1229

61

61.1  IntroduCtIon

Drying is one of the most energy-intensive unit operations in industrial processing. In case of evaporative drying, a large part of the required energy is used in the form of thermal heat (i.e., heat generated directly by combustion of fuel or indi-rectly from steam or using hot oil through a heat exchanger) in the drying process. The drying system would include infeed material handling and loading components, dryer unit, out-feed handling and unloading components, and heat plant and accessories. Drying is a very diverse unit opera-tion in chemical engineering. Some distinguishing features of the drying unit operation are the variation in the material size and shape, variety of drying media used, and the wide range of drying times. This is why there are over 400 differ-ent types of dryers reported in the literature and over 100 dis-tinct types of dryers commonly available (Mujumdar, 2006).

Drying is a coupled heat and mass transfer process. The essential mechanisms involved with any drying are removal of moisture, and this water vapor is carried away by forced or natural airflow in the dryer. This can either be enhanced or decreased depending on the air-conditions employed sur-rounding the material to be dried. The overall drying system selection should focus on the heating system and any pre- or postdrying treatment in addition to the drying process itself based on techno-economic and environmental evaluation. The three main requirements of a dryer are the supply of heat (with either hot air or air–steam mixture or pure steam), a carrier to remove the water evaporated from the material dur-ing drying, and the exposure of the wet surface to the drying medium by agitation or other means.

Traditionally, drying systems have been developed based on the engineering design and technical and economic fea-sibility. However, recently, the life cycle assessment (LCA) methodology has been used to evaluate the environmental performance of a process in addition to conventional techno-economic considerations. This method is a recognized tool for environmental assessment of a product or process using international standards (ISO 14044, 2006). This tool origi-nated in the early 1990s with initiatives from the Society of Environmental Toxicology and Chemistry (SETAC) and the United Nations Environment Program (UNEP SETAC Life Cycle Assessment Initiative, 2012). A review of historical development of LCA and issues in the context of mining and mineral industry has been studied (Yellishetty et al., 2012).

LCA can be potentially used to assess performance of existing drying processes and to develop new drying technologies so that they meet the sustainability criteria. However, LCA of the drying system can be challenging since the drying technology can be used for processing in diverse industry sectors, including food and dairy, wood, paper, bio-mass, pharmaceuticals, textile, and mineral concentrates, each having its own unique characteristics. There are con-ventional drying chambers, bed dryers, rotary dryers, flash dryers, and fluidized bed dryers. The drying medium can be hot air, hot air–steam mixture, superheated steam, radiofre-quency-based heating, or a combination of any of these. The fuel or heating source can be the sun, wood waste, natural gas, oil, coal, or electricity.

The objective of this chapter is to introduce LCA in the context of drying systems and to demonstrate its application using typical drying processes. The application of the LCA

Life Cycle Assessment of Drying Systems

Nawshad Haque, Sachin V. Jangam, and Arun S. Mujumdar

Contents

61.1 Introduction ................................................................................................................................................................... 122961.2 Life Cycle Assessment ................................................................................................................................................... 1230

61.2.1 Goal and Scope Definition ..................................................................................................................................123161.2.2 Inventory Analysis ..............................................................................................................................................123161.2.3 Life Cycle Impact Assessment ............................................................................................................................123161.2.4 Interpretation and Application ............................................................................................................................1231

61.3 Example Case with LCA Application in Drying Context ..............................................................................................123161.3.1 Wood Drying ..................................................................................................................................................... 123261.3.2 GHG Footprint of Drying of Radiata Pine ........................................................................................................ 123361.3.3 Other Impacts .................................................................................................................................................... 1235

61.4 Example: Drying of Sewage Sludge .............................................................................................................................. 123761.5 Summary of Other LCA Analysis of Dryers ................................................................................................................. 1238References ............................................................................................................................................................................... 1238

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1230 Handbook of Industrial Drying

methodology to evaluate some of the drying systems will be demonstrated with more case studies.

61.2  lIFe CyCle assessMent

LCA has been a recognized method for measuring the envi-ronmental impacts of products, processes, and services. It provides a scientifically sound method of comparing between products and processes on common grounds to identify the so-called hot spots and opportunities for reducing environ-mental impacts. Ideally, LCA evaluates the total environmen-tal burdens and benefits over the entire life cycle for a product from cradle to grave, including material and energy used during extraction and processing of raw materials, manufac-turing, transportation, reuse, recycling, and end-of-life fate. Depending on the scope of a study, the life cycle sometimes is bounded between cradle-to-gate or gate-to-gate process boundary. In the commercial sense, LCA assists environ-mental assessment such as eco-labeling, environmental prod-uct declarations (EPD), and so-called carbon footprints and greenhouse gas (GHG) accounting and management, finally with the aim to develop eco-efficiency, design for environ-ment, or the discipline of industrial ecology that tries to mimic closed-loop processes of natural ecology. Recently, some con-ceptual guides have been developed with the term life cycle sustainability assessment (LCSA), which covers conventional environmental LCA, life cycle costing (LCC), and social LCA (s-LCA) following the framework of international standards (UNEP SETAC Life Cycle Assessment Initiative, 2011). For these studies, environmental, cost, and socially relevant unit process data are collected, impact indicators are selected, and overall sustainability is assessed. In this chapter, we have focused only on the environmental LCA.

