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Thursday, 15 December 2016
Potentials for cascading of recovered wood from building deconstruction—A case study for south-east Germany
Published Date doi:10.1016/j.resconrec.2015.10.030 Author
Karin Höglmeier,
Gabriele Weber-Blaschke
Klaus Richter
Technische Universität München, Holzforschung München, Winzererstr. 45, 80797 München, Germany
Received 8 February 2013. Revised 8 July 2013. Accepted 8 July 2013. Available online 21 November 2015.
Highlights
•
New approach to determine wood in building stock of south-east Germany.
•
Wood contained in the building stock of south-east Germany more than assumed.
•
One third of recovered wood from buildings is suitable for high-value recycling.
•
Potential waste wood for cascading is considerably higher than currently utilized.
Abstract
Increasing scarceness of primary raw materials leads to a heightened focus on secondary resources. Deposits from urban infrastructure, mainly the building stock, are a potential major source of secondary resources. However, reliable information concerning available volumes and qualities is lacking. We analyzed incorporated amounts of wood in the building stock of south-east Germany, and calculated resulting streams of recovered wood in order to quantify potentially available volumes for an environmentally beneficial cascading utilization of these secondary resources. By applying a new method using data from sample buildings in regard to the quantity and quality of incorporated wood and statistical data concerning the building stock, the stock of wood based materials in buildings and the recovered wood resulting from demolishing for the year 2011 were calculated.
We found that considerable amounts of recovered wood in suitable condition for a resource-efficient use in cascades can be expected to originate from the building stock: 25% of the recovered wood is suitable for re-use and 21% could be channeled into other high-value secondary applications.
These first initialized concepts of a cascading utilization of recovered wood should be further refined and extended to utilize the existing potential to its optimum.
Keywords
Building stock
Cascading
Demolition waste
Secondary resources
Urban deposit
Waste wood
1 Introduction
1.1 Problem statement
The Europe 2020 publication of the European Commission states increasing resource efficiency as a major strategy for generating economic growth, to fight against climate change and limit the adverse environmental impacts of resource use (European Commission, 2010). The corresponding flagship initiative enumerates the re-use of valuable materials which would otherwise be wasted as a favorable measure in order to reduce the pressure on primary materials, such as raw wood from forests (European Commission, 2011).
On a national level, the German Government aspires to a doubling of resource productivity by the year 2020, compared to 1994 as a baseline (BMU, 2002 and BMU, 2012).
In addition to focusing on resource efficiency, the finite nature and instability of fossil resources supply has led to a heightened awareness concerning the importance of renewable resources for an additional and more sustainable supply for both energy-related and material use.
This paper focuses on wood as a versatile renewable resource with a high potential relating to the mitigation of climate change: First, wood products act as a carbon pool during their lifetime. Second, these products can substitute for others produced from scarce and potentially more energy-intensive resources. Third, they can substitute for fossil fuels, when used for combustion with energy recovery after their service life (Werner et al., 2005 and Richter, 2009).
In addition, however, also wood as a regrowing and thus renewable resource is not available infinitely with respect to volumes and regional availability. In recent years, an increasing competition for wood, intensified by rising prices for fossil fuels, can be detected (Schwarzbauer and Stern, 2010). To ensure a stable supply for multiple purposes and to meet the growing demands, the efficiency of the use of wood as a resource has to be enhanced and additional sources for wood have to be identified.
As described by Sirkin and ten Houten (1994), resource cascading is a method to enhance the efficiency of resource utilization by a sequential re-utilization of the same unit of a resource for multiple high-grade material applications followed by a final use for energy generation. Thereby, primary raw materials are saved and positive effects due to the substitution of finite materials by renewable resources can be increased (Gustavsson and Sathre, 2011).
