Ecological Assessment of Clay Brick Manufacturing in China Using Emergy Analysis

: Clay brick is a commonly used building material in China. Due to the enormous land destruction and excessive consumption of resources, such as materials and energy in the manufacture of clay brick, it is important to study its overall sustainability, i.e., in terms of impact on the environment, services, and economy. In this study, emergy analysis is employed, which o ﬀ ers a holistic perspective, unlike typical environmental studies. A series of emergy indices such as renewability rate (R%), nonrenewability rate (N%), unit emergy values (UEVs), emergy yield ratio (EYR), environmental loading ratio (ELR), and emergy sustainability index (ESI) were used to study manufacturing of clay brick. In addition to calculating UEVs of clay brick manufacturing in China (7.18 × 10 12 sej / kg), our detailed analysis shows that the nonrenewable resources and imported energy have a dominant impact on the emergy contribution (50.6%) and within the nonrenewable resources, clay is the foremost item, accounting for 33.5% of local emergy inputs. Given di ﬀ erent electricity UEVs, the change ranges of clay brick system UEVs are 14.9% (scenario 1), 7.24% (scenario 2), 8.91% (scenario 3), and 6.94% (scenario 4). Furthermore, several policy suggestions are discussed for improving the sustainability of the evaluated system, involving the energy structure adjustment, recycling material replacement, and promotion of energy-saving systems.


Introduction
Before the advent of cement-based materials for buildings, clay bricks were one of the most commonly used materials for more than two thousand years in China, as seen in ancient buildings [1,2]. Despite China's rapid growth from the reform in the 1980s [3], clay bricks continued to be used in building construction in rural areas. While much attention has been aimed at cementitious bricks in terms of sustainability assessments, the research community has rarely focused on the overall sustainability, i.e., in a holistic perspective, of clay bricks in China.
Sustainability demonstrates the concept of the triple bottom line, which is a comprehensive dimension study related to the environment, society, and economy. Sustainability is an important concept and central issue which is widely used in various fields such as supply chain management [4], engineering analysis [5], building construction program [6], information aspect [7], tourism growth [8], agricultural directions [9], national sustainability [10], urban assessments [11], groundwater sustainability assessment [12], dryland regions [13], and campus culture [14].

Emergy Methodology
Emergy analysis offers a holistic ecological evaluation of a system or product, including materials, energy, resources, and service over the entire lifespan of that system or product. As a unique concept, emergy was used to elucidate the sustainability of materials.

Emergy Sustainability Indicators
The following emergy indices have been used in this study: (1) Basic indicators: Renewability rate (R%), a ratio with a renewable emergy part and total emergy part. A higher renewability rate represents a better ecological level.
Nonrenewability rate (N%) signifies the ratio of the nonrenewable emergy part and total emergy part. A higher nonrenewability rate implies a worse ecological level.
Other related indicators are UEVs, imported resources, imported energy, and labor and service.
(2) Emergy yield ratio (EYR): EYR is defined as the ratio between total emergy and imported emergy, and it represents the ability to generate emergy. The less input of purchased emergy will lead to lower EYR, which expounds the competitive ability and economic status of the targeted object.
(3) Environmental loading ratio (ELR): ELR is the ratio between nonrenewable resources and imported emergy to renewable emergy. ELR has been used to formulate the ecological pressure of targeted ecosystems. Environmental loads are made up of non-resource emergy, imported emergy, and others, such as gas emissions, waste discharge, etc. Some researchers have already given the related standard for different environmental load situations [31].
(4) Emergy sustainability index (ESI): ESI is the ratio of two emergy indicators, EYR and ELR. It implies the integral sustainable situation of one system, which can be used to assess the overall effect. Generally speaking, in view of the literature [31], there are three numerical segments to signify the sustainable degrees, which contain ESI < 1 (sustainable), 1 < ESI < 5 (medium sustainability), and ESI > 5 (unsustainable), in the long term.

