DIENER et al 2012 Waste heat recovery from cement production for faecal sludge drying

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DIENER et al 2012 Waste heat recovery from cement production for faecal sludge drying
Waste heat recovery from cement
production for faecal sludge drying
This report was compiled in the framework of the SPLASH Sanitation Programme which is funded by the
Austrian Development Cooperation (ADC), Department for International Development (DFID), Ministère des
Affaires Étrangères et Européennes (MAEE), Swedish International Development Cooperation Agency (SIDA),
Swiss Agency for Development and Cooperation (SDC), Bill & Melinda Gates Foundation
Eawag: Swiss Federal Institute of Aquatic Science and Technology
Sandec, Dept. of Water & Sanitation in Developing Countries
Überlandstr. 133
CH-8600 Dübendorf
Tel. +41 44 823 5286
ISE: Institut de Sciences de l’Environnement ISE
Université Cheikh Anta Diop
Dakar, Senegal
Bibliographic reference:
Diener, S., Reiser, J.C., Mbéguéré, M., Strande, L., 2012, Waste heat recovery from cement production for faecal
sludge drying, Eawag: Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland
Photos title page:
Sococim cement plant, Rufisque, Senegal
Sludge emptying truck, Senegal
Faecal sludge drying bed, Senegal
Sococim cement plant, Rufisque, Senegal
All photos by Sandec/Eawag
In Africa and Asia, 65–100 % of urban residents are served by on-site sanitation facilities, such as
septic tanks or latrines. These systems are typically emptied with suction trucks or manual labour. The
contents are most commonly dumped directly into the environment, or disposed of in a treatment
plant if one is available and affordable. This practice has its origin in the common perception that FS is a
waste product without any value. However, this is a misperception, as faecal sludge not only contains
nutrients for use as agricultural fertilisers but also could replace fossil fuel sources in industrial
processes, such as boilers and kilns. Use of alternative fuel sources (e.g. tires, animal meal, sewage
sludge, waste oil) in industrial kilns and boilers is a recent trend driven by increasing fossil fuel costs.
However, before using FS which was passively dried on sand and gravel filter beds, it would need to
undergo an additional drying process before being blown into a burner. A possible option for an energy
efficient approach to eliminate excess moisture is the use of waste heat from the clinker production
process in cement factories. The objective of this study was to assess the technical and economic
feasibility of using waste heat from a cement factory in Rufisque, Senegal, to enhance the dryness of
faecal sludge. Two sources of waste heat are generally available in a clinker production line: i) recovery
of hot gases and ii) radiant heat loss from the kiln’s surface. In this case, the flue gas from the kiln was
already used directly in the preheater and for drying the raw material, leaving the flue gas from the
chimney as the only hot gas stream available for recovery. Its heat transfer rate is 2.8 MW, which would
be sufficient to evaporate 2.5–3.1 tons of water per hour. If the total FS production of Dakar was
dewatered on filter beds to 80% solids content, the recovered waste heat would be sufficient to
achieve a 90% dry solids content. The resulting 26.7 tons of dried faecal sludge would cover ~2% of the
daily energy requirements of the factory.
