Embodied energy and CO2 in UK dimension stone
N. Crishna*, P.F.G. Banfill** S. Goodsir*
*SISTech Ltd, Heriot-WattUniversity, Edinburgh, EH14 4AS
**School of the Built Environment, Heriot-WattUniversity, Edinburgh, EH14 4AS
Corresponding author: Professor PFG Banfill, School of the Built Environment, Heriot-WattUniversity, Edinburgh, EH14 4AS, UK.
Phone +44 131 451 4648. Fax: +44 131 451 3161. Email
Abstract
A process based life cycle assessment of dimension stone production in the UK has been carried out according to PAS 2050. From a survey of eight production operations, on a cradle-to-site basis for UK destinations the carbon footprint of sandstone is 77 kgCO2e/tonne, that of granite is 107 kgCO2e/tonne and that of slate is 251 kgCO2e/tonne. These values are considerably higher for stone imported from abroad due to the impact of transport. Reducing the reliance on imported stone will contribute to emissions reduction targets as well as furthering the goals of sustainable development.
Keywords
Stone; Embodied CO2; Life cycle assessment; Carbon footprint
1. INTRODUCTION
Stone masonry is characteristic of the built environment in the north of the UK, and especially Scotland.Indeed, for many centuries dimension stone[1]has playeda major role in Scotland’s economic development and cultural heritage.In the 1850s there were over 700 operational quarries but by 2000 this number had dropped to around 50 (Scottish Executive, 2006). This decline has been linked to a loss of craft skills, a greater demand for industrially produced building materials such as brick and concrete, and increasing imports of building stone. However these changes have had a cost in terms of environmental impact and, in particular, carbon dioxide emissions. Despite this decline, stone continues to be needed for new construction, where existing vernacular styles are to be maintained, and for repair, maintenance and extension of historic buildings and structures, in accordance with internationally recognised best conservation practice.
The Climate Change (Scotland) Act 2009 lays out the Scottish Government’s commitment to reduce greenhouse gas emissions in Scotland by 80% in 2050 (Scottish Government, 2009). The relative roles of imported and indigenous stone in achieving this target are unclear because, while there are clear guidelines and regulations for new construction and there is established data about the environmental impacts of materials,there is no similar data inventory for traditional structures. In particular, there is little existing information on the impact of building stone (Hammond and Jones, 2008b).
This paper reports a study aiming to understand the impact, in terms of energy use and greenhouse gases, of the quarrying and processing of natural stone used in the repair of traditional buildings and construction of new buildings. It reports a process-based life cycle assessment (LCA), carried out on data collected from quarries and stone yards, to calculate the energy and carbon dioxide embodied in sandstone, slate and granite dimension stone, and considers the role of imported stone. It presents, for the first time, high quality data on Scottish and UK stone production, linked to a consideration of transport and logistics.
2. LITERATURE REVIEW
2.1. Life cycle assessment and embodied carbon accounting
Accounting for the carbon footprint of products and buildings has become a well developed approach to quantifying the success of climate change mitigation initiatives (BSI, 2008a, 2008b). In the built environment, the focus is either on the emissions during the operational life of the asset or on the embodied emissions defined as ‘the total carbon dioxide equivalent that is emitted during the different stages of extraction, processing, use and disposal of the material’ (UKWIR, 2008). Embodied energy contributes 10-20% of the lifecycle energy consumption of conventional buildings but the trend towards lower operational energy in highly efficient ‘green’ buildings is increasing this proportion (Ramesh et al, 2010), with contributions reported up to 40% (Chen et al, 2000) and 45% (Thormark, 2002) over a 50 year period. By definition an embodied carbon analysis originates from a life cycle assessment (LCA). A recent development in life cycle carbon accounting is the PAS 2050 standard (BSI, 2008a, 2008b).