To maximize the effects of a cascading utilization, secondary resources should be used in the application with the highest possible quality for which they are intrinsically suitable (Fraanje, 1997 and Haberl and Geissler, 2000). For example, high quality recovered wood from the building sector in large dimensions and without contamination, such as solid beams, should first be used to produce timber of smaller dimensions, such as lamellas, which after a service time as flooring can be chipped and used in a second cascade step as particle- or fiberboards, and finally as an energy carrier, rather than being immediately used for energy production after the first product life as a beam (Fig. 1).
Fig. 1. Cascading use of wood to improve resource efficiency (based on EPEA, 2009).
Previous studies have shown various benefits from cascading. Fraanje (1997)examined the effects on primary resource use by the cascading of pine wood in the Netherlands. He found that large savings of primary resources were possible and the time a resource is used could be extended considerably by cascading. Dornburg and Faaij (2005) compared cascading chains with wood from short rotation poplar, considering land use, CO2 emission reduction and economic performance. They concluded that cascading has the potential to improve both CO2 emission reductions per hectare and CO2 mitigation costs of biomass usage. Sathre and Gustavsson (2006) analyzed the energy and carbon balances of cascade chains for recovered lumber with several post-recovery options such as particleboard production, re-use and burning for energy recovery. They compared the balances of cascaded products to the use of virgin wood and to the use of non-wood products. Cascading was found to have positive effects on the balances, especially due to a reduced demand for non-wood products when wood is cascaded and owing to energy savings by direct cascade effects. Gärtner et al., 2012 and Gärtner et al., 2013 conducted Life Cycle Assessments (LCA) of different wood cascade chains. They concluded that generally the impact on the environment decreases with more cascade steps of using wood as a material resource before a final use for energy production.
However, most studies also discussed restrictions of the benefits of cascading. Sathre and Gustavsson (2006) concluded that benefits from cascading are minor, if virgin biomass were available yet remains unused due to cascading of recovered wood. However, this is not the case in the study area of Bavaria, despite a continuing increase of overall forest stocks. A considerable part of the increase of stocks is contained in small privately owned forests and a mobilization of these resources could not be achieved, despite great efforts over the last years. Furthermore, recovered wood today mainly substitutes virgin wood of lower quality as raw material for wood panel manufacturing (mainly particleboard). These assortments are also the main input for the growing wood use in domestic heating, which has led to rising raw wood prices and competition for these assortments between material use and use for energy production in recent years (Friedrich et al., 2012). An increase in the use of recovered wood by cascading would therefore alleviate this competition and prevent rising prices for manufactured wood products, which ultimately would lead to a partial displacement by non-wood products. This also is in accordance with the conclusion of Sathre and Gustavsson (2006) that a cascading use of recovered wood avoids the use of more energy intensive non-wood products, if forest resources are limited.
Gärtner et al. (2013) detect possible constraints in the delay of the energy recovery step for several decades which would be caused by an extensive cascading of recovered wood. This may lead to the replacement of possible cleaner energy sources in the future, when the cascaded wood finally reaches its end-of-life. Nonetheless, those possible negative environmental effects are counterbalanced by the prolonged carbon storage in the wood due to cascading, thereby contributing to the mitigation of climate change. It is evident that a cascading utilization of wood is not to be favored uncritically. Yet, when taking into account legislative requirements and the situation in the study area regarding wood demand and availability, it seems a concept worth encouraging.
Currently, in Germany, cascading of wood as a strategy to extend the material lifetime has not yet been implemented sufficiently. Close to 80% of the total amount of recovered wood is incinerated, mainly in large-scale power plants with effective flue gas cleaning. Regarding the use of recovered wood as a secondary raw material, particleboard is the only noticeable industrial application (Friedrich et al., 2012 and Mantau et al., 2012). Re-use is common in small amounts with old furniture. Other technically feasible applications, e.g. as input for other wood-based composites such as MDF (Medium Density Fiberboard) and OSB (Oriented Strand Board), or raw material for the pulp and paper production are not practiced, mainly due to only minor cost benefits, necessary technological process adaptations, and concerns of customers toward materials generated from “waste”.