Waste heat recovery for sludge drying │ 1
Strauss et al. (2000) estimate that 65–100% of urban dwellers in Africa and Asia and some 20–50%
of urban dwellers in Latin America are served by on-site sanitation systems such as septic tanks or
latrines. In low and middle-income countries, these onsite sanitation systems produce enormous
amounts of faecal sludge every day. In Kampala, Uganda, for example, cesspool emptiers fetch 300 m3
of faecal sludge every day (NETWAS Uganda, 2011), in Dar-es-Salaam, Tanzania, the faecal sludge
removed from pits and septic tank adds up to 700 m3 per day (Chaggu et al., 2002) and it is estimated
that in Dakar, Senegal, 6,000 m3 of fresh faecal sludge is produced daily (Bill & Melinda Gates
Foundation, 2011). High costs for households are a major impediment for emptying, transport and
proper treatment of the faecal sludge and often leads to improper operation of on-site sanitation
systems. Problems include septic tanks with reduced hydraulic retention times due to sludge
accumulation and pit latrines that are flood-washed with rainwater into the street during the rainy
season. On-site systems are typically emptied with suction trucks or manual labour. The faecal sludge is
commonly dumped directly into the environment, or disposed of in a treatment plant if it is available
and affordable. This practice has its origin in the common perception of faecal sludge being a waste
product without further value. But this perception is wrong. For example, faecal sludge contains
nutrients (N, P, K) for agricultural purposes, can be used as feedstock for biogas plants or may replace
industrial heat consuming
processes such as boilers or
Alternative fuel in
cement industry
The use of alternative
fuel sources in industrial
kilns and boilers is a recent
trend driven by increasing
costs for fossil fuel and/or
environmental aspects in
Figure 1: Stock of old tires to be used as alternative fuel in a cement
factory in Würenlingen, Switzerland
2 │ Waste heat recovery for faecal sludge drying
company policies (Figure 1). The cement industry especially shows an increasing interest in the use of
alternative fuels, as a large part of production costs are directly linked to the energy intensive
production of clinker. Today, even state-of-the-art cement kilns (rotary kilns with preheater and
precalciner) have an energy demand of ~3,000 MJ/t clinker produced (Madlool et al., 2011; Schneider
et al., 2011) which corresponds to ~70 kg of crude oil per ton of clinker (net calorific value of crude oil =
42 MJ/kg, (European Commission, 2010)). However, alternative fuels such as tires, animal meal, sewage
sludge, waste oil or waste paint have to fulfil certain minimum criteria not only concerning their
energetic composition but also their physical characteristics. Solid fuels most often are fed into the kiln
by air flows and have therefore to have a minimum degree of dryness (≥90% DM). It has never been
evaluated whether faecal sludge could be used as fuel, but this seems promising based on the common
use of sewage sludge as an alternative fuel (Groupe Lafarge, 2007; Murray and Price, 2008; Schneider et
al., 2011). The dryness of faecal sludge, passively dried on uncovered sand drying beds depends on
climatic conditions but varies between 60% and 80% (Badji et al., 2009). It would therefore need to
undergo an additional drying process before fed into the burner. One possibility for an energy efficient
approach to eliminate the excess moisture would
be the use of waste heat from the clinker
production process.
This study was conducted at the Sococim
Cement factory in Rufisque, Senegal. The
objective was to evaluate the possibility of
recovering waste heat from cement, and to
evaluate the technical and economic feasibility of
using the waste heat to enhance the drying of
faecal sludge, thereby using two waste streams
to provide an efficient fuel.
Faecal sludge in Dakar
Dakar, the capital of the west African country
Senegal is located on a peninsula at the very
western point of the country. Of the 2.5 million
inhabitants of Dakar, 30% are served by a
Figure 2: Truck unloading FS at FS treatment
plant in Niayes, Dakar.
Waste heat recovery for sludge drying │ 3
centralised sewer system and a wastewater treatment plant. The remaining 1.8 million residents are
served by a faecal sludge management system, including cistern/pour flush toilets connected to septic
tanks on household level (Strande et al., 2012). The sludge from the septic tanks is transported to the
wastewater treatment plant in Cambérène, where the faecal sludge is co-treated with wastewater, or it
is unloaded at the faecal sludge treatment plants in Niayes or Rufisque (Figure 2). The treatment plants
have settling/thickening tanks followed by unplanted drying beds, with the leachate at Cambérène
going to the wastewater treatment plant. According to an estimation by the Bill & Melinda Gates
Foundation (2011), greater Dakar generates 2.2 million m3 of faecal sludge per year. However, the
solids content of faecal sludge in Dakar is very low, in the range of 3.5–4.5 g/l (Dème et al., 2009;
Tounkara, 2007).