Clay Brick Manufacturing Process and Data in China
As clay bricks are still used as a primary building material in China, there are over 60,000 clay brick manufacturing plants in operation today, producing 800 billion clay bricks on an annual basis [32]. For the purposes of this work, a medium-sized clay brick manufacturing plant located in Shijiazhuang City of Hebei Province in China was selected. The manufacturing plant has an area of 5000 square meters. The annual manufacturing of this plant is approximately 6 million standard sized bricks (240 mm × 115 mm × 53 mm) in a year [33]. Data used in this work were obtained from this manufacturing plant. Figure 1 shows the process of the manufacturing of clay brick.
As clay bricks are still used as a primary building material in China, there are over 60,000 clay brick manufacturing plants in operation today, producing 800 billion clay bricks on an annual basis [32]. For the purposes of this work, a medium-sized clay brick manufacturing plant located in Shijiazhuang City of Hebei Province in China was selected. The manufacturing plant has an area of 5000 square meters. The annual manufacturing of this plant is approximately 6 million standard sized bricks (240mm × 115mm × 53mm) in a year [33]. Data used in this work were obtained from this manufacturing plant. Figure 1 shows the process of the manufacturing of clay brick.  The traditional manufacturing processes of brick product (Figure 1) are composed of the raw material preparation process, crushing process, mixing process, drying, and firing process. The entire process can be simplified to four major processes, namely raw material, crushing, mixing, and drying and firing processes, in that order [34]. Figure 2 shows the emergy diagram of manufacturing of clay brick. The system boundary of clay brick manufacturing includes renewable, nonrenewable, and imported resources, imported energy, and labor and service. Energy is used in four main processes of brick manufacturing, including the ingredient, crushing, and mixing processes, as well as the drying and firing process. The traditional manufacturing processes of brick product ( Figure 1) are composed of the raw material preparation process, crushing process, mixing process, drying, and firing process. The entire process can be simplified to four major processes, namely raw material, crushing, mixing, and drying and firing processes, in that order [34]. Figure 2 shows the emergy diagram of manufacturing of clay brick. The system boundary of clay brick manufacturing includes renewable, nonrenewable, and imported resources, imported energy, and labor and service. Energy is used in four main processes of brick manufacturing, including the ingredient, crushing, and mixing processes, as well as the drying and firing process.

Emergy Analysis and Calculation of UEV
The sustainability assessment of clay brick manufacturing takes into account environment, service, and economical factors. For this work, in addition to typical emergy analysis, we can include air pollutant emission and its environmental impact (in sej), ecological service accounting, and economy accounting with an emphasis on disability-adjusted life years (DALYs) lost [24].

Air Pollutants and Related Emissions
According to the China's National Mandatory Standard [35], there are three main air pollutants in building brick manufacturing, namely dust, sulfur dioxide (SO2), and nitrogen oxides (NOx). The exhaust can cause severe effects on human health and ecosystem sustainability, specifically by

Emergy Analysis and Calculation of UEV
The sustainability assessment of clay brick manufacturing takes into account environment, service, and economical factors. For this work, in addition to typical emergy analysis, we can include air pollutant emission and its environmental impact (in sej), ecological service accounting, and economy accounting with an emphasis on disability-adjusted life years (DALYs) lost [24].

Air Pollutants and Related Emissions
According to the China's National Mandatory Standard [35], there are three main air pollutants in building brick manufacturing, namely dust, sulfur dioxide (SO 2 ), and nitrogen oxides (NOx). The exhaust can cause severe effects on human health and ecosystem sustainability, specifically by causing respiratory disease and disrupting ecological balance aspects. To obtain a careful consideration, the ecological service and economic loss calculations have been performed in this study. The initial emission data contain 80 mg/m 3 (dust), 20 mg/m 3 (SO 2 ), and 50 mg/m 3 (NOx) based on the GB 3095-2012 standard.