Adequate primary process data and database for every industry sector such as energy, transport, mining and metals, agriculture, building construction, materials, and chemicals are fundamental for LCA studies. There are initiatives in var-ious countries to collect such data or develop databases (often referred to as life cycle inventory [LCI]) for LCA, which are

sometimes interlinked under the leadership of government or private organizations or learned societies. One such initia-tive in Australia is called the Australian Life Cycle Inventory Database Initiative (AusLCI), led by the Australian Life Cycle Assessment Society (ALCAS, 2012). The challenges are big because of the complexity of many interlinked pro-cesses and an enormous amount of resources are required for the development of such databases.

There has been significant LCI data development of several thousand industrial processes in Europe. There was also an attempt to make some datasets of world average technology for a particular process and customizing these data sources for var-ious regions of the world, including North America. One such database set is called ecoinvent (Swiss Centre for Life Cycle Inventories, 2012). With sound judgment, modification, and cus-tomization by the LCA practitioners, these databases have been packaged in recently developed standard LCA software such as SimaPro developed by the Dutch company PRé Consultants (PRé, 2012) and can be adjusted for specific regions. The eco-invent Centre owns, maintains, and updates the ecoinvent data-base and sells licenses as part of LCA software sales. Another LCA software, GaBi (PE International, 2012), also contains similar datasets, which are maintained and owned by the com-pany. Given that LCA is an evolutionary process, any study can be adjusted once precise data sources become available for a specific technology or geographical location. There are other similar software such as openLCA and databases available that readers can explore further (OpenLCA, 2012). The LCA disci-pline has publications covering studies in established scientific journals, predominantly in the International Journal of Life Cycle Assessment, the Journal of Cleaner Production, and the Journal of Industrial Ecology.

International standards (ISO 14040, 1998) provide prin-ciples and framework for undertaking LCA. LCA should include the definition of goal and scope, inventory analysis, impact assessment, and interpretation of results as its phases are illustrated in Figure 61.1. International standards series (14041, 1999; 14042, 2001; 14043, 2001) provide further details on each of these steps and stages.

Goal andscope

de�nition

Inventoryanalysis

Impactassessment

Interpretation

Direct applications:Process/product development andimprovementsStrategic planningPublic policy makingMarketingOther

FIgure  61.1  Phases of an LCA study. (From ISO 14040, Environmental Management—Life Cycle Assessment—Principles and Framework. International Organization for Standardization, ISO, Geneva, Switzerland, 1998.)

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1231Life Cycle Assessment of Drying Systems

61.2.1  GoaL and ScopE dEfinition

The goal and scope of an LCA study should unambiguously state the intended application, the purpose of this study, and intended audiences. The scope of the study should define the functional units and justification of this selection, the unit processes considered, the boundary, allocation procedures if multiple products are considered, types of impact and impact methods, assumptions, and input data quality and limita-tions. The information flow through the life cycle of a prod-uct in the context of LCA is shown in Figure 61.2.

61.2.2  inVEntory anaLySiS

The LCI analysis is the heart of the LCA process since it involves with actual data collection, validation, and calcula-tion procedures. Data collection sheets should be prepared and data need validation process by process experts as reviewers. After validation, data can be converted to units per functional unit basis. The functional unit is the reference unit of a product or process for which quantified performance is measured in an LCA study. Where allocation or partition-ing of impacts are required due to multiple products or reuse and recycling in the process, the impacts can be shared based on mass allocation of the flow by weight basis or economic allocation based on annual average prices over a few years. According to international standards, allocation should be avoided where possible by expanding the process system. If it cannot be avoided, the basis of allocation and procedures should clearly be stated.

61.2.3  LifE cycLE impact aSSESSmEnt

Life cycle impact assessment aims to examine the pro-cess or product system from an environmental perspective using impact categories and category indicators connected with LCA results. The characterization models are used for developing units of impact indicators. For example, one com-mon characterization model for global warming potential (GWP) is the Intergovernmental Panel on Climate Change

(IPCC) model. According to this model, GHG emission is reported in carbon dioxide equivalent unit. This is a quantity that describes for a given mixture and amount of GHG the amount of CO2 that would have the same GWP when mea-sured over a specified timescale (generally, 100 years). The CO2 equivalent of methane emission is 25 according to this model. This is the characterization factor for methane, which means 1 kg of methane emission will have the same GWP impact as 25 kg of CO2.