2.2 Previous work on dimension stone
Much effort has been devoted recently to documenting the environmental impacts of the different materials used in construction and most of the results are incorporated in commercial software, handbooks (Anderson, 2002; Anderson et al, 2009), websites (e.g. and tools (e.g. BREEAM) which are widely used by academia and industry. The ‘Environmental Profiles’ database for materials, produced by BRE, is an attempt to produce standardised environmental data on construction materials in the UK (Anderson et al, 2009).Embodied energy coefficients of materials were initially produced forNew Zealand (Alcorn, 1998, 2001, 2003). The Inventory of Carbon and Energy (Hammond and Jones, 2008a, 2008b) summarises embodied energy and CO2coefficients for building materials, using data collected from primary and secondary sources in the public domain, and employs a cradle-to-gate analysis for the majority of the materials included. However, sandstone is not listed.
In contrast to these comprehensive listings for other building materials, there are fewer studies of dimension stone in the public domain and those that exist differ in the use of boundaries and resulting carbon and energy estimates. Alshboul and Alzoubi (2008) and the University of Tennessee (2008a, 2008b, 2008c) have published some figures fromJordan and USA respectively. The latter is an ongoing LCA study that has collected industry data from 15 stone quarries and operations across the US and has published results for granite, limestone and slate. In Scotland, a preliminary study of a sandstone quarry by Venkitachalam (2008) found that transportation emissions were a high proportion of the total carbon footprint.
Table 1 summarisespublished embodied energy and CO2 values and supports the view of Hammond and Jones (2008b) who state that the data for stone LCA results are ‘generally poor’. Table 2 comparesthe activities and sources of CO2included within the system boundaries of these studies, and highlights the inconsistency across the different work.It is worth noting that the ISO 14040 and PAS 2050 standards recommend the use of wider boundaries,but in practice most studies have focused on first and second order impacts (Alshboul and Alzoubi, 2008; Venkitachalam, 2008), and commonly exclude the embodied CO2associated with the manufacture and maintenance of capital equipment and infrastructure.
Table 1. Reported embodied energy and CO2 values for stone
Source Study / Type of stone / Embodied Energy(MJ/kg) / Embodied CO2
(kgCO2/kg) / Boundaries
Alcorn (2003) / General / 0.656 / n/a / Cradle-to-grave
Alshboul and Alzoubi (2008) / General / 0.309 / n/a / Cradle-to-site
Venkitachalam (2008) / Sandstone / 0.122 / 0.0095 / Cradle-to-site
University of Tennessee (2008a) / Granite / 5.908 / 0.62 / Cradle-to-gate
University of Tennessee (2008b) / Slate / 0.208 / 0.028 / Cradle-to-gate
University of Tennessee (2008c) / Limestone / 0.964 / 0.105 / Cradle-to-gate
University of Bath ICE (2008b) / Granite / 0.1 to 13.9 / 0.006-0.78 / Cradle-to-gate
University of Bath ICE (2008b) / Limestone / 0.3 / 0.017 / Cradle-to-gate
Table 2. LCA System Boundaries used by different standards and studies
ISO 14040 / PAS 2050 / BRE Material Profiles1 / University of Tennesseestudies2 / Stone Study: Scotland3 / Stone Study: Jordan4Boundaries include / Cradle-to-grave / Cradle-to-grave / Cradle-to-site / Cradle-to-site / Cradle-to-site / Cradle-to-gate
Materials (used in the production process) / / / /
Energy generated onsite / / / / /
Use of electricity / / / / / /
Use of fuels on site / / / / / /
Use of fuels off site (transport) / / / / / /
Energy embodied in fuels / /
Energy use in offices and factories / / /
Treatment and disposal of waste products / / /
Recovery of used products (including reuse, recycling and energy recovery) / /
Manufacture of ancillary materials /
Manufacture, maintenance and decommissioning of capital equipment /
Manufacture, maintenance and decommissioning of capital infrastructure /
Any other processes within the life cycle which are associated with GHG emissions /
1 Anderson et al, 2009; 2 University of Tennessee 2008a; 2008b; 2008c; 3 Venkitachalam, 2008; 4 Alshboul and Alzoubi, 2008
3. METHODOLOGY
3.1. System boundaries
A process-based LCA approach to extraction and processing has been usedto quantify the carbon footprint of dimension stone. In order to produce consistent, comparable results, the boundaries and guidelines specified in PAS 2050 have been followed as far as practicably possible withany deviations from the PAS 2050 methodology identified below.