Economic Loss Accounting
In this paper, the economic loss due to the air pollutants of brick manufacturing needs to be considered and computed. Given the study of Bakshi (2002), human health influence has been accounted for by using DALYs (Table 1), as shown in Equation (4) [36]. L represents economic emergy loss. i is the exhaust gas species. Wi shows the waste gas values. DALY is disability-adjusted life years on the basis of exhaust gas, a/kg of emission (in Table 1). α means damage parameter for one year: 1.68 × 10 16 sej/(a·person) [24].

Ecological Services Accounting
There is a close relationship between the ecological services calculation and environmental impacts. Therefore, three air pollutants have been chosen (dust, SO 2 , and NO x ) for emergy assessment. For the estimate of the ecological service, as a first step, the waste gas amount must be estimated according to Equation (5) [24].
Secondly, the emergy of ecological services is calculated as below.

Emergy Table Calculations
The emergy analysis calculations of clay brick manufacturing in China are given in Table 2. The calculated items comprise renewable resources, nonrenewable resources, imported resources, imported energy, and labor and service. In the evaluated system, five renewable inputs are considered, such as sunlight, rain (chemical energy), rain (geopotential), wind (kinetic energy), and geothermal heat. Nonrenewable resources consist of clay and shale. Imported resources are mainly purchased resources, gypsum, limestone, steam, and fly ash. Imported energy is electricity. Based on local economic conditions, labor and service were included in emergy calculation.  19 sej/year for imported energy, and 1.11 × 10 20 sej/year for labor and service.
The emergy results shown in Table 2 reveal that the most important factors are nonrenewable resources (50.7%), followed by labor and service (25.8%) and imported energy (18.1%). Compared with those items, renewable resources play a minor role in the input emergy.
For nonrenewable resources, clay is the major contributor, accounting for about 66.1% of nonrenewable resources in emergy terms and 33.5% of the total emergy needed. Shale emergy, in second place, uses 33.9% and 17.1%, respectively. For the imported resources, the contribution ratios are 1.78% for limestone emergy, 1.78% for gypsum emergy, and 1.63% for fly ash emergy. Low quantities of renewable resources were used as inputs. Overall, results show that, in emergy terms, clay, shale, electricity, and labor and service are dominant components for clay brick manufacturing in China.

Emergy-Based Indicators
A series of indicators, namely, renewability rate, nonrenewability rate, emergy yield ratio, environmental loading ratio, and emergy sustainability index, were calculated and are shown in Table 3; details are as follows. (1) Renewability rate (%R) The renewability rate of clay brick manufacturing in China is calculated as 0.01%. This value symbolizes the sustainable level for the evaluated system and represents the proportion of renewable resources. The renewability rate is too low for clay brick manufacturing in China. Because of the high emergy costs of energy and resources, a large number of nonrenewable resources are used in the brick plant, resulting in a high non-renewable rate.
Therefore, enhancing the renewability rate is an urgent task for clay brick manufacturing plants in China. In light of recent research results, some measures have been put into practice to improve the clay brick manufacturing process, such as the use of rainwater recycling technology [41] and wastewater reuse technology [42,43]. For example, if the renewable resource can be increased by 10%, %R can be improved by 50%.
(2) Nonrenewability rate (%N) The nonrenewability rate of clay brick manufacturing in China is 50.6%. This large percentage shows the share of negative strain on the sustainability of the evaluated system. The higher the nonrenewability rate, the worse its sustainability. There are two approaches to mitigate this situation, i.e., reduce %N, namely, by enhancing energy utilization efficiency and using alternative materials to replace nonrenewable resources. For example, if energy utilization efficiency can be enhanced by 10%, the nonrenewability rate can be reduced by 0.76%; meanwhile, if 10% of the clay can be replaced by brick fragments, the nonrenewability rate will be reduced by 3.21%. In the case of alternative materials, for instance, when clay is decreased by 10%, the nonrenewability rate (%N) can be reduced by 5.07%. Therefore, using alternative materials is critical to improve %N.
Other studies have implemented similar approaches to realize sustainable development. According to Prafulla et al. [44], the activated alumina was used to substitute clay to for brick making, and, to a certain extent, the sustainable development model of the brick factory was improved. Taking advantage of fly ash and fly ash wastes, eco-friendly clay brick manufacturing was attempted by Mucahit et al. [45].