1.3 Recovered wood from building deconstruction – situation in Germany
With declining stocks of primary resources, resources already incorporated in anthropogenic stocks will become increasingly important for a future resource supply (Müller, 2006). The building stock with both its mineral and organic components is one of the most important human-made reservoirs for secondary resources (Bringezu, 2012). Also for recovered wood as a secondary resource, the building sector, especially the deconstruction and demolishing of buildings, is essential. Previous studies concerning the recycling of building waste, including the wood fraction, already described the ecological benefits (Thormark, 2001 and Coelho and Brito, 2012). However, no studies heretofore have applied the concept of cascading to building waste or a subsection of it.
For an effective cascading of recovered wood from building deconstruction, verified information relating to the expected amounts and the quality of the recovered wood is essential. Up to now, only little comprehensive, recent information is available on national or regional level. Studies with different methodological approaches were carried out for the Netherlands, Japan and the city of Vienna (Fraanje, 1999, Merl, 2007 and Weng and Yashiro, 2003) and Kroth et al. (1991) assessed wood in the housing stock of Germany. Studies focusing on the overall material flow of the building stock are more common, though lacking more detailed information regarding the wood fraction (Baudirektion Kanton Zürich, 2010, Bringezu, 2012, Görg, 1997, Kloft et al., 1996, Kohler et al., 1999, Rubli and Schneider, 2007 and Wittmer and Lichtensteiger, 2007). Yet, such information is crucial to assess the suitability of recovered wood for a use in cascades, as both the quantity and the quality determines its potential secondary use.
Recovered wood as an additional source of wood is already used in Germany with estimated recovered annual amounts around 6.3 million tons (Mantau et al., 2012).
This volume represents the share of the total post-consumer wood which is collected separately. An additional approximately 3.1 million tons are combusted as part of the municipal waste, co-fired with coal or burned in small-scale furnaces in housing. Friedrich et al. (2012) state a total of 1.25 million tons of recovered wood for Bavaria in 2010.
Due to the implementation of a landfill ban regarding biomass based materials in 2003 and a rather strict legislation in regard to waste management, the recovered share of wood with at least one prior application can be seen as close to 100% in Germany. The Act for Promoting Closed Substance Cycle Waste Management(German Government, 2012) issued in 2012 regulates a five-step waste hierarchy, thereby implementing the concept of resource cascading on a legislative level. Which of the options of the hierarchy is to be favored depends on the quality of the recovered material. In general, the application highest in the pyramid which is suitable for a respective material has to be chosen.
In the case of recovered wood, the quality requirements to select the paths of either energy utilization, re-use or recycling are defined by the German Waste Wood Act from 2003 (German Government, 2003), which offers four different categories based on former treatment of the wood products with paints, preservatives, or other chemical substances (Table 1).
Table 1. Quality classes of recovered wood according to the German Waste Wood Act (German Government, 2003).
Class
Description
Intended application
A I
Untreated or only mechanical treatment
Material use (energy possible)
A II
Glued or painted wood
Material use (energy possible)
No halogen-organic compounds or preservatives
A III
Wood containing halogen-organic compounds;
Energy use (material use only with prior processing)
No preservatives
A IV
Contaminated wood, including halogen-organic compounds
Energy use in large-scale combustion facilities
No PCB
PCB
PCB treated wood
Non-hazardous disposal
Studies focusing on the origin of recovered wood in Germany are rare. Lang (2004)states a share of 33% originating from the building sector. Other major contributors are packaging (14%) and wooden parts of municipal waste (31%). A study by the Bavarian Environmental Institute states the share of recovered wood from buildings as 44% (LFU, 2012).
1.4 Objectives
To enable a cascading use, information relating to the properties of the recovered wood is crucial, as it determines the possible secondary applications and helps to steer each part of the recovered wood stream to its optimal utilization.
