The Carbon Math Behind Compressed Biogas

 

For a Compressed Biogas (CBG) project especially one utilizing feedstock like Napier grass or municipal waste carbon credit calculation is a process of quantifying the "net climate benefit."

In simplified terms, your project earns credits by being "cleaner" than the alternative. Here is the detailed breakdown of how this is calculated.

1. The "Golden Equation" for CBG

The number of credits you generate is the difference between what would have happened and what actually happens:

Net Credits = Avoided Emissions - Project Footprint

A. Avoided Emissions (The Plus Side)

CBG projects typically claim two types of avoidance:

1. Methane Avoidance: In the baseline, organic waste (like agri residue, MSW, Pressmud, industrial sludge or dung) might rot in the open, releasing Methane. Since Methane is 28x more potent than CO2, capturing it in a digester creates a massive credit gain.

2. Fossil Fuel Displacement: The Compressed Biogas (CBG) replaces Diesel, LPG, or Natural Gas. You calculate the CO2 that would have been emitted by those fossil fuels.

B. Project Footprint (The Minus Side)

You must subtract any emissions your plant creates:

• Grid Electricity: CO2 from the power used to run agitators, pumps, compressors and scrubbers.

• Transportation: Emissions from trucks bringing Feedstock to the plant and taking aways products and byproducts to point of use.

• Physical Leaks: Small amounts of methane that might escape from the digester or valves or purification system losses or digestion of undigested slurry at composting yard piles or Liquid FOM lagoons etc but not limited to (usually estimated at 1–2%).

2. Step-by-Step Calculation Logic

To make this practical, let's look at the variables for a typical 6 TPD (Tons Per Day) project:

Step 1: Establish the Baseline

If you weren't making CBG, where would the energy come from?

• Activity: 6,000 kg of CBG replaces approximately 7,200 liters of Diesel from energy value.

• Calculation: 7,200  liters x 2.68 kg CO2/liter = 19.3  tons  CO2/day. 

Step 2: Calculate Methane Avoidance (Optional but Lucrative)

If using waste that would otherwise decay anaerobically:

• Formula: Tons of Waste x Methane Generation Potential x 28  (GWP of Methane).

Note: This is often the largest source of credits for MSW (Municipal Solid Waste) projects.

Step 3: Deduct Project Emissions

• Electricity: If your plant uses 1,000 kWh/day: 1,000 x 0.8  kg CO2/kWh = 0.8  tons CO2 /day.

• Fuel for Logistics: If tractors/trucks use 100 liters of diesel: 0.27 tons CO2/day

Step 4: Final Tally

Net Credits = 19.3  (Avoided) - 1.07 (Project Usage) = 18.23 Credits per day 

Note : Values are placeholders for a simple understanding of calculation. In practice, they involve numerous specific factors for each item used in production, along with recording and monitoring provisions.

3. Key Methodologies to Follow

To sell these credits on the international market, you must follow specific "recipes" (Methodologies). The most common for CBG are:

4. Simplified "Rules of Thumb"

• 1 Ton of CBG typically generates between 2 to 5 Carbon Credits, depending on the feedstock and what fuel it replaces.

• Napier Grass, Pressmud and MSW projects are currently high-value because they also allow for "Soil Carbon Sequestration" credits if you can prove the grass roots are storing carbon in the ground, Methane Avoidance if not treated etc.

• Verification is Key: You cannot just claim these numbers. An independent auditor (VVB) must visit the site to verify your gas flow meters and electricity bills.