(3) Unit emergy values (UEVs)
The unit emergy value of building clay brick manufacturing is calculated as 7.18 × 10 12 sej/kg, which is different from other researchers' results, see Table 4. The UEV based on this study differs from existing studies due to the emergy baseline used (12.0 × 10 24 sej/year), evaluated system boundaries, specific manufacturing processes, UEVs of input items, brick types, labor and service situations, energy structure and efficiency, etc. (4) Emergy yield ratio (EYR) The Emergy yield ratio of clay brick manufacturing in China is 18.3. This signifies that the imported emergy item is far less than total emergy. The local resource (clay) is the main source of non-resource emergy causing serious clay waste and vast environmental pollution.
(5) Environmental loading ratio (ELR): The environmental loading ratio for clay brick manufacturing in China is 486, which is far greater than the relevant standard and empirical values, which indicates serious environmental burden. It is to be noted that, based on emergy analysis, clay brick manufacturing will bring greater damage to the local ecological environment. One option is to, among others, to use sustainable raw materials for brick manufacturing to reduce the impact to the environment. The emergy sustainability index of clay brick manufacturing in China is 0.038. Based on the ESI, the manufacturing process is not a sustainable state in the long run. In order to improve the unsustainable state of the brick factory, several environmentally friendly brick products (shale, gypsum, limestone, etc.) can be used as alternatives.

Sensitivity Analysis
As shown in Table 2, the total emergy input is divided into five parts, involving renewable resources, nonrenewable resources, imported resources, imported energy, and labor and service. The accuracies of basic data and UEVs can be impacted by various sources. Therefore, it is necessary to conduct the sensitivity analysis.
For the purposes of sensitivity analysis, the nonrenewable resources and imported energy are prioritized owing to their major influence on total emergy. Here, four scenarios are tested for sensitivity; scenario 1: the UEVs of nonrenewable resources change by −5% (i.e., reducing the nonrenewable resources by 5%); scenario 2: the UEVs of nonrenewable resources vary by +5%; scenario 3: the UEVs of imported energy are the target that have a −5% range of change; and scenario 4: the UEVs of imported energy are altered by +5%. The resultant sensitivity data of four scenarios are shown in Figures 3 and 4.
Buildings 2020, 10, x FOR PEER REVIEW 8 of 17 As shown in Table 2, the total emergy input is divided into five parts, involving renewable resources, nonrenewable resources, imported resources, imported energy, and labor and service. The accuracies of basic data and UEVs can be impacted by various sources. Therefore, it is necessary to conduct the sensitivity analysis.
For the purposes of sensitivity analysis, the nonrenewable resources and imported energy are prioritized owing to their major influence on total emergy. Here, four scenarios are tested for sensitivity; scenario 1: the UEVs of nonrenewable resources change by −5% (i.e., reducing the nonrenewable resources by 5%); scenario 2: the UEVs of nonrenewable resources vary by +5%; scenario 3: the UEVs of imported energy are the target that have a −5% range of change; and scenario 4: the UEVs of imported energy are altered by +5%. The resultant sensitivity data of four scenarios are shown in Figures 3 and 4.     Scenario 1 has the biggest fluctuation (−3.43%), followed by scenario 2 (2.84%), scenario 3 (−1.87%), and scenario 4 (1.19%), in this order. Based on −5% changes of four scenarios, the UEVs of nonrenewable resources have a greater influence than imported energy (from −3.43% to −1.87%). It  Scenario 1 has the biggest fluctuation (−3.43%), followed by scenario 2 (2.84%), scenario 3 (−1.87%), and scenario 4 (1.19%), in this order. Based on −5% changes of four scenarios, the UEVs of nonrenewable resources have a greater influence than imported energy (from −3.43% to −1.87%). It means that a 5% reduction of nonrenewable resources and imported energy can cause 3.43% and 1.87% decreases for the entire emergy input, respectively. For a +5% variation in EYR, a similar trend is seen, as shown in Figure 3 (from 2.84% to 1.19%). Nonrenewable resources play a major role, as can be seen in the emergy contribution ratio, accounting for 50.6% of total emergy in the evaluated system. The higher the emergy ratio, the greater the sensitivity results. In addition, based on EYR and ELR calculations, the ESI can be computed through a formula (EYR was divided by ELR). Because ELR is much larger than EYR, this results in an essentially unchanged ESI for this paper.
To improve overall sustainability of clay brick manufacturing, we recommend the use of clean energy as well as increased use of recycled materials.