The main influencing factors are type of the recovered wood (e.g. solid wood, engineered wooden products), size and volume, purity and composition of the wood with other materials (hybrid materials). Finally, the overall collected amount of a certain homogenous part of the recovered wood is also decisive, as most secondary utilizations require minimum volumes to be financially viable.
In order to determine the potential for the cascading of recovered wood, this paper aims to provide information concerning the following questions:
(1)
What quantities of wood are embodied in the Bavarian building stock?
(2)
Which building products are the biggest contributors to wood stock?
(3)
What amounts of recovered wood can be expected to originate from the building stock per year?
(4)
Which potentials for cascading of recovered wood, based on the quality of the wood in the building stock, can be expected?
The area of the case study presented in this paper is the Federal State of Bavaria in south-east Germany. As the biggest of the German federal states and representing a mixture of both rural and urban building structures, it is suitable for generating outcomes also potentially applicable to other areas. Furthermore, the statistical data concerning building stocks and activities available for this region is consistent and sufficient for the purpose of this case study. We aim to evaluate the potentials for an optimal utilization of recovered wood from building deconstruction in Bavaria.
2 Methods
2.1 Case study and research approach
The first part of the study focuses on calculating the volume of wood contained in the building stock of Bavaria in south-east Germany. Based on this quantification, amounts and quality of recovered wood originating from the demolishing of buildings were derived and used to determine the potential for different secondary applications of the wood part of building waste in Bavaria (Fig. 2).
Fig. 2. Process of analysis of the wood in the Bavarian building stock by using sample buildings.
A bottom-up approach based on data from the German Architects Association was applied. For 86 buildings constructed between 1948 and 2008, datasets specifying detailed summaries of all the used building materials were available, including interior furnishings such as staircases and doors. The type of materials is described in detail in these datasets, as they are normally used by the German Architects Association in a software tool to estimate the costs of building projects by comparing the planned object to already existing similar ones (BKI, 2011).
Based on these 86 model buildings, the volumes of wood in the housing stock of Bavaria were calculated by using statistical data of construction and demolishing of buildings provided by the Bavarian Statistical Office (BayLfStaD, 2012).
For the year 2011, amount and composition of recovered wood resulting from all demolishing of buildings were calculated. We determined the quality according to the categories of the German Waste Wood Act (Table 1) and analyzed the waste wood in regard to additional quality factors:
•
The amount of rather large, formerly structurally used wooden elements which would potentially be suitable for re-use or other utilizations conserving the material integrity, and
•
the share of solid wood without major contamination, which is a suitable secondary raw material for engineered wood products such as particleboards.
The potential for a use in cascades of these waste wood fractions was then determined, allowing the assessment of possibilities in Bavaria for improving resource efficiency by cascading.
2.2 Wood in the building stock
Each dataset contains a comprehensive list of all components embodied in the respective building on the level of single parts (i.e. beams, windows, floorings). Since only the surface area of the elements was given, average thicknesses were applied in order to calculate the volume.
The volume in cubic meters of the various building components was then summarized for each building, according to the methodology previously applied by Weber-Blaschke et al., 2006a and Weber-Blaschke et al., 2006b. However, in contrast to this study, our work focuses only on the wood fraction of the building materials. Additionally, a more detailed analysis concerning the type and quality of the used wood was carried out in order to provide suitable information to quantify waste wood streams.
35 of the datasets were for residential buildings, both single-family dwellings and multi-family apartment buildings with up to 16 units. For non-residential buildings, 51 datasets were available, representing school, office and commercial as well as agricultural buildings.
For both groups of buildings, the average amount of each of the building product groups (doors, load-bearing structures, etc.) per cubic meter of gross cubic content (GCC) was calculated. By applying the number of buildings and the average gross cubic content per building, volumes of wood for the whole Bavarian building stock were calculated according to Formulae (1).
The term gross cubic content describes the volume enclosed by the building structure, including walls, roof and foundation slab. It is a key figure in terms of financing of building activities and is calculated according to the German standard DIN 277 (German Institute for Standardization, 2005).
equation1
with Wstock = wood contained in the stock [m3]; WGCC = average amount of wood per cubic meter of GCC derived from the sample buildings [m3/m3]; GCC = average gross cubic content per building [m3]; n = number of buildings in the total stock.