Sococim Industries, Rufisque
The Sococim Industries (VICAT group, since 1999) factory is located in Rufisque, about 30 km east of
Dakar. It is the largest cement producer in West Africa and the factory has the capacity to produce up
to 7,500 tons of clinker per day in three production lines. The most recent kiln plant (3,500 t/day) has
been in operation since 2010 and consists of a 5-stage, 2-string DOPOL®’90 preheater with a PREPOL®AS-CC calcining system, a rotary kiln and a POLYTRACK® clinker cooler. The lime for the clinker
production is mined directly on Sococim’s premises and since 2007, the factory runs its own power
plant enabling the cement plant to be self-sufficient in terms of electric power.
Clinker production process
In a typical dry rotary kiln system (Figure 3), raw material (crushed limestone, iron, bauxite,
quartzite and/or silica) is preheated in a cyclone type pre-heater. The last stage of the pre-heater would
act as a precalciner, where a big part of the raw material is calcined (decomposition of CaCO3 to CaO
and CO2). The rotary kiln itself is a lined tube with a diameter up to 6 m. It is generally inclined at an
angle of 3–3.5° and rotates 1–2 times per minute. At the lower end of the kiln, the product falls into the
clinker cooler. Fast cooling enables waste heat recovery and improves the product quality (Engin and
Ari, 2005).
4 │ Waste heat recovery for faecal sludge drying
Raw material
Limestone Flue gas
Flue gas
Flue gas
Rotary kiln
Flue gas
Figure 3: Overview of the clinker production process.
Waste heat recovery technologies
Generally, there are two sources of waste heat available in a clinker production line: i) waste heat
recovery from hot gases and ii) radiant heat loss from the kiln surface. One of the most effective and
simple waste heat recovery methods is the preheating of the raw material prior to it being fed into the
kiln. To achieve this, the hot flue gas from the preheater is directed unaltered to silos where the raw
material is stored, to eliminate excess moisture. Sometimes such straightforward use of the excess heat
is not possible, either because the physical (e.g. humidity, particles) or chemical (e.g. toxic elements)
characteristics of the air stream do not comply with the secondary utilization. In this case, heat
exchangers are indispensable to transfer the heat to another medium such as water or oil. The
temperature of exhaust air from the clinker cooler is about 215–250°C (Madlool et al., 2011;
Trenkwalder, 2010), allowing for the use of a variety of heat exchange technologies. For example, the
heat can be directed through a waste heat recovery steam generator (WHRSG). The generated steam is
then used to power a steam turbine driven electrical generator, and the produced energy can then
replace a large part of the production facility’s electrical energy demand (Madlool et al., 2011). Another
possibility is an air to thermal oil heat exchanger. The oil is used to transport the thermal energy to
applications in other locations. In most cases, the thermal oil is used to convey heat within a premise,
but it can also be tapped using an oil/water heat exchanger if long transport distances are required. For
example, contributing to a municipal heating network or to external companies. An example of such an
Waste heat recovery for sludge drying │ 5
application is illustrated in Figure 4, where a private company in Würenlingen, Switzerland, uses the
waste heat from a cement factory for the drying of dewatered sewage sludge, which then is sold back
to the cement factory as alternative fuel in their clinker production. A similar system is described by
Trenkwalder (2010) for a cement plant in Karlstadt, Germany, where the entire fuel stream for the
clinker production is provided for with the use of alternative fuels. In this case, dried sewage sludge
makes up ~10% of the total firing heat capacity.
