Allocation Procedures in Product Carbon Footprinting
Allocation Hierarchy
The GHG Protocol Product Standard and PAS 2050 specify a hierarchy of allocation methods:
- Avoid allocation through subdivision of processes or system expansion
- Physical allocation based on relevant physical relationships
- Economic allocation as a last resort
1. Avoiding Allocation
The preferred approach is to avoid allocation whenever possible through:
Process Subdivision
Example: Factory Energy Use
Instead of allocating total factory energy use between products, install sub-meters to measure energy consumption for each production line.
- Before: Total factory electricity = 1,000,000 kWh/year
- After subdivision:
- Product A line = 400,000 kWh/year
- Product B line = 600,000 kWh/year
System Expansion
Example: Combined Heat and Power (CHP)
Instead of allocating emissions between electricity and heat:
- Calculate emissions avoided from conventional heat production
- Subtract from total CHP emissions
- Remaining emissions attributed to electricity
2. Physical Allocation
When allocation cannot be avoided, physical allocation should be used based on relationships that reflect the underlying physical relationships between products.
Mass Allocation
Example: Chemical Production
Process produces two chemicals with total emissions of 1,000 kg CO₂e:
- Product A: 600 kg output (60%)
- Product B: 400 kg output (40%)
Allocation:
- Product A: 1,000 × 60% = 600 kg CO₂e
- Product B: 1,000 × 40% = 400 kg CO₂e
Energy Content Allocation
Example: Oil Refinery
Refining process with 10,000 kg CO₂e emissions:
- Gasoline: 40 MJ/kg × 200 kg = 8,000 MJ (40%)
- Diesel: 45 MJ/kg × 267 kg = 12,000 MJ (60%)
Allocation:
- Gasoline: 10,000 × 40% = 4,000 kg CO₂e
- Diesel: 10,000 × 60% = 6,000 kg CO₂e
3. Economic Allocation
Economic allocation should only be used when physical allocation is not feasible or appropriate.
Example: Agricultural Products
Farm process with 5,000 kg CO₂e emissions produces:
- Wheat grain: 1000 kg at $200/tonne = $200 (80%)
- Straw: 2000 kg at $25/tonne = $50 (20%)
Allocation:
- Wheat: 5,000 × 80% = 4,000 kg CO₂e
- Straw: 5,000 × 20% = 1,000 kg CO₂e
Recycling Allocation
Recycling presents special challenges for allocation. Different approaches include:
Example: Closed-Loop Recycling of Aluminum
Production of 1 kg aluminum can with 50% recycled content:
- Virgin aluminum: 0.5 kg × 10 kg CO₂e/kg = 5 kg CO₂e
- Recycled aluminum: 0.5 kg × 1 kg CO₂e/kg = 0.5 kg CO₂e
- Total footprint = 5.5 kg CO₂e
Sensitivity Analysis
Different allocation methods can significantly affect results. Always conduct sensitivity analysis to understand the impact:
Allocation Method | Product A Emissions | Product B Emissions |
Mass | 600 kg CO₂e | 400 kg CO₂e |
Energy | 450 kg CO₂e | 550 kg CO₂e |
Economic | 700 kg CO₂e | 300 kg CO₂e |
Best Practices
- Document and justify all allocation decisions
- Use consistent allocation methods for similar processes
- Consider industry-specific guidance
- Validate allocation results with sensitivity analysis
- Review allocation choices with stakeholders
- Update allocations if circumstances change significantly
Key Takeaway: Allocation choices can significantly impact product carbon footprints. Always follow the allocation hierarchy, document decisions thoroughly, and validate results through sensitivity analysis.