Carbon Dioxide Balance of Wood Substitution: Comparing Concrete- and Wood-Framed Buildings

In this study a method is suggested to compare the net carbon dioxide (CO₂) emission from the construction of concrete- and wood-framed buildings. The method is then applied to two buildings in Sweden and Finland constructed with wood frames, compared with functionally equivalent buildings constructed with concrete frames. Carbon accounting includes: emissions due to fossil fuel use in the production of building materials; the replacement of fossil fuels by biomass residues from logging, wood processing, construction and demolition; carbon stock changes in forests and buildings; and cement process reactions. The results show that wood-framed construction requires less energy, and emits less CO₂ to the atmosphere, than concrete-framed construction. The lifecycle emission difference between the wood- and concrete-framed buildings ranges from 30 to 130 kg C per m² of floor area. Hence, a net reduction of CO₂ emission can be obtained by increasing the proportion of wood-based building materials, relative to concrete materials. The benefits would be greatest if the biomass residues resulting from the production of the wood building materials were fully used in energy supply systems. The carbon mitigation efficiency, expressed in terms of biomass used per unit of reduced carbon emission, is considerably better if the wood is used to replace concrete building material than if the wood is used directly as biofuel.     

Global warming potential factors and warming payback time as climate indicators of forest biomass use

A method is presented for estimating the global warming impact of forest biomass life cycles with respect to their functionally equivalent alternatives based on fossil fuels and non-renewable material sources. In the method, absolute global warming potentials (AGWP) of both the temporary carbon (C) debt of forest biomass stock and the C credit of the biomass use cycle displacing the fossil and non-renewable alternative are estimated as a function of the time frame of climate change mitigation. Dimensionless global warming potential (GWP) factors, GWP_bio and GWP_biouse, are derived. As numerical examples, 1) bioenergy from boreal forest harvest residues to displace fossil fuels and 2) the use of wood for material substitution are considered. The GWP-based indicator leads to longer payback times, i.e. the time frame needed for the biomass option to be superior to its fossil-based alternative, than when just the cumulative balance of biogenic and fossil C stocks is considered. The warming payback time increases substantially with the residue diameter and low displacement factor (DF) of fossil C emissions. For the 35-cm stumps, the payback time appears to be more than 100 years in the climate conditions of Southern Finland when DF is lower than 0.5 in instant use and lower than 0.6 in continuous stump use. Wood use for construction appears to be more beneficial because, in addition to displaced emissions due to by-product bioenergy and material substitution, a significant part of round wood is sequestered into wood products for a long period, and even a zero payback time would be attainable with reasonable DFs.     

Methane Emissions from Landfills and Carbon Dynamics of Harvested Wood Products: The First-Order Decay Revisited

The First-Order decay method for carbon stocks is derived from first principles, and applications in the context of national greenhouse gas inventories are discussed. For methane emissions from landfills a method is developed that in principle is more accurate than the method currently recommended by the IPCC, and systematic errors are estimated numerically. The First-Order decay method is further applied to derive the permanent part of the carbon pool of harvested wood products.     

Carbon dynamics in wood products

The carbon (C) reservoir of wood products in Finnishconstruction and civil engineering was estimated by three inventoriesincluding the years 1980, 1990 and 1995. The inventory method ismainly based on the statistics of Finnish building stock. The use of differentconstruction materials in different parts of buildings is estimated for eachbuilding type. Information collected through building permits includes thematerials of bearing frames and facades. More information about the useof wood products in construction is gathered by many enquiries. The mixof construction materials has changed during each decade. Furthermore, thetimber stocks in construction not subject to permission and in civilengineering (e.g. bridges) were estimated. The C reservoir is calculated onthe basis of dry matter content of wooden construction materials. The timeparameters of a simple exponential decay model and a more detailed Cbalance model of wood products were calibrated to the inventory resultsusing the estimated wood flows to construction as model inputs.According to the inventories the C pool in sawn wood and wood-basedpanels of the Finnish building stock was 8.7 Tg C in 1980, 10.7 Tg C in1990 and 11.5 Tg C in 1995. The mean annual increases, 0.20 Tg Cfrom 1980 to 1990 and 0.15 Tg C from 1990 to 1995, areapproximately 1.3% and 0.8% of the fossil fuel C emissions in Finlandduring the same periods. When also taking into account construction notsubject to permission and civil engineering works, the estimated C stock ofwood products in Finland was 16.5 Tg C in 1995, which is about 3.3 MgC per capita and approximately 2.4% of the C reservoir in Finnish forestbiomass. The total C reservoir of wood products (excluding wood wasteand paper products) coming from Finnish forests might be as much as 7%of the standing biomass if exported wood products are also included. Theaverage lifetime of sawn wood in Finnish construction is less than 40 years.     

Equal Opportunity for Biomass in Greenhouse Gas Accounting of CO2 Capture and Storage: A Step Towards More Cost-Effective Climate Change Mitigation Regimes

Carbon dioxide capture and permanent storage (CCS) is one of the most frequently discussed technologies with the potential to mitigate climate change. The natural target for CCS has been the carbon dioxide (CO₂) emissions from fossil energy sources. However, CCS has also been suggested in combination with biomass during recent years. Given that the impact on the earth's radiative balance is the same whether CO₂ emissions of a fossil or a biomass origin are captured and stored away from the atmosphere, we argue that an equal reward should be given for the CCS, independent of the origin of the CO₂. The guidelines that provide assistance for the national greenhouse gas (GHG) accounting under the Kyoto Protocol have not considered CCS from biomass (biotic CCS) and it appears that it is not possible to receive emission credits for biotic CCS under the first commitment period of the Kyoto Protocol, i.e., 2008–2012. We argue that it would be unwise to exclude this GHG mitigation alternative from the competition with other GHG mitigation options. We also propose a feasible approach as to how emission credits for biotic CCS could be included within a future accounting framework.     

Fossil carbon emissions associated with carbon flowsof wood products

Specific fossil carbon (C) emissions and primary energy useassociated with the manufacture of different wood product groups inFinland are estimated and expressed as emissions or energy use per amountof wood-based C in raw material and per amount in end product. Thecalculation includes both emissions from supplied fuels within the forestindustries, and from electricity and district heat purchased from externalsources. The results are compared to fossil C emissions from the wholelifecycle of harvested wood products. The results of the study show, forinstance, that the emission of fossil C per wood-based C in end products(MgC/MgC) is of the order of 0.07 for sawn wood and 0.3–0.6 for paperin the manufacturing stage. The primary energy use per wood-based C inend product is of the order of 2 MWh/MgC for sawn wood, whereas forvirgin paper grades the figure is between 17 and 19 MWh/MgC. Theprimary energy content is highest in papers based on chemical pulping, butaround 60% of the energy used is produced in this case from by-productwood wastes (black liquor, bark etc.). The specific fossil C emission andprimary energy divided by the estimated service life of the wood productare measures for the relative burden of maintaining the corresponding woodproduct pool. These figures should be kept in mind when considering woodproducts as a potential C sink option.