Separate calculations were carried out for residential and non-residential buildings. In the case of residential buildings, the calculations were conducted separately for 7 building age classes. As the majority of the sample buildings were constructed later than 1975, correction factors of the wood per unit of GCC were derived from Kloft et al. (1996). Data availability was reasonably good for residential buildings. Numbers of annually constructed as well as deconstructed buildings were available (BayLfStaD, 2012). Thus, a calculation of the current number of buildings and their age composition could be carried out. The age classes reflect the change in building tradition over time. For example, buildings constructed before 1918 consist of nearly four times the amount of wood, compared to buildings constructed later than 1980 (Kloft et al., 1996).
Calculation of the stock based on statistical data was not possible for non-residential buildings, as data for the time before 1987 are lacking. Still, a recent study on behalf of the German Ministry of Construction (BMVBS, 2011) made an estimation of the stock of non-residential buildings in Germany based on statistical data and capital invested in building stock which we used as a basis for our calculations. To obtain the Bavarian share of the German total, numbers of inhabitants and the gross domestic product in Bavaria in relation to Germany were applied. The amount and composition of wood in the building stock was also calculated according to Formulae (1). However, no age class distribution could be made.
2.3 Recovered wood from building deconstruction
In order to quantify potential amounts for cascading, the annual stream of recovered wood from the building sector and its composition has to be calculated. In our study, the situation of the year 2011 was calculated, as deconstruction rates were fairly stable over the last years and the results of this exemplary year can be seen as representative for the present situation.
For both residential and non-residential buildings the number of demolished and deconstructed buildings per age class and year were available from the Bavarian Statistical Office (BayLfStaD, 2012). Consequently, deconstruction rates for each of the age classes could be calculated. By applying the average amounts of wooden building products derived from the sample buildings, the amount of recovered wood from the building sector was also calculated according to Formulae (2).
equation2
with Wrec = recovered wood from building demolition [m3]; d = annual deconstruction rate [% of total stock].
Again, these calculations were carried out separately per age class for residential buildings and as a total for non-residential buildings.
From the detailed information regarding the type, it is possible to derive the quality and intended use of all wooden materials available for the sample buildings, the quality class of the recovered wood to be expected in the future by decomposition of the buildings. A prerequisite for this was the classification of each wooden component of the building according to the four categories of the German Waste Wood Act (Table 1). As virtually no wood of class A I and A III accrues from the building stock, these classes were subsumed under the classes A II and A IV respectively. The amount of recovered wood per class in 2011 was calculated according to Formulae (2), using respective values of WGCC for each of the two quality classes of recovered wood.
Mainly two factors determine the potential recovered wood quality: First, the inherent properties of the wood or wooden material used to construct the building are decisive. Especially materials combined permanently with the wood, such as plastic overlays, glues, adhesives or paints are to be mentioned here. Second, influences on the quality taking place during maintenance and repair over the service life of the wood, i.e. subsequent treatment with preservatives and paints have to be taken into account. In our study, we assumed that the wood products were treated only with the necessary minimum of additives: Only if safety standards would require the treatment with preservatives or paint, was a downgrading into the respective lower waste wood class applied. Consequently the amounts of clean and untreated wood we calculated represent an optimum situation in regard to recycling of recovered wood. In reality higher shares of contaminated wood may occur. Wood from load-bearing structures such as roof beams was graded into class A IV according to legal stipulations, despite this wood in reality partially not being treated with preservatives.
2.4 Potential for cascading recovered wood
Besides a classification and quantification of wood in the building stock, the assessment and evaluation of potential for cascading recovered wood was a major goal of our study.
