Waste that can be burned is dealt with in LCA by “system expansion”. See Fig. 7.6. The basic idea is to add an extra step to the chain (the blue step in Fig. 7.6), where the waste is burned, either in a electrical power plant or in a municipal waste incinerator. To calculate the gross heat, the Lower Heating Value of the waste has to be applied (ISO 14044).
The following efficiencies are to be applied in the eco-costs system (the best practice in Western Europe):
– 45% to convert the LHV to electricity in a power plant for use in manufacturing plants (medium voltage, i.e. 1 KV to 35 kV, power supply in the range of 160 kWh to 40.000 kWh)
– 25% (= 55% of 45%) to convert the LHV to electricity in a municipal waste incinerator, medium voltage
– 95% to convert heat input to heat output.
The moisture content (MC) in wood must be evaporated, leading to the following net LHV values:
– 20 MW per kg dry wood (this is an average for softwood, the LHV for hardwood is approx. 10% higher)
– 17,3 MW per kg wood MC 12%, wood in houses (= 0,88 x 20 – 0,12 x 2,25 MW per kg)
– 8,9 MW per kg wood MC 50%, fresh wood (=0,5 x 20 – 0,5 x 2.25 MW per kg)
For other moisture contents, the LHV formula (MW/kg) is: ‘weight per kg’ x (1 – ‘moisture comtent’) x 20 – ‘weight per kg’ x ‘moisture content’ x 2,256
Since wood is a natural product, the CO2 (and SO2) emissions of combustion are not counted in the eco-costs system (see FAQ 2.1) : these emissions are part of a closed loop when the wood stems from plantations (which is the case for European wood types).
For combustion of plastics, the situation is basically the same, see Fig. 7.7. The difference with wood is that most of the plastics which are applied in products are based on fossil fuels. Therefore the eco-costs of CO2 must be counted. The result is that the positive effect of the generation of electricity (or heat) is counterbalanced by the CO2 emissions. The net result for electrical power plants is slightly positive for some plastics, but negative for many others. See the Excel File on eco-costs for Products, tab Idemat and Ecoinvent. For municipal waste incineration, the result is always negative, because of the lower efficiency. So burning plastics is a municipal waste incinerator is not a good solution for the environment: plastics should be recycled.
When plastics are made from renewable resources (“bio-plastics”), combustion is a good option (since the CO2 is not counted, like wood).
Note that combustion is a better solution than uncontrolled bio-degrading, since uncontrolled bio-degrading has the risk of CH4 emissions (a greenhouse gas, 20-25 times stronger than CO2). There are two types of controlled bio-degrading:
a. Controlled bio-degrading by anaerobic bacteria in a closed storage tank, where the CH4 is collected and burned. For small rural communities in the 3rd world, this seems to be a good local solution to generate methane for cooking. In the Western world it seems to be that it is not a good solution, since the overall eco-efficiency is lower than combustion in an electrical power plant.
b. Controlled bio-degrading by aerobic bacteria in a closed building, where the CH4 emissions are minimised and captured. This method is applied in Western Europe (the Netherlands, UK, Germany, etc.) for municipal waste. In countries like the Netherlands, there is more compost production than the local market can absorb, so there is a tendency to limit compost production and use the biomass for combustion.
Note: The Ecoinvent database in combination with the Ecoindicator 99 and Recipe has the characteristics that the benefit of combustion of wood cannot be taken at End of Life. The debate is that this would result in double counting, since biomass is already taken into account at the “market mix” of electricity. From macro-ecologic point of view this is right. However, the issue is that LCA is normally applied to micro-ecologic issues: when a designer decides on wood since it can be burned at the EoL, it is a good decision as such, regardless of the fact what happens on average in Europe. It is an issue of applying marginal instead of integral mass-balances.
The eco-costs system is meant to be for designers, purchasers, business people and consumers who have to take their marginal decisions. Therefore, the eco-costs system incorporates the positive effect of combustion of wood at the EoL stage.