Once reliable LCI data have been collected, based on the impact method chosen and depending on the regional or national requirements, impact categories should be selected. The main categories often are the gross energy requirement (GER) or embodied energy, which is used to estimate GWP or GHG emissions in CO2 equivalent units, solid waste bur-den, water usage and depletion, human toxicity, freshwater or marine ecotoxicity, acidification, eutrophication, carcin-ogens, biodiversity depletion, land use, fossil fuel, mineral resource use, and even socioeconomic impact. Each of these impact categories has different units of measure. Some of the metrics are well established, whereas some others have been under development. These impact categories can be broadly grouped under that detrimental to human health, ecosystem damage, and resource depletion. Finally, an area that is under development within the LCA community and that is largely subjective is putting a weight factor for each of these areas (i.e., human, environment, socioeconomic) based on agreed percent by the stakeholders depending on specific circum-stances. The common impact categories of the so-called midpoint and endpoint based are shown in Figure 61.3. The midpoint categories are between the LCI and the final dam-age, whereas the endpoint indicators are the resultant impacts of an activity.

61.2.4  intErprEtation and appLication

The main objectives of life cycle interpretation are to analyze results, reach conclusions, state limitations, and provide rec-ommendations. The essential aim is to gain as much insight as possible about the processes throughout the life cycle to identify opportunities to reduce impacts. The interpretation covers the process activity as defined with the goal and scope of the study and released to the intended audiences.

61.3   exaMPle Case WItH lCa aPPlICatIon In dryIng Context

The goal of this LCA is to demonstrate the utility of LCA in the wood drying process context. It is a relatively simple pro-cess based on gate-to-gate scenario where there are not many unit processes involved and interlinked. The use of materials is limited, although the energy use is relatively high. These energy inputs have been categorized as thermal energy for heating and drying and electrical energy for forcing airflow using electricity-driven motor-operated fans. The intended audiences are the researchers and practitioners of wood dry-ers. LCI data have been collected from published literature

Energy

Material

MaterialProduction Use

Recycle

Reuse

Land�ll

Incineration

Processing

FIgure  61.2  Information flows needed for a complete LCA. (From UNEP/SETAC LCI, Towards a life cycle sustainability assessment: Making informed choices on products, Job Number DTI/1412/PA, Stockholm, 86p, 2011.)

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1232 Handbook of Industrial Drying

and have also been predicted using a mathematical kiln drying model. Initially, GHG had been chosen as the main impact, but other impacts have also been presented on life cycle basis. This is an LCA study in the sense that the upstream impacts of fuel and electrical energy generations have been accounted for. The GHG impact of wood drying from both published literature and model prediction has been compared.

61.3.1  wood dryinG

Wood drying is an inevitable part of forest and timber industry and is also highly energy intensive. Wood drying is the removal of moisture from wood to make it suitable for use in building and construction and for making furniture. The objective is to minimize movement in wood in service. The moisture content of wood is reduced to equilibrium moisture content of the sur-rounding environment where wood will eventually be used.

There are varieties of wood species categorized as either hardwood or softwood. Hardwood and softwood are not based on their respective hardness or softness but are based on their origin of sources from the types of plant they come from. Eucalyptus wood is termed as hardwood derived from broad-leaved trees, whereas wood from coniferous pine trees is called softwood. The microstructure, initial moisture con-tent, and physical properties such as density, permeability, and diffusion coefficients are different for both wood types. Thus, their drying properties are different. The typical dry-ing times of eucalyptus hardwood are in the order of weeks and months, whereas softwood such as radiata pine can be dried within several hours or in a day in an industrial kiln. A  relatively recent textbook covers the various aspects of industrial kiln drying of timber (Keey et al., 2000).

The two main factors of timber drying are wood resource types, drying process conditions that include dry-bulb tem-perature, relative humidity of drying air, and the airflow over and through the stack of boards. The other main factor is the kiln control and performance of the equipment to maintain the desired set point conditions. The kiln conditions are often called as a schedule. This can be time based or moisture content based. A schedule may run over a period of time to achieve the target moisture content. Once the target moisture content is reached, the schedule is terminated and the drying kiln is stopped for postdrying treatment for stress relief from timber. Sometimes the condition is successively changed based on the change in moisture content of wood as it dries.

