The following information is taken from the Bureau of Transport & Communications Economics (BTCE) Model, based on a steady-state Monterey Pine plantation forest. The figures are also applicable to Tasmanian Blue Gums. These are referred to as 'Model' trees in the calculations.

Greenfleet has a policy of undertaking environmental plantings, where the original ecosystems are permanently restored. Likely comparative CO₂ absorption rates are advised by the Centre for Forest & Tree Technology based on individual sites selected. As a general guide a factor of 2.5 is applied to the calculation for the number of 'Model' trees required. So for every 'Model' tree needed, Greenfleet will plant 2.5 'Biodiversity' trees.

Assumptions

• Carbon content of steady state forest is 219 tonnes per ha (BTCE working paper 23), comprised of 176 tonnes of carbon per ha of standing timber and 43 tonnes of undecayed wood product.
• The plantation density is 1,200 trees/ha
• Average fuel consumption of cars, rigid trucks, articulated trucks, buses and average distance travelled have been used (ABS & BTRE estimates).

Notes

• Tree absorption is based on steady state assumption = 183 kg carbon/tree ie. 219,000kg carbon/1200 trees.
• Converting to C to CO₂ (molecular weight of CO₂ is 44, molecular weight of carbon is 12, so to convert C to CO₂ multiply weight of C by 44/12), so 183 kg carbon/tree is equivalent to 669 kg CO₂/tree.
• Fuel consumption per year = average fuel consumption per 100*km x average distance travelled per year.
• Diesel conversion factor 38.6 megajoules/litre * 75.2 grams CO₂/megajoule = 2.9 t CO₂/kL
• Petrol conversion factor 34.2 megajoules/litre * 72.3 grams CO₂/megajoule = 2.5 t CO₂/kL
• LPG conversion factor 26.2 megajoules/litre * 65.5 grams CO₂/megajoule = 1.7 t CO₂/kL

Additional Calculations

• 14,200Km @ 11.2 litres per 100 Km = 1,590 litres of petrol used.
• 1,590 litres of petrol = 4.0 tonnes CO₂-e
  
• 1 Model Tree can lock up 669 kg CO₂-e, equivalent to 267.6 litres of petrol.
• 6.0 Model Trees absorb 4.0 tonnes CO₂-e, equivalent to 1,590 litres of petrol. 
  
• The average Biodiversity Tree can lock up 268 kg CO₂-e
• 14.9 Biodiversity Trees can absorb 4.0 tonnes CO₂-e, equivalent to 1,590 litres of petrol.
  
• Biodiversity trees for 1,590 litres cost $51, so trees for 1 litre cost 3.2c.

CO₂ EMISSION FACTORS & LIQUID FUEL ENERGY DENSITIES BY FUEL TYPE

Estimation of Air Depleted of Oxygen by Consumption of 1 Litre of Petrol

The volume of air depleted of O₂ by direct (end-use) combustion » 9.5 m3/L petrol +/- 0.2

The true impact of fuel useis calculated over the full fuel cycle (ie including extraction, refining and transport of the fuel to the end-user). A factor of 1.114 is used by the Bureau of Transport Economics to multiply end-use energy by to obtain full fuel cycle energy. The same factor can be used in this case.
>>The volume of air depleted of O₂ on a full fuel cycle basis » 1.114 x 10.1 = 11.3 m³ / L gasoline

Basic Data

Petrol (Automotive Gasoline):

• Energy density = 34.2 MJ/L [AGO Factors & Methods Workbook, Dec 2006]
• Specific Gravity = 0.74 kg/L [AS2877-1986]
• Fuel oxidation factor = 99% [ibid]
• CO₂ emission factor = 72.4g/MJ [AGO] = 2.5 kg/L
• Full fuel cycle factor = 1.114 [Bureau of Transport Economics]
• Molar/volume composition of air = 21% O₂, 79% N₂
• Gravimetric (mass) composition of air = 23.2% O₂, 76.8% N₂ [from steam tables]
• Latent heat of vaporisation of water at 100°C = 2.261 MJ/kg

Assumptions

• No oxygen consumed by non-CO₂ emissions (SO_x, NO_x)
• Ambient conditions of 25°C

Calculation

1. O₂ carbon consumption factor = 32/(12+32) x 2.5kg/L= 1.82 Kg of O₂ /L petrol
2. O₂ hydrogen consumption factor - This can be estimated approximately from assuming:
   higher heating value - lower heating value = latent heat of condensation of water HHV-LHV¹ » 3.3MJ/kg +/- 0.2
   >> mass of water from 1 kg fuel = 3.3/2.261 » 1.5kg
   >> mass of water from 1 litre fuel = 1.5 x 0.74 = 1.1 kg/L
   >> O₂ hydrogen consumption factor =16/(16+2) x 1.1kg/L = 0.98 kg/L +/- 0.06
3. Total Oxygen consumed = 1.82 + 0.98 = 2.8 kg O₂ / L of fuel
   Density of air at 25°C = 1.185 kg/m³ [interpolated from steam tables]
   >> Density of O₂ in air at 25°C = 1.185 x 0.232 = 0.275 kg/m³
   >> Vol. air depleted of O2 by direct (end-use) combustion = 2.8/0.275 = 10.2 m³/L petrol +/- 0.2
   >> Vol. of air depleted of O₂ on full fuel cycle basis (BTE) » 1.114 x 10.2 = 11.3 m³/L gasoline

Sources: BTRE estimates, ABS statistics (9208.0, Oct 2007),  Centre for Forest & Tree Technology, National Greenhouse Accounts Factors (Jan 2008) - Department of Climate Change.