To enable cascading, each part of the recovered wood stream has to be assigned to an appropriate secondary use, depending on its quality. To assess potential amounts for the different secondary use options, which constitute the recovered wood cascade, the wooden building products were categorized: First, according to their application in the building (load-bearing structures, doors, windows, flooring, stairs, boards etc.) and second, in regard to their inherent properties. Together, they define possible secondary use options. In the latter, the amounts of structural components possibly suitable for re-use and the fraction of solid wood products were determined.
In each case, current stocks and amounts to be expected in the recovered wood stream per year were calculated, applying the described methodology.
3 Results and discussion
3.1 Wood in the sample buildings
Values of wood content were calculated for each of the 86 sample buildings. They varied considerably, due to the size of the buildings, different construction methods and differing choices with interior fittings and windows (Fig. 3).
Fig. 3. Mean values and standard deviation of wood in the sample buildings (BKI, 2011); left: amounts calculated with gross cubic content of sample buildings; right: amounts calculated with statistical Bavarian average of gross cubic content (see text), no separate values for OF and MF available (RB total: residential buildings total; OF/MF: One-/multi-family buildings; NRB: non-residential buildings).
The representativeness of the sample buildings in regard to the building stock in general can be assessed by the following characteristic values: (1) the share of wooden buildings in the total building stock, since those buildings incorporate a considerably higher amount of wood per unit of gross cubic content compared to conventional buildings (Kroth et al., 1991), and (2) the average GCC, as it directly influences the calculation of the total amount of wood in the stock when operating with statistical building data.
In a study for the German Ministry of Construction, Diefenbach et al. (2010) report a share of 7.2% for wooden residential buildings in the stock of southern Germany. In our sample, 2 out of 35 residential buildings were wood constructions (6%), representing the actual share fairly well. Diefenbach et al. (2010) present no data for the share of wooden non-residential buildings in Germany. However, it can be expected to be considerably lower than the number for residential buildings. In our sample 2% of those buildings were wood constructions, confirming this assumption.
The second characteristic value, the average GCC per building, shows a considerable difference between the statistical value for Bavaria and the average value of our building sample. Therefore, for calculating the amounts of wood in the total building stock, the average Bavarian GCC was used, instead of the average one of the sample buildings, as the latter is considerably higher both with residential and non-residential buildings (see Fig. 3 and Table 2).
Table 2. Characteristic parameters of the sample buildings used to determine wood in the building stock of Bavaria. Data from BKI (2011) (GCC: gross cubic content).
Residential buildings
Non-residential buildings
Total
Single-family
Multi-family
Number of sample buildings
35
19
16
51
Year of construction (from-to)
1948–2009
1948–2009
1989–2008
1989–2008
Average GCC [m3]
of sample buildings
2317
913
3983
11,187
Bavarian average
1169
–
–
4112
Wooden buildings
2
2
0
0
A reason for this discrepancy might be that architects can choose if they input one of their building projects into the planning tool we derived our data from (BKI, 2011). It can be assumed that they do this more often with high value buildings, which normally also have a higher than average GCC. Another reason may be the high number of multi-unit houses in our sample buildings, which increases the overall average GCC. To obtain an accurate overall value of incorporated wood, we assumed all buildings to have the average Bavarian GCC, however, with differing amounts of wood contained per cubic meter of GCC, as derived from the sample buildings.
3.2 Wood in the building stock
The quantitative and qualitative assessment of wood in the Bavarian building stock was one major goal of the study. The Bavarian building stock is dominated by residential buildings. Although non-residential buildings account for nearly one third of the overall gross cubic content (GCC) of the Bavarian building stock (Table 3), residential buildings contribute over 90% of the wood contained in the total stock. This is due to the fact that residential buildings embody roughly 1.5 times the amount of wood compared to non-residential buildings. Per unit of GCC, the rates differ even more (Table 4).
Table 3. Description of the Bavarian building stock in 2011 (Column 1–4: data from BayLfStaD, 2012 and derived values; Columns 5 + 6: own calculations based on sample buildings and statistical data).
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