There are various drying schedules applied in the kilns depending on the wood species or the purpose of the intended use. The low-temperature schedules are generally below 70°C, accelerated conventional temperature (ACT) schedules are between 70°C and 90°C, medium-temperature schedules (MT) are 90°C–110°C, high-temperature (HT) schedules are 120°C–160°C, and ultrahigh-temperature (UHT) schedules are between 160°C and 200°C. Naturally, the heating up period, air velocity, and typical drying times are all different for each schedule. Generally, hardwoods are dried at low to conventional schedule because their permeability is low and cannot be dried at high temperature without unacceptable degrade. Softwood such as radiata pine can be dried very fast using HT or UHT schedules. However, the intended timber grade of use for HT-/UHT-dried timbers are structural com-ponent in timber-framed house construction. However, for use in furniture or high-value appearance grade timber, even the highly permeable radiata pine is dried using ACT schedules.

Environmentalinterventions

Impactcategories

Damagecategories

= Endpoints

Human health

Are

as o

f pro

tect

ion

Resource depletion

Ecosystem quality

= Midpoints

Climate change

Resource depletion

Land use

Water use

Human toxic effects

Ozone depletion

Photochemicalozone creationEcotoxic effects

Eutrophication

Acidification

Biodiversity

Raw materialextraction

Emissions(in air, water, and soil)

Physical modificationof natural area(e.g., land conversion)

Noise

FIgure 61.3  LCA midpoint and endpoint impact categories. (From UNEP/SETAC LCI, Towards a life cycle sustainability assessment: Making informed choices on products, Job Number DTI/1412/PA, Stockholm, 86p, 2011; Jolliet, O. et al., Int. J. Life Cycle Assess., 8(6), 324, 2003.)

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1233Life Cycle Assessment of Drying Systems

The LCA use in drying timber is a relatively new area and no study is available in the literature. In this section, the use of LCA has been demonstrated using an example of radiata pine drying. The drying schedules, drying time, and energy foot-print data have been used from a published study (Ananias et al., 2012). These data have further been judged, analyzed, and validated independently for use in this LCA study.

A typical flow sheet of the timber drying process is shown in Figure 61.4. In this process, green boards are stacked in a layer separated by sticks for passing air through the gaps using automatic stacker in large production sawmills. The stack of board is moved on Travis track using overhead elec-trical systems. This forms the loading system, although in smaller operations, mobile front-end loaders are also used to move timber stack inside the kiln. Once the kiln is loaded, the door is closed with seals and the drying schedule is started from a computer, loaded with controlled software such as DrySpec™. DrySpec™ kiln control software was developed by the New Lealand Forest Research Institute in the 1990s. The kiln is fitted with sensors for recording dry-bulb and wet-bulb temperatures. The dry bulb is the actual temperature in the kiln where wet-bulb temperature is used to estimate the relative humidity. The end tip of the thermo-couple sensor used for wet-bulb temperature is wrapped with wet cotton wick and kept moist at all times with a water flow. The airflow is also maintained around the wet wick at cer-tain minimum velocity to maintain evaporation and cooling. The airflow in the kiln is maintained by electrically operated fans. In modern kilns, airflow is reversed and regulated to achieve uniform drying and can also be varied with variable frequency drives of fan motors. The effect of drying process variables has been extensively studied and reported (Haque et al., 2007) using mathematical models and experiments.

From the LCA point of view, the materials and energy in the kilns are generally electrical energy for fan motors and thermal energy to heat the kiln and maintain at the desired set point temperature. The other material used in the kiln is minimal and negligible. The effect of infrastructure on dry-ing is expected to be low since the life of the kiln is often over 50 years and these kilns would have a very high throughput over this time. However, where the contribution of infrastruc-ture is likely to be high, it should be included in the LCA study. The electricity of the loading and unloading system has been estimated for a typical kiln site. A boiler burns a partic-ular fuel to heat water and produce steam. This steam is used

directly in the kiln or indirectly through the heat exchanger. In some kilns, hot oil is used as heating medium and passed through finned or unfinned tube heat exchangers. In most heating systems for wood drying kilns, on-site- generated wood waste from sawmill and dry mill is used as a fuel source for the boiler. However, some kilns use natural gas if available cheaply. Nonetheless, the GHG emission would be significantly higher if gas is used instead of wood waste that originates from sustainably managed forest plantations.

61.3.2  GhG footprint of dryinG of radiata pinE

Among the LCA impact categories, the use of GWP or GHG emission in CO2 equivalent (CO2 e) unit has been the most popular in the industrial sectors. The GHG footprints of dry-ing of radiata pine using various schedules have been esti-mated here. The reported energy data by Ananias et al. (2012) have been used to estimate the GHG emission footprint. The source of electricity is assumed to be black coal fired, and the source of thermal energy is from burning typical wood waste in a boiler as fuel generated on-site from a sawmill. These schedules are shown in Table 61.1. Energy for loading and unloading using rail tracks has also been estimated. Emission from kiln construction is a crude estimate based on the results from ecoinvent database in SimaPro software. The results are shown in Figure 61.5. The component of GHG contribution from the electricity use was 74%–94% of total energy for vari-ous schedules. The GHG emission due to thermal energy was 7%–26% depending on the schedule. The contribution from loading, unloading, and kiln construction is negligible com-pared to that of GHG contribution from thermal heating using wood waste and fan energy for forcing airflow through the kiln using black coal–fired electricity. The GHG footprint for ACT schedules was significantly higher compared with the HT schedule mainly because of longer drying time in case of ACT schedules, thus with longer fan running using a higher amount of electricity. The implication of this result is that the fan power may be optimized using variable frequency drive for ACT schedules to reduce the emission of GHG. The effect of the reduction of velocity toward the later stages of drying may not increase drying time but will result in lower GHG emission due to less electric power consumption.

