Environmental Impacts of Multi-Storey Buildings Using Different ...

- 62 -In the steel recycling scenario, all recoverable structural steel (estimated to be 250 tonnes)was recycled. The amount of recycled steel was then assumed to replace virgin steel andcredited to the building. For the Concrete building, the recoverable structural concrete(estimated at 2,180 tonnes) was assumed to be recycled to produce gravel. The production ofthe same amount of virgin gravel was then credited to the building.The total mass of wooden materials was the same as in the landfilling scenario which includestimber, LVL, plywood, and MDF (see Table 6.3). It was assumed that all these materialswould be burnt in a cogeneration plant with an efficiency of 60% (Connell Wagner, 2007; seealso the Bioenergy Knowledge Centre calculator, www.bkc.co.nz/tools). This means that 60%of the calorific value of the wood (i) is recovered as useful energy (ii) through combustionwith a ratio of electricity to heat of 1:3 (Connell Wagner, 2007). Thus 60% of the totalcalorific value of the timber was converted into electricity (iii) and heat (iv). It was assumedthat this amount of electricity and heat replaces electricity from the national grid and heatfrom burning natural gas and therefore displaces fossil fuels (0.067 kg CO 2 e per MJ heat fromnatural gas [a universal coefficient] and 0.078 kg CO 2 e per MJ electricity (GaBi 4.3, adjustedfor electricity generation in the NZ grid) and primary energy (1.42 MJ per MJ heat fromnatural gas and 2.25 MJ per MJ electricity (GaBi 4.3)).Table 6.3: Energy recovered from wood combustion and CO 2 displaced from avoiding the use oftraditional energy sources (natural gas and electricity)Building type Concrete Steel Timber Timber+Material (t)Timber 11.57 10.94 61.28 164.96LVL 343.94 343.94Plywood/MDF 6.09 6.09 56.45 125.07Total wood waste 17.66 17.03 461.67 633.97Retained energy (GJ)(i) Calorific Value 276.91 267.03 7,238.99 9,940.65(ii) at 60% efficiency 166.15 160.22 4,343.39 5,964.39(iii) Metered Electricity 41.54 40.05 1085.85 1491.10(iv) Metered Heat 124.61 120.16 3,257.54 4,473.29CO 2 e displacement (t)Electricity 3.23 3.12 84.45 115.97Natural gas 8.31 8.01 217.17 298.22Total 11.54 11.13 301.62 414.19Primary energy displacement(GJ)Electricity 93.46 90.12 2,443.16 3,354.97Natural gas 176.53 170.23 4,614.85 6,337.16Total 269.99 260.35 7,058.01 9,692.13This displacement can be explained more clearly by tracking the path of the carbon, withinthe wooden products, from cradle to grave.