Emission estimate guide for primary aluminum producers: chapter 7


7.6 Casting centre emissions

7.6.1 Process description

The liquid aluminium produced during electrolysis is sent to the casting centre where it is skimmed before being solidified in moulds to make aluminium ingots, pigs,  billets, etc. The dross are the impurities found on the surface of the metal in the foundry furnaces. These impurities are usually removed and sent to a subcontractor, which extracts the aluminium and returns it to the plant.

Key operations: receiving the molten metal, casting the metal, skimming and cooling the pigs, ingots, aluminium billets, alloying, etc.

7.6.2 Emissions

TPM, PM10, PM2.5, NOx, CO, SO2

7.6.3 Emission calculations

Sample calculation for nitrogen oxide (NOx) emissions:

A facility wants to estimate the amount of NOx released annually if its casting centre burns 4,000,000 m3 of natural gas. (In most cases, the calculation of emissions from fuel combustion will be for the entire facility, not for individual processes.)

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

EF(NOx) = 1,600 kg/106 m3

The following equation can be used to calculate the emissions:

EF(NOx) = EF * QTY * 1t/1,000kg

Where:

  • ENOx: NOx emissions (tonne/yr)
  • EF = Emission factor for NOx from natural gas combustion (kg/m3)
  • Qty = Quantity of natural gas per year (m3/yr)

Therefore, the NOx emissions from the casting centre are:

ENOx = 1,600kg/106 m3 * 4,000,000m3/yr * 1t/1,000kg = 6.4 tonnes/yr

7.7 Emissions from related processes

Related processes are associated with electrolysis and have similar characteristics in terms of operations (storage, handling, etc.) and particulate matter emissions. 

 7.7.1 Process description

This part applies to the following processes:

Butt storage:

  • Butts are what remain of anodes after they have been used in the pots. The spent anode assembly is usually cooled by natural ventilation in a storage area.
  • Key operations: storage and handling

Bath treatment:

  • The bath recovered from the anode butts is ground, processed and stored, then sent to electrolysis for reuse in the process.
  • Key operations: grinding, ground bath silo and bath treatment

Butt processing:

  • The anode butts (spent anodes) are cleaned, ground and completely reused to produce new anodes.
  • Key operations: the butts are cleaned and shotblasted, the casting is ground and the rods are brushed

Anode sealing:

  • Grey iron is poured to attach the rod-leg assemblies to the carbon anodes.
  • Key operations: a thimble stripping station, rod maintenance, induction oven for melting the grey iron

Crucible cleaning and relining operations:

  • Crucible cleaning. Some of the residues recovered are recycled in the bath circuit.
  • Key operations: storage and handling

Dross storage:

  • The dross is placed in a bin and stored until it is shipped to a subcontractor that processes the waste.
  • Key operations: storage and handling

Pot lining and delining shops:

  • Lining: installation of the inside pot lining, the refractory bricks, cathode blocks and ramming paste
  • Delining: removal of the interior lining of used pots in order to replace them
  • Key operations: storage, handling, conveyor and steel shot blasting

Spent Pot Liner storage:

  • The spent pot liner is stored as hazardous waste material.
  • Key operations: handling spent pot liners and storing them in the warehouse

7.7.2 Emissions

Particulate matter (TPM, PM10 and PM2.5). Other potential emissions such as HF and ammonia can also be produced during these activities.

7.7.3 Emission calculations

The emission factors for fans and dust collectors presented in the spreadsheet can be used to estimate particulate emissions from related operations.

The spreadsheet does not yet contain an emission factor for estimating the release of certain substances from these related processes; users can therefore use one of the approaches mentioned above in Table 2.

7.8 Boiler emissions

7.8.1 Process description

Boilers are combustion or heat recovery systems that produce steam.