The same schedules used by Ananias et al. (2012) have been used by a drying model (Haque et al., 2007) to predict drying times and energy use. The predicted heating energy

Wet product Loading systemDrying

kiln

Heat plant

Fuel Water Water

Unloading system

Dry product

Electricity Electricity

Electricity

FIgure 61.4  Schematic drying process boundary used for this LCA.

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1234 Handbook of Industrial Drying

use over the drying regime for HT schedules and ACT schedules is shown in Figures 61.6 and 61.7. The results show some interesting findings and help to identify opportunities to reduce specific thermal energy consumption. The energy during kiln equipment and wood heat-up period is very high compared with the later part of drying, which is expected. The energy demand is very high in the beginning with heat-up and other components together but low toward the end of the drying period when only minimal evaporation occurs with only heat loss component. Once the kiln and material is heated up and the set point temperature is achieved, the energy components required are for evaporation of water and drying and the heat loss. Overall, when total energy consumption and total drying times are taken into account, the specific energy consumption per tonne of dry timber was similar for all schedules ranging from 4.6 to 4.8 GJ/t. The results indicate that a two-track continuous drying kiln in countercurrent flow mode is likely to be more energy-effi-cient compared with a batch kiln because the energy demand can be balanced between the stages of drying. In this mode,

heat from exiting hot stack is exchanged with opposing cold stack entered through the door on kiln tracks. Both doors act as entry and exit for timber stack loads in a countercurrent fashion. The energy to heat up the kiln component may not be high once heated up as the process is continuous and does not require natural cooling down and heating up between batches of production. This would likely result in efficient boiler and optimized heat plant and also can help reduce GHG emission from drying.

These predicted energy data have been used to estimate GHG footprint of dry timber (Figure 61.8). Although there are similarities in general pattern, there are significant dif-ferences also. The differences are deficiencies in experi-ments in reported data by Ananias et al. (2012) and some inadequacy in the drying to predict reality exactly. For example, the drying times reported by Ananias et al. (2012) are different for the same schedules (HT1 and HT2). When the drying time decreases, even the air velocity increases while other conditions remain the same. This is unex-pected unless the wood type is different from sapwood to

taBle 61.1drying schedules used for estimating gHg emission (ananias et al., 2012) and used by the kiln Model for Prediction of drying time for this study

schedulesdry-Bulb 

temperature (°C) Wet-Bulb 

temperature (°C) air Velocity 

(m/s) Board 

thickness (mm) drying 

time (h) drying time (h) 

Predicted by kiln Model 

HT1 120 70 8 17 6.5 4.67

HT2 120 70 8 17 3 4.67

HT3 120 70 6 17 5.5 5.34

HT4 112 70 7.2 17 2.5 5.5

HT5 120 70 7.2 17 4 4.84

ACT6 90 60 7.2 25 18 12.5

ACT7 90 60 7.2 25 18 12.5

ACT8 85 65 6 38 18 31.67

ACT9 100 80 7.2 38 17.5 26.34

0

50

100

150

200

250

300

HT1 HT2 HT3 HT4 HT5 ACT6 ACT7 ACT8 ACT9

kg C

O2 e

/t dr

y tim

ber

Drying schedules

Kiln constructionLoad and unload electricityFan electricityHeat

FIgure 61.5  GHG footprint of drying of radiata pine using various schedules.

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1235Life Cycle Assessment of Drying Systems

heartwood. Heartwood has generally very low initial mois-ture content compared with sapwood. Wood types, wood physical properties, sawing pattern, ring angles, perme-ability data, and conditions such as frequency of airflow reversals are not reported by Ananias et al. (2012). These input data are used in the model and can affect drying time. There would be some differences expected because heating up time, airflow reversals, and other factors are not exactly the same between these two sets of data. Furthermore, the model does not predict reality exactly as for an industrial kiln, although historically generally good agreement was found between predicted drying time by this model and con-trolled experimental runs or industrial runs when adjusted for heat-up time (Haque and Sargent, 2008). The contribu-tion from the kiln construction and loading and unloading components on GHG is same for both that are generally very low compared with the contribution of electricity use and heat energy. Although the GHG emission occurs mainly

due to fan electricity use and constitutes a large component of total emission, the contribution of thermal energy use is the largest component when the drying cost per unit timber is considered. The implications of these findings are that to reduce drying cost, the reduction of thermal energy is a more dominant factor compared with electricity. However, to reduce GHG emission from drying, the reduction of use in electricity in kiln should be the focus or the source of electricity should be changed to less GHG-intensive ones, such as renewable or hydroelectricity.