As a first step all the materials, activities and processes that contribute to the production of dimension stone were identified anda process map developed andverified in discussion with quarry and stone yard owners. This defines the flows of energy through the life cycle (Figure 1).
Figure 1. Process map of the life cycle of dimension stone
The second step was to define the system boundaries for the cradle-to-gate and cradle-to-site LCA. The boundaries of the cradle-to-gate analysis conducted in this research are consistent with the Business-to-Business (B2B) approach outlined in PAS 2050 and the main inputs are shown in Figure 2. Four sources of energy use were excluded from this study: (i) the manufacture and maintenance of machinery and vehicles, (ii) water
Figure 2: System boundaries - Inclusions and exclusions in each stage of the life cycle
use on site, (iii) construction and maintenance of buildings and (iv) the production of black powder / explosives. Excluding energy embodied in the production of black powder was due to lack of existing information on explosives. As the amount of explosive used at the quarry (compared to the energy use) is minimal this exclusion has not made a material difference to the results of this work and isconsidered to be in accordance with PAS 2050[2].For the cradle-to-site analysis, the cradle-to-gate figures have been supplemented with additional data associated with the transport of stone to a number of example destinations in Scotland, considering only direct fuel use, excluding manufacture and maintenance of vehicles.
The functional unit adopted for this work is kilograms of carbon dioxide equivalent per tonne of finished dimension stone (kgCO2e/tonne) and it is assumed that the output product consists of regular blocks finished flat on all six faces, but without significant decorative treatment. The only process emissions associated with the production of stone are from the combustion of black powder at the quarry. The emission factors for fuels and electricity used include associated NO2and CH4 emissions.
3.2. Data collection and quality
Based on the process map (Figure 1) survey questionnaires were developed for owners of quarries and stone processing facilities to report fuel use and production for calendar year 2008 and to state the main destinations of their stone products. Sandstone, granite and slate operations across Scotland, the UK and in the Republic of Ireland were contacted, with an emphasis on sandstone quarries, and the responserate is shown in Table 3. Eight operations, covering all three stone types and a range of scale of operation, were visited to contextualise the data. The relatively low response rate (38%) is probably due to the small scale and seasonal nature of some operations, anda perception by operators thatfinding data would take significant time out of their business. The majority of the activity data from the 12 returns received was based on actual records kept by the organisation, rather than estimates. However, the data on main destinations for stone products was based largely on estimates.
Table 3. Number of UK stone operations contacted and responses received
Type of stone / No. of operations contacted / Responses received / LocationQuarry and yard / Stone yard / Quarry and yard / Stone yard
Sandstone / 13 / 6 / 4 / 3 / Scotland and Derbyshire
Slate / 6 / n/a / 2 / n/a / Cumbria and Wales
Granite / 6 / n/a / 3 / n/a / Cornwall and Ireland
The primary data was supplemented by secondary data on emission factors drawn from standard sources, including Defra, European Commission and industry-specific sources (see Table 4). The only exception to this was the emission factor for black powder which was developed stoichiometrically.
3.3. Calculation of embodiedCO2
For each stage in the life cycle, the energy used for each activity was multiplied by the appropriate emission factor (Table 4) and summed. The overall figure was then allocated among products and by-products using the system expansion method (BSI 2008b) and the total tonnage of quarried stone or stone product. Overall Scottish and UKembodied carbon for each stone type is an output-weighted average of all the quarries and stone yards who provided data.
Table 4. Sources of secondary data used in this study
Emission Factors / Type of factor / Applied to / SourceFuels: diesel, gas oil, fuel oil, LPG / Combustion factor / Cradle-to-Gate analysis / Defra / DECC (2009)
Fuels: diesel, gas oil, fuel oil / Life-cycle factor / Cradle-to-Gate analysis / European Commission (2008)
Fuels: LPG / Life-cycle factor / Cradle-to-Gate analysis / World LP Gas Association (2008)
Freight Transport: road (UK) / Tonne.km factor / Gate-to-site analysis / Defra / DECC (2009)
Freight Transport: road (worldwide) / Tonne.km factor / Gate-to-site analysis / IFEU (2008). (This work was commissioned by various EU freight and rail organisations.)
