COMPARATIVE CO2 EMISSIONS CONSIDERATIONS IN STEEL CONSTRUCTION

Transcription

1 4 th International Conference on Steel & Composite Structures Wednesday 21- Friday 23 July 2010 Sydney, Australia COMPARATIVE CO2 EMISSIONS CONSIDERATIONS IN STEEL CONSTRUCTION M. RODDY 1, R. SERRETTE 2 1 Researcher, Pacific Energy Management, Vashon, Washington, USA greenframe@aol.com 2 Associate Professor, Department of Civil Engineering, Santa Clara University, Santa Clara, California, USA rserrette@scu.edu 1. RESEARCH GOALS There is a shortage of reliable information on CO2 emissions of homebuilding materials, for a variety of reasons. We chose to focus on an American house s structural system, and to engineer and design a single house, but with three common structural systems: cold-formed galvanized steel, timber, and concrete masonry units (CMU). This approach is not new, since the United States (US) timber industry s CORRIM report made a similar effort in However, the results from that report, which included a Life Cycle Inventory, do not appear to be consistent with currently available carbon accounting methods. Neither the engineering nor the carbon accounting methodology adopted in the CORRIM report was performed in a sufficiently thorough and scientifically sound manner. This was particularly the case for the timber house analysis where logging site CO2 emissions were apparently underreported and harvested wood products (HWP) sequestration over-reported. In the same report, emissions for steel appear to be exaggerated, since three of the four reference mills were older basic oxygen furnace (BOF) facilities, and not reflective of advancements in the industry. For lack of a detailed alternate analysis, CORRIM at times has been relied on by architects and specifiers, to the detriment of credible accounting for the different products emissions burdens. The analysis in this paper will attempt to report each industry s emissions burden in a way that is consistent with current technical knowledge, and corresponds with typical building practices. Several strategies were employed in order to achieve this goal.

2 2. HOUSE DESIGN AND ENGINEERING A modest single-story house, Figure 1, located in Texas was chosen for this comparative analysis a ranch style home containing three bedrooms, two baths, and a garage, common in the US. Living space was approximately 116 square meters, plus an attached 47 square meter garage and storage area. Figure 1: Single-story house The structural systems for all three construction materials cold-formed steel framing, timber framing, and CMU block were designed by profession engineers. General contractors then developed cut lists and block totals from engineered drawings. For the steel version of the house, two different approaches were taken: standard profiles, using mostly 0.84 mm (33 mil) material for field framing and high tensile strength 414 MPa (60 ksi) steel combined with more advanced roof truss geometries. Both approaches are common in the US. We then obtained an average total steel weight figure from the two designs. Wood sheathing for the roof was not included, since these elements would be identical for all three houses. For the wood house, two general contractors (unknown to each other) were asked to develop lumber takeoffs based on the US prescriptive code. Wind and seismic loads were minimal in this location. The lumber takeoffs generated by the contractors were averaged and the average total board footage figure was employed in this analysis. Since emissions and not cost were being calculated, we added 15% for lumber yard and site cull a figure deemed conservative. One weakness in the wood calculation is the difficulty in analyzing the relationship between finished wood products and industrial roundwood consumption, which does not allow for mill waste and other factors. By using a 25% figure for mill waste, in addition to cull, we arrived at a CO2 emissions estimate with a standard deviation of 2.5 tons of CO2 emissions for the sample house. The block house was engineered using empirical codes for the same location. We then submitted the drawings to a masonry contractor, who developed a block materials list, plus a wastage allowance. Only one block takeoff was collected, assuming variances in builders block totals for walls would be minimal.

3 A 5% breakage figure was added. The roof was framed in steel, so that total structural emissions could be calculated. Units of measurement were kilograms of steel, cubic meters for wood, and kilograms for CMU blocks plus steel trusses. This corresponds to building practices and emissions calculations in US Environmental Protection Agency (EPA) emissions reports. 3. CO2 EMISSIONS ANALYSIS METHODOLOGY In the US, industry CO2 emissions are calculated by the respective trade associations, in compliance with EPA reporting requirements. The American steel industry, whose total pollutants are monitored regularly, submits emissions reports that are practically non-controversial. The number of steel mills in the US is relatively small, and each is subject to regular inspections by EPA officials. Concrete block manufacturing is more dispersed and variable, but rough industry emissions averages are obtainable from the concrete and masonry trade associations. Timber emissions resulting from logging operations in a variety of circumstances are more difficult to calculate. Unlike other countries, the US timber industry reports only net sequestration in emissions reports, so there is no separate category for logging produced emissions. Our three emissions estimate approaches for the different material industries follow. 3.1 Cold-formed steel emissions estimates Data from the three most recent available years was collected: 2006, 2007, and Tonnage was averaged for these years, yielding a figure of 93.8 million metric tons (Mmt). About 57% of this steel is produced by electric arc furnace (EAF) mills and 43% by BOF integrated mills. Average annual CO2 emissions for the US steel industry for were Mmt per year 1. This overall industry figure, with a CO2 emissions intensity of 1.19, was used to calculate emissions for the steel elements in the Texas house. While emissions vary among the different steel technologies, and even in different mills, we decided to use the gross US industry figure. It is not possible to accurately calculate the percentage of coiled sheets for construction elements that originate from the various production sources. This is particularly true because as steel s market share increases, EAF mills will produce more of the material than they do currently, reversing existing ratios. 3.2 CMU industry emissions estimates 2006 concrete emissions, for both process and energy use, were reported at Mmt CO2 in EPA reports to the Energy Information Agency 2. The market share of CMU is not available, so we relied on recommended weight formulas in order to estimate annual emissions. The weight of CMU block for the sample house was estimated at 76 metric tons of concrete and mortar. By calculating 8% emissions by weight of CMU, a reasonable emissions figure can be determined. Steel trusses for the CMU house were added to the total emissions burden of the CMU house.