61.3.3  othEr impactS

LCA also includes other impact categories apart from GHG, although this is the most common use found in the industrial sector as mentioned earlier. The other midpoint and endpoint impact categories (including climate change as indicated by GHG emission) are shown in Table 61.2. It has been shown

0

100

200

300

400

500

600

700

800

900

0 1 2 3 4 5 6

MJ/

t dry

tim

ber

Time (h)

HT1HT3HT4HT5

FIgure 61.6  Predicted heating energy use over drying regime for HT schedules (note HT2 and HT3 schedules are the same, thus not shown).

0

50

100

150

200

250

300

350

400

450

500

550

0 5 10 15 20 25 30 35

MJ/t

dry

tim

ber

Time (h)

ACT6ACT8ACT9

FIgure 61.7  Predicted heating energy use over drying regime for HT schedules.

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1236 Handbook of Industrial Drying

taBle 61.2environmental Impacts of 1 t of dry Wood using lCa

Impact Category unit/t dry Wood Ht1 Ht2 Ht3 Ht4 Ht5 aCt6 aCt7 aCt8 aCt9

Midpoint categories

Climate change kg CO2 eq 73 58 51 66 51 215 193 270 224

Ozone depletion kg CFC-11 eq 3.8E-08 2.8E-08 2.5E-08 2.8E-08 2.2E-08 1.2E-07 1.1E-07 1.5E-07 1.3E-07

Terrestrial acidification kg SO2 eq 0.58 0.51 0.46 0.69 0.52 1.37 1.26 1.74 1.46

Freshwater eutrophication kg PO4 eq 1.3E-04 1.0E-04 8.6E-05 1.0E-04 7.8E-05 4.3E-04 3.8E-04 5.4E-04 4.5E-04

Marine eutrophication kg N eq 0.03 0.03 0.02 0.04 0.03 0.05 0.05 0.06 0.05

Human toxicity kg 1,4-DCB eq 11.17 11.35 10.28 17.92 13.36 17.17 17.09 22.66 19.35

Photochemical oxidant formation

kg NMVOC 1.25 1.25 1.13 1.95 1.46 2.05 2.01 2.69 2.29

Particulate matter formation kg PM10 eq 0.80 0.81 0.73 1.26 0.94 1.27 1.26 1.67 1.43

Terrestrial ecotoxicity kg 1,4-DCB eq 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01

Freshwater ecotoxicity kg 1,4-DCB eq 0.18 0.14 0.12 0.14 0.11 0.59 0.52 0.74 0.61

Marine ecotoxicity kg 1,4-DCB eq 0.19 0.15 0.13 0.17 0.13 0.55 0.49 0.69 0.57

Ionizing radiation kg U235 eq 0.06 0.04 0.04 0.04 0.03 0.19 0.17 0.24 0.20

Agricultural land occupation m2/year 271 282 256 455 339 374 381 499 429

Urban land occupation m2/year 2.64 1.98 1.71 1.98 1.54 8.57 7.59 10.69 8.86

Natural land transformation m2 1.3E-04 9.5E-05 8.2E-05 9.5E-05 7.4E-05 4.1E-04 3.7E-04 5.2E-04 4.3E-04

Water depletion m3 0.11 0.08 0.07 0.08 0.06 0.35 0.31 0.43 0.36

Metal depletion kg Fe eq 0.08 0.06 0.05 0.06 0.05 0.26 0.23 0.32 0.27

Fossil depletion kg oil eq 5.76 4.32 3.73 4.32 3.36 18.71 16.58 23.35 19.35

Endpoint categories

Human toxicity: noncarcinogenic

DALY 1.7E-05 1.8E-05 1.6E-05 2.8E-05 2.1E-05 2.6E-05 2.6E-05 3.5E-05 3.0E-05

Human toxicity: carcinogenic

DALY 9.9E-07 9.9E-07 9.0E-07 1.5E-06 1.2E-06 1.6E-06 1.6E-06 2.1E-06 1.8E-06

Freshwater aquatic ecotoxicity

DALY 3.2E-12 2.6E-12 2.3E-12 3.2E-12 2.4E-12 8.7E-12 7.9E-12 1.1E-11 9.2E-12

Marine aquatic ecotoxicity DALY 5.8E-09 4.3E-09 3.7E-09 4.3E-09 3.4E-09 1.9E-08 1.7E-08 2.3E-08 1.9E-08

Terrestrial ecotoxicity DALY 6.9E-12 7.0E-12 6.4E-12 1.1E-11 8.3E-12 1.0E-11 1.0E-11 1.4E-11 1.2E-11

CFC-11, chlorofluorocarbon; 1,4DCB, 1,4-dichlorobenzene; NMVOC, nonmethane volatile organic carbon compound; PM, particulate matters; DALY, dis-ability adjusted life years.