Freight Transport: sea / Tonne.km factor / Gate-to-site analysis / Defra / DECC (2009)
‘Black Powder’ explosive / Combustion factor / Cradle-to-Gate analysis / Developed stoichiometrically from process chemistry.
3.4. Modelling transport impact of imported stone
It was particularly important to understand the carbonimpact of transport because of the volume of stone now being imported from abroad. Although HM Revenue and Customs maintains detailed data on imports and exports, there is very little collated, published information on the volume of stone imports into the UK. Industry reports (Natural Stone Specialist, 2006) and interviews with project stakeholders identified Portugal, Spain, Italy, and Poland in the EU and Brazil, India and China as principal sources, so four representative countries were chosen– Spain, Poland, India and China.
Inconsistent disaggregated data and the variable data qualitymade it unfeasible to calculate top-down CO2 figures for these countriesso their dimension stone industries were investigated. A desk-based review, followed up with phone interviews with industry professionals in the UK and in each country established the structure and scale of the stone industry, the types of stone produced and exported, the location of the main stone producing areas in each country, the modes of transport and the routes followed within the country and to the UK. The average road and shipping distances were used to model the CO2associated with the transport of stone to two hypothetical destinations in Scotland: Edinburgh, for all three stone types, and Aberdeen, for granite only. (Granite is the predominant stone in Aberdeen’s built environment.)It is assumedthat stone is quarried and processed within the country of origin before export to UK and that the shortest practicable transport routes are followed.
4. RESULTS AND COMMENTARY
4.1. Carbon footprint of natural stone from the UK
Table 5 presents the embodied CO2associated with sandstone, granite and slate produced by quarries and stone processing facilities in the UK. The figures are output-weighted and based on data collected for the calendar year 2008.The cradle-to-gate values in Table 5 reflect the energy used in quarrying and processing of the stone, while the cradle-to-site figures reflect the transport of stone within the UK.
Table 5. Cradle-to-gate and cradle-to-site embodied carbon
Stone type / Embodied carbon (kgCO2e/tonne)Cradle-to-gate / Cradle-to-site
Sandstone / 64.0 / 77.4
Granite / 92.9 / 107.5
Slate / 232.0 / 251.8
Slate has the largest footprints of the three stones and its cradle-to-gate footprint is inflated by the amount of waste that is associated with quarrying and processing. Quarry operators’ estimates for slate wasteare 85% compared to29% for sandstone and 47% for granite. Taking these proportions into account to calculate the footprint on a total production basis (i.e. including waste) reverses the order with slate having the lowest footprint. The scale of wastage can be attributed to the thinly bedded and easily breakable nature of slate, requiring much more bed rock to be quarried and processed in order to produce slate product, as compared to sandstone andgranite. We return to the issue of waste later.
The cradle-to-site figures are based on data collected from each stone yard on the mode of transport and destinations for stone processed in the yard in 2008, and therefore reflect the average impact of transport to construction sites (mostly within the UK). For slate this is slightly higher than granite and sandstone comparators due to the clustering of slate quarries in the study (Cornwall and Cumbria) and the UK wide distribution (and associated carbon cost of transport)of the slate product from these operations. For most of the Scottish sandstone operators, distribution is largely regional or local.
Figure 3 shows the disaggregated impacts for each stone type and the allocation of CO2to the main stages in the life cycle of the stone. The largest component is attributable to processing, mostly due to the number of stages and the variety of machinery associated with each stage in processing. The largest source of processing emissions is the electricity to run stone preparation machinery, dust extraction devices and water pumps.
Figure 3: Disaggregated embodied CO2in UK stone
4.2. CO2associated with the import of natural stone
Table 6 presents the results of the transport modelling from various countries of origin on the cradle-to-site CO2 emissions. The impact of transport increases with distance from the country of origin, with transport fromChina resulting in an over 550% increase while transport from Spain increases the embodied CO2by 7% for slate and 2% for granite which is equivalent to transporting stone end to end in the UK.For stone sourced from Spain and Poland, most of the transport impact is attributable to road journeys within each country, and although the journey by sea to the UK is considerably longer the CO2impact is lower, due to the lower CO2impact (per tonne-kilometre) of shipping. For stone sourced from India and China, the largest contribution is attributable to shipping over the long distances involved.