Different Electricity UEV Effects Based on Multiple Literature Sources
In terms of Table 2, there are four primary inputs, which are clay, shale, electricity, and labor and service, accounting for 33.5%, 17.1%, 18.1%, and 25.9% of the total emergy in the brick plant, respectively. Owing to the large variations in different pieces of literature within the context of China, four groups of new electricity UEVs were selected and calculated from other studies in the last five years [49][50][51][52]. Specific data can be found in Table 5. Based on different electricity UEV selections from other studies (in Table 5), the four scenarios previously mentioned were considered and calculated in this paper. All input proportions of the four new scenarios have been computed to analyze the disparate electricity UEV impacts on the sustainability of clay brick plants. Table 6 lists the final emergy sustainability results and Figure 5 shows the comparison of the scenarios. Firstly, given the small renewability rate (R%) input for all scenarios, its proportion has no change. In all cases, it is 0.01%. In addition to renewable inputs, other cases have different variations, such as %N (from 18.38% input to 21.69%), EYR (from 18.39% input to 21.7%), ELR (from 18.38% input to 21.69%), and ESI (from 16.83% input to 23.65%). On the whole, the ranges of variation are from 3.31% to 6.82%, which demonstrate that different electricity UEVs of multiple literature sources have a greater impact on the assessment results. In this paper, taking the ESI as an example, the biggest gap was 6.8%, resulting in inaccurate sustainability results.   Table 7 reveals the UEVs of a clay brick system on the basis of five scenarios. All calculations were conducted in view of the latest baseline in this paper. Taking the original version as the calculation basis, four new change scenarios are illustrated in Figure 6, which are 85.1% for scenario 1, 92.76% for scenario 2, 91.09% for scenario 3, and 93.04% for scenario 4. The change ranges of clay brick system UEVs are from 6.94% (scenario 4) to 14.9% (scenario 1). Hence, based on four new scenarios from different literature data, single input element UEVs have a significant influence on the accuracy. In order to enhance the accuracy of calculation results, all unit UEVs of input elements should be considered carefully in the calculation.   Table 7 reveals the UEVs of a clay brick system on the basis of five scenarios. All calculations were conducted in view of the latest baseline in this paper. Taking the original version as the calculation basis, four new change scenarios are illustrated in Figure 6, which are 85.1% for scenario 1, 92.76% for scenario 2, 91.09% for scenario 3, and 93.04% for scenario 4. The change ranges of clay brick system UEVs are from 6.94% (scenario 4) to 14.9% (scenario 1). Hence, based on four new scenarios from different literature data, single input element UEVs have a significant influence on the accuracy. In order to enhance the accuracy of calculation results, all unit UEVs of input elements should be considered carefully in the calculation.   Table 7 reveals the UEVs of a clay brick system on the basis of five scenarios. All calculations were conducted in view of the latest baseline in this paper. Taking the original version as the calculation basis, four new change scenarios are illustrated in Figure 6, which are 85.1% for scenario 1, 92.76% for scenario 2, 91.09% for scenario 3, and 93.04% for scenario 4. The change ranges of clay brick system UEVs are from 6.94% (scenario 4) to 14.9% (scenario 1). Hence, based on four new scenarios from different literature data, single input element UEVs have a significant influence on the accuracy. In order to enhance the accuracy of calculation results, all unit UEVs of input elements should be considered carefully in the calculation.