0

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HT1 HT2 HT3 HT4 HT5 ACT6 ACT7 ACT8 ACT9

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Drying schedules

Kiln constructionLoad and unloadAir­owHeat

FIgure 61.8  GHG footprint of drying of radiata pine using various schedules using a kiln drying model.

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1237Life Cycle Assessment of Drying Systems

to demonstrate that there are other impact categories that should be considered in addition to GHG when undertaking LCA of drying. In this drying context, many of the impacts are relatively low, but when these impacts are high, the objec-tive of the process or technology developer and designer should be to minimize these impacts. These impacts can also be compared against the best-practice benchmark process to identify opportunities for reduction when contributions are shown from each of the unit processes of the overall activity throughout the life cycle of the process.

61.4  exaMPle: dryIng oF seWage sludge

Industrial sludge drying has been very important in recent years. The quantity of sewage treated is increasing the amount of sewage sludge, which needs attention. The untreated munic-ipal sludge is considered to be hazardous waste material due to its chemical content. A proper treatment of sludge is very important for its acceptability. Previously used sludge dehy-dration and sludge condensation processes are not enough to satisfy the requirements. Hence, decreasing moisture con-tent by thermal means is the primary objective for all sludge management approaches. The high moisture in sludge also poses other problems such as difficulties in transport. The commonly used thermal dehydration methods include paddle dryers, agitated bed dryers, impinging stream dryers, super-heated steam dryers, and, recently, pulse combustion dryers. A detailed discussion on sludge drying techniques is provided in Chapter 44. Fry drying is another recently used technique

for removal of moisture from sewage sludge. This technique consists of immersing the mechanically dewatered sludge in a large volume of hot oil maintained at a temperature more than the boiling point of water (Peregrina et al., 2006). Fry drying avoids the plastic phase during water removal, which is an important problem in sludge drying using conventional dry-ing techniques. In addition, the fry drying technique provides dried sludge with better calorific value as a result of impregna-tion of oil (Table 61.3).

It is well known that sludge drying is highly energy inten-sive because of various complications associated with it. Hence, it is essential to carry out the LCA of a process before implementing it. Peregrina et al. (2006) carried out LCA of fry drying of sludge. The impact categories considered in their study were abiotic depletion of resources, climate change, acidification, and eutrophication. After selecting the boundaries, the structure used for LCA comparison con-sisted of a dryer, transportation of partially dried sludge, a combustor, transportation of combustion ashes, and, finally, landfilling. The fry dryer was compared with the conven-tional paddle dryer. Although the experiments were per-formed on a small dryer, several assumptions were made for a comparison at full scale. After comparison in four different categories, fry drying was found to be a better environmental option in terms of abiotic depletion of resources, while the acidification impacts were equivalent. Fry drying was found to perform worse in the two remaining categories, namely, climate change and eutrophication. For a detailed discussion, readers may refer to the work by Peregrina et al. (2006).

taBle 61.3summary of lCa applied to drying

details of lCa study  Purpose  Method  observations  reference 

Supercritical drying of silica aerogel

To investigate if aerogel as an insulation technology for the building sector provides a measurable environmental benefit over its life cycle

Three steps were considered for the LCA study: gel preparation, aging, and drying.

Low-temperature and HT supercritical drying were compared.

CO2 burden and energy costs were the main parameters for comparison.

Low-temperature supercritical drying method for aerogel manufacture had the longest environmental payback as a result of a higher amount of solvent used.

Environmental impact could be reduced if larger batches were used, if more energy-efficient equipment is used, and if the solvent is recycled.

Dowson et al. (2012)

LCA of spray drying process

To compare the environment load produced by laboratory- and industrial-scale spray dryer

The impact was presented in terms of human health, ecosystem quality, and resources.

Three different cases were compared at different life cycle stages: 1 h work of industrial- and laboratory-scale dryer; comparison performed for 1 year of operation of two dryers and comparison of one industrial dryer with the number of laboratory-scale dryers for the same evaporation rate.

The outcome depends on the functionality of the system.

In case of 1 h operation, the industrial-scale dryer shows dominant environmental impact.

For 1-year operation, both dryers show similar environmental impact with a larger impact during usage stage.

For the same evaporation rate, the industrial dryer has much better performance than laboratory-scale dryers.

