SO2 emissions from fuel combustion during anode baking:

Assume that 17,000 litres per year of fuel oil (Bunker No. 2) are used for anode baking. The fuel has 0.5% sulphur content and a mass volume of 820 kg/m3.
How much sulphur is released?
The following equation can be found under the “SO2” tab of the spreadsheet in the NPRI toolbox:

ESO2=p * %S * V * MWSO2/MWS * 1t/1,000kg

 Where:

  • ESO2 = SO2 emissions (t/yr)
  • ρ = volumetric mass (kg/litre)
  • %S = percentage of sulphur in fuel
  • V = number of litres of fuel (litre)
  • MWSO2 = molecular weight of SO2(64 g/mol)
  • MWS = molecular weight of sulphur (32 g/mol)

Therefore, SO2 released through combustion is:
ESO2= 820kg/1,000litres * 0.5 * 17,000litres/yr * 64/32 * 1t/1,000kg = 13.94tonnes/yr

 


7.5 Emissions from electrolysis

7.5.1 Process description

Production of aluminium by electrolysis of alumina dissolved in a cryolite bath. The carbon anodes are immersed in the bath and electricity is supplied, breaking the aluminium–oxygen bond. The aluminium is deposited at the bottom of the pot and siphoned off at regular intervals.

Key operations: changing the anodes, siphoning the metal, siphoning the bath, pot maintainence, and pot start up and shut down.

7.5.2 Emissions

Primary emissions (stacks): Most of the emissions are captured in the pots and sent to the dry (GTC) or wet scrubbers.

Secondary emissions: Fugitive emissions not captured in the pots (opening of pot panels, siphoning, anode butt tray).

Emissions: PAHs, HF, SO2, TPM, PM10, PM2.5, CO, VOCs, COS (carbonyl sulfide) and TRS (total reduced sulphur).

7.5.3 Emission calculations

Sample calculation for carbon monoxide (CO) emissions :

A prebaked anode facility wants to determine the amount of CO released from the pots, knowing that the plant produces 400,000 tonnes of aluminium annually and its Faraday efficiency is 93%.

Begin by retrieving the equation shown below under the “CO” tab of the spreadsheet in the NPRI toolbox.

ECO=PA1 * [100-F(%)/F(%)] * 84/54

Where:

  • ECO = Carbon monoxide emissions (tonne/yr)
  • PAl = Aluminium production (tonne/yr)
  • F = Faraday efficiency (%)

Therefore, the annual CO emissions from the pots are:

ECO = 400,000t/yr * (100-93/93) * 84/54 = 46833.93 tonnes/yr

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