4 3.3 Timber industry emissions estimates The US timber industry does not report logging emissions to either Intergovernmental Panel on Climate Change (the US is not a Kyoto signatory) or to EPA. Values were arrived at through several methods: assessing comparable softwood timber producers reported emissions, in this case Canada and Sweden 3, evaluating rough submittals in EPA annexes 4 and relying on the professional literature. One such study by Heath and Birdsey, US Forest Service carbon scientists, estimated the long term sequestration value of a no logging scenario in the US at billion tons of CO2 emissions annually 5. Figure 2 illustrates forest carbon flux after a clearcut harvest. Note that the site vegetation does not begin to achieve net sequestration for a number of years, since wood products, slash, and litter are decaying faster than growth occurs. It takes 200 years or more before the site sequesters the amount of carbon held previously. This and related data should provide incentives for the steel industry to move toward promoting product substitution as a way to help slow global warming- a claim that the timber industry makes in reverse, but without evidence. Figure 2: Land use effects on spatial and temporal patterns of carbon storage and their flux in PNW forests (adapted with permission from a slide presentation by David Wallin, University of Western Washington, 2004) CO2 intensity for logging is roughly estimated at 0.96 Mmt of CO2 per cubic meter of industrial roundwood production, based on typical country forestry emissions reports. The US Department of Agriculture 6 reported 2005 US wood products consumption at 599,523 cubic meters, a figure which includes consumption of imported wood products. US CO2 emissions from consuming wood products are therefore estimated at Mmt annually. This figure does not include site emissions from soil, and follows Kyoto protocols in not allowing for harvested wood products (HWP) sequestration. These two data input omissions approximately balance out. HWP carbon sequestration is only 15% of site emissions according to accepted carbon science calculations (see Figure 3).

5 Figure 3: Percent of carbon stored in wood products (adapted with permission from a slide presentation by Ann Ingerson, The Wilderness Society, US Forest Carbon and Climate Change, 2007 Soil emissions tend to be higher than 15%, but since this figure cannot be easily quantified due to the many variables, the 575 Mmt of CO2 emissions attributed to US wood products consumption is a reasonable figure. Emissions caused by logging are measured on site and are not controversial among scientists, with minor exceptions regarding measurements and methodology. To the authors knowledge, this information is not released by the US timber industry in the US and may be one contributing factor to often citied smaller energy intensity of timber in terms of CO2 emissions burdens. 4. EMISSIONS ESTIMATES FOR SAMPLE HOUSE Slightly different methods were used to arrive at each structural material s CO2 emissions score reported in Table 1. Since wood panel roof sheathing is assumed to be identical for each house, this material was not included in the emissions calculations. Steel and masonry calculations are roughly accurate, but there are minor uncertainties in the timber calculation, which are reflected in the standard deviation. CMU Steel Framed Wall Steel Trusses Timber Framed Material 4.45 (tons) 68.9 (tons) 2.16 (tons) 24.5 m 3 ± 2.5 m 3 CO2 Intensiy 1.19 /ton 0.08 /ton 1.19 /ton 0.96 /m 3 CO2 Emissions 5.30 (Mt) 5.51 (Mt) 2.57 (Mt) 23.5 (Mt) CO2 Index Table 1: CO2 emissions for steel, CMU and timber houses

6 5. DISCUSSION The methodology discussed in this paper follows US government reports, and will vary in other countries, depending on steel, concrete, and wood technologies employed, but the rough ratios and trend would be the same. Long term, trends will favor steel, as energy intensity is on a downward trend, and less so timber, since climate disruption is increasing tree mortality worldwide. Degraded forest habitat from industrial logging is a very significant factor in this mortality, but this is not factored into the above emissions calculations. 6. FUTURE CONSIDERATIONS The international housing industry has an historic opportunity to make substantial contributions toward reducing CO2 emissions into the atmosphere by considering a more accurate and complete assessment of emissions locally and globally. To enable this opportunity, a task force should be established to provide extensive scientific justification, and standard measurements and methods to corroborate the claims made in this paper. This will require independent studies in a number of countries, to include local forest sequestration opportunities and targeted markets. Together we can develop this exciting opportunity. Persistence, funding, and disciplined in seeking out the highest quality science are the main requirements. 7. REFERENCES [1] American Iron and Steel Institute, March 2010 private communication re EPA submittals. [2] US Energy Information Administration, 2007 Emissions and Sinks [3] UNFCC, Estimation, Reporting, and Accounting of Harvested Wood Products, GCCC/7P/2003/7, Oct 2003 [4] US Environmental Protection Agency, 2007, LULUCF & Annex 3, Table 200 [5] Heath and Birdsey, Carbon trends of productive temperate forests of the coterminous United States, Water, Air, and Soil Pollution, Volume 70, Oct [6] United States Department of Agriculture, Statistical Tables, 2007