Ciesielski and Zbicinski (2010)

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1238 Handbook of Industrial Drying

61.5   suMMary oF otHer lCa analysIs oF dryers

In the last few decades, the demand for quality dried prod-ucts has increased, which is associated with the increased energy uses of already energy-intensive unit operation. The study of environmental impact of drying process has been of great importance in recent years. The LCA is a very useful tool to quantify the environmental impact associated with a particular process. This can include energy, use of resources, and disposal of waste to environment and health effects as well. Considering the increased emphasis on sustainability in recent years, one should evaluate the environmental impact of a particular drying operation before implementing it. LCA is a simple technique that can help understand these issues and compare different options available before making a final choice.

reFerenCes

ALCAS. (2012). Australian Life Cycle Assessment Society and AusLCI Database Initiative. http://www.alcas.asn.au [accessed 23 August 2012].

Ananias, R.A., Ulloa, J., Elustondo, D.M., Salinas, C., Rebolledo, P., and Fuentes, C. (2012). Energy consump-tion in industrial drying of radiata pine. Drying Technology 30(7):774–779.

Ciesielski, K. and Zbicinski, I. (2010). Evaluation of environmen-tal impact of the spray-drying process. Drying Technology 28(9):1091–1096.

Dowson, M., Grogan, M., Birks, T., Harrison, D., and Craig, S. (2012). Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying. Applied Energy 97:396–404.

Haque, M.N. and Sargent, R. (2008). Standard and superheated steam schedules for radiata pine single-board drying: Model prediction and actual measurements. Drying Technology 26(2):186–191.

Haque, N., Riley, S., Langrish, T.A.G., and Pang, S. (2007). Predicted effect of process variables on kiln drying of Pinus radiata. Drying Technology 25(3):455–461.

ISO 14040. (1998). Environmental Management—Life Cycle Assessment—Principles and Framework. International Organization for Standardization, ISO, Geneva, Switzerland.

ISO 14041. (1999). Environmental Management—Life Cycle Assessment—Goal and Scope Definition and Inventory Analysis. International Organization for Standardization, ISO, Geneva, Switzerland.

ISO 14042. (2001). Environmental Management—Life Cycle Assessment—Life Cycle Impact Assessment. International Organization for Standardization, ISO, Geneva, Switzerland.

ISO 14043. (2001). Environmental Management—Life Cycle Assessment—Life Cycle Impact Interpretation. International Organization for Standardization, ISO, Geneva, Switzerland.

ISO 14044. (2006). Environmental Management—Life Cycle Assessment—Requirements and Guidelines. International Organization for Standardization, ISO, Geneva, Switzerland.

Jolliet, O., Margni, M., Charles, R., Humbert, S., Payet, J., Rebitzer, G., and Rosenbaum, R. (2003). IMPACT 2002+: A new life cycle impact assessment methodology. International Journal of Life Cycle Assessment 8(6):324–330.

Keey, R.B., Langrish, T.A.G., and Walker, J.C.F. (2000). Kiln-Drying of Lumber. Springer, Berlin, Germany, 326p.

Mujumdar, A.S. (2006). Chapter 1—Principle, classification and selection of dryer. In: Handbook of Industrial Drying, 3rd edi-tion. CRC Press, Taylor & Francis Group, New York.

OpenLCA. (2012). The Open LCA Project. http://www.openlca.org/index.html [accessed 23 August 2012].

PE International. (2012). GaBi Software—A software solution by PE International. http://www.gabi-software.com [accessed 23 August 2012].

Peregrina, C.A., Lecomte, D., Arlabosse, P., and Rudolph, V. (2006). Life cycle assessment (LCA) applied to the design of an inno-vative drying process for sewage sludge. Process Safety and Environmental Protection, 84(B4):270–279.

PRé. (2012). Life cycle consultancy and software solutions, SimaPro software. http://www.pre.nl [accessed 23 August 2012].

Swiss Centre for Life Cycle Inventories. (2012). Ecoinvent data-base. http://www.ecoinvent.ch [accessed 23 August 2012].

UNEP SETAC Life Cycle Assessment Initiative. (2011). Towards a life cycle sustainability assessment: Making informed choices on products. Job Number DTI/1412/PA, Stockholm, 86p.

UNEP SETAC Life Cycle Assessment Initiative. (2012). International life cycle partnerships for a sustainable world. http://lcinitiative.unep.fr [accessed 9 August 2012].

Yellishetty, M., Haque, N., and Dubreuil, A. (2012). Issues and challenges in life cycle assessment in the minerals and met-als sector: a chance to improve raw materials efficiency. In: Sinding-Larsen, R. and Wellmer, F. (eds.) Non-Renewable Resource Issues: Geo-Scientific and Societal Challenges, Series as part of the International Year of Planet Earth, Springer, New York, pp. 229–246.

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