Policy Implications
In light of the studies in this article, three policies are suggested to improve the overall sustainability of clay brick manufacturing in China: (1) Adjustment of primary energy resources Unbalanced energy resources are one of the main problems for clay brick manufacturing in China. To date, there are five types of power energy resources in China: thermal power (75.08%), UEVs changes Figure 6. UEV changes of clay brick system within different electricity UEV scenarios.

Policy Implications
In light of the studies in this article, three policies are suggested to improve the overall sustainability of clay brick manufacturing in China: (1) Adjustment of primary energy resources Unbalanced energy resources are one of the main problems for clay brick manufacturing in China. To date, there are five types of power energy resources in China: thermal power (75.08%), hydroelectric power (13.15%), wind power (6.1%), nuclear power (4.17%), and the other types (1.49%) [53]. In this paper, the main source of electricity is thermal power generation. If 10% of electricity supplied by thermal energy (primarily generated using coal) was replaced by wind power, the environmental load ratio can be reduced by approximately 3.1%. Hence, several renewable energy resources should be carefully considered, including solar power [54], hydropower [55], and wind power [56], which are the most common and important sources of renewable energy in China. However, a few shortcomings restrict the use of these renewable energy resources, particularly those related to huge investment, professional and technical barriers, and geographical conditions, which can be only managed effectively by local governments in China. For instance, necessary financial subsidies and advantageous tax policies needs to be offered so as to strengthen the promotion of clean energy.
(2) Recycling materials Owing to over dependence on clay in clay brick manufacturing, heavy environmental loading can be observed. Material substitution is currently proving to be effective, especially focusing on industrial waste and by-products. If 10% of clay is replaced by construction material waste while still maintaining strength, as per, the UEVs will be reduced to 3.99 × 10 12 sej/kg; this is a 5.67% reduction from the current UEV calculation. Moreover, the environmental loading ratio will be lowered by 4.1%. Several researchers have attempted material substitution in the brick industry [57][58][59][60]. Munz et al. (2014) reviewed sustainable construction material wastes for the fired clay brick industry and 44 sustainable alternative wastes with an aim to achieve sustainable brick manufacturing [57]. Taking asphalt mix as an added material, Eduardo (2018) carried out a study to design an innovative and new building brick [58]. Armando (2017) made use of construction wastes to bring about the manufacturing of ecological bricks [59]. Mamdooh et al. (2019) used aluminum recycling raw material to manufacture ecological clay bricks [60]. We suggest that China's clay brick factories should continue to increase the proportion of alternative material applications.
(3) Promotion of energy-saving systems According to the sustainability assessment of clay brick manufacturing, imported energy is the third largest contributor to the unsustainable state, so it is necessary to take some measures to mitigate the negative effect. In this study, if electricity consumption was reduced by 5%, the factory's sustainability indicators may be improved by 1.1%. The residual energy reutilization of advanced systems should be deemed as the optimal sustainable approach and be adopted to realize the reduction of imported energy [61][62][63]. Other new energy-saving systems can be given priority for application and promotion, which consist of waste water recovery systems, waste heat recovery systems, cooling recovery systems, kiln top heat exchange and reuse systems, etc.

Conclusion
In this paper, emergy analysis was used to assess manufacturing of clay brick in China. The main conclusions are summarized as follows.