7.8.2 Emissions

TPM, PM10, PM2.5, CO, SO2, NOx

7.8.3 Emission calculations

The spreadsheet  does not yet contain an emission factor for estimating releases specifically from boilers; users can therefore use the same approach as the one used for fuel or one of the approaches mentioned above in Table 2 or appropriate spreadsheets available in the NPRI toolbox.

7.9 Solvent and paint emissions

7.9.1 Process description

Solvent use includes degreasing with a solvent, used solvent recovery, fugitive emissions generated during product preparation and use of commercial solvents. Many solvents contain VOCs that are emitted by evaporation during use.

Paints used for coating storage tanks or any other products that also contain VOCs must be reported to the NPRI.

7.9.2 Emissions

VOCs

7.9.3 Emission calculations

Releases can be estimated by assuming all VOCs in paint and solvents are released. The percentage of total VOC emissions and individual VOCs in paint and solvents is normally indicated on the product MSDS. If not, you can obtain this information from your supplier.

The total release will equal the percentage of VOCs multiplied by the total weight of the paint used. There are also generic emission factors for paint and coatings. (references: Chapter 4.2 of document AP-42 and www.epa.gov/ttn/chief/eiip/techreport/volume02/ii07_july2001.pdf).

Emission factors, mass balances and engineering calculations are often used to calculate VOC emissions from the use of solvents (reference: www.epa.gov/ttn/chief/ap42/ch04/final/c4s06.pdf).

7.10 Dust emissions from unpaved roads

7.10.1 Process description

Gravel surfaced roads and roads with thin membrane bituminous surface treatments and bituminous cold mix surfaces are referred to as unpaved roads. Even if a dust suppressant is applied to an unpaved road, this section of road is still considered an unpaved road.

7.10.2 Emissions

Particulate matter: TPM, PM10 and PM2.5

7.10.3 Emission calculations

Dust emissions can be estimated using the following general equation:

Ex = VK * EFx * COR * (1 - EC/100)

Where:

  • EX = Emission of contaminant x, (kg/year)
  • VK = Total annual vehicle kilometres travelled (km)
  • EFX  = Emission factor of contaminant x, (kg/KV)
  • COR = Correction factor for the days of precipitation, snow cover and frost
  • EC = Effectiveness of dust suppression method (%)

Source: USEPA, 2006

The following equation is used to calculate the number of vehicle kilometres travelled (VKT) per year:

VK = T * L * N

Where:

  • VK = Total annual vehicle kilometres travelled (km)
  • T = Average daily traffic
  • L = Length of unpaved road
  • N = Number of days of use per year

The average daily traffic is equal to the total number of vehicles that travel on a section of road every day. The length of the roads at the facility and the number of vehicles accessing the facility during a typical day. If such data is unavailable, surveys can be conducted during representative operating days during the year to estimate total VKs. The VK calculation is described in detail in the Facility Information sheet in the Unpaved Industrial Road Dust Calculator spreadsheet. (Environment Canada, 2009)

The emission factor, expressed in metric units (kilograms/VK) is calculated using the following equation:

EF = k(s/12)a * (W/2.72)b

Where:

EF = Size-specific emission factor (kg/VK)

s = Surface material silt content (%)

W = Mean vehicle weight, metric tonnes

k, a, b = Numerical constants (EC, 2007)

Source: Document AP 42, Chapter 13: Miscellaneous Sources, Section 2.2 (USEPA, 2006).

Note: For more detailed information, please refer to Guidance on Estimating Road Dust Emissions from Industrial Unpaved Surfaces in the NPRI toolbox.

7.11 Accidental releases

7.11.1 Process description

All accidental (unplanned) releases into the atmosphere other than normal emissions from stacks or point releases and releases attributable to storage or handling and fugitive emissions.

7.11.2 Emissions

All substances used on the site that can be released accidentally.

7.11.3 Emission calculations

The spreadsheet does not contain an emission factor for estimating releases from accidental spills; users can therefore use one of the approaches mentioned above in Table 2.

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