Air/Oil heat exchanger
Community heating
Pre-heating of heavy oil
Cement factory
Clinker cooler
Evaporation rate
• 1.4 tons H2O/h
Water/Air heat exchanger
Dried sewage sludge
• 90% solids
• 0.63 tons/h
Thermal energy
• 1,500 kWh
sewage sludge
• 20-45% solids
• 2 tons/h
Belt dryer
Sludge drying company
Oil/Water heat exchanger
Electric energy
• 150 kWh
Figure 4: Use of waste heat from clinker production for the drying of sewage sludge in Würenlingen,
As an alternative to traditional energy recovery methods, using the radiant heat loss from the
rotary kiln has been proposed in literature (Ari, 2011; Caputo et al., 2011; Engin and Ari, 2005). Up to
15% of the total energy input is lost through radiation and convection from the kiln surface (Engin and
Ari, 2005). Major obstacles for the implementation of this new technology are the high investment
costs and the limited access to the rotary shell for monitoring and maintenance due to complex
mounting of new equipment.
6 │ Waste heat recovery for faecal sludge drying
More common technologies for waste heat recovery are air-to-air heat exchangers. Heat can be
recovered from low temperature air streams (100–200°C) using flat plate air-to-air heat exchangers
(recuperator) or rotary heat exchangers (Oǧulata, 2004). These air-to-air heat exchangers utilize
relatively simple technology. For the production of electricity it has to be considered that using water as
a working fluid in a cycle that converts heat into work (Rankine Cycle) cannot be operated in an
economically feasible manner with waste heat below 370°C (Hung et al., 1997). In this case an Organic
Rankine Cycle (ORC) containing working fluids with a much lower boiling point such as NH3, Benzene,
R134 or R11, has to be considered.
Possibilities for heat recovery at Sococim
The situation of low temperature air streams also applies to the most recent production line of
Sococim. The flue gas has a temperature of 115–140°C with an air flow stream ̇ of 450,000–500,000
Nm3/h. The heat transfer rate ̇ for this air stream can be calculated applying equation (1). To provide
the most conservative estimate, the inlet temperature of 115°C and a decrease in temperature of 25K
(∆T), typical for air-to-air plate heat exchangers, have been assumed. A temperature of 115°C and
ambient pressure of 1 bar was assumed to set the values of relative density (ρ = 0.9 kg/m3) and the heat
capacity (cp = 1.0138 kJ/kgK) of the air.
̇ = ̇   ∆
The 450,000 Nm3/h eventually bear a heat transfer rate of 2.8 MW which can be used for the
conditioning of the faecal sludge before it is used as fuel in the cement factory.
For the efficient use of the faecal sludge, Sococim requires a minimum dryness of 90%. Given the
high water content of raw faecal sludge, and the costs associated when transporting water weight long
distances from where it is collected to the cement factory (~30 km), it was assumed for these analyses
that faecal sludge undergoes first a dewatering and drying process in a semi-centralized treatment
plant near the city.
60–80% dryness can be achieved on a sand bed (Badji et al., 2009). The energy required for the
evaporation of the remaining water before the faecal sludge can be fed to the burner therefore varies.
Theoretically, the energy needed to evaporate one kilogram of water is 2.26 MJ (= 627 Wh). However,
in reality, depending on the efficiency of drying systems, the energy demand varies from 3.3–4.0 MJ/kg
of water (= 916–1,100 Wh/kg) (Water Environment Federation, 2004). This means that using the flue
Waste heat recovery for sludge drying │ 7
gas, 2.5–3.1 tons of water can be evaporated per hour. Based on this the amount of faecal sludge that
can be dried using this waste heat source can be calculated (Table 1).
Table 1: Maximum hourly amount of faecal sludge that can be dried using flue gas from Sococim
cement plant depending on water content. Assumptions: heat transfer rate = 2.8 MW; Energy
demand to evaporate water = 920 Wh/kg
Solids content of
incoming FS
Amount of FS
processed per hour
Final product
(90% DM)
The flue gas energy would be sufficient to treat the total 6,000 m3 of faecal sludge produced in
greater Dakar area per day (DM 0.4%) if the sludge was dewatered to 80% solids content prior to drying
at Sococim. The resulting 26.7 tons of dried faecal sludge (DM 90%) would cover ~2% of Sococim’s daily
energy requirements.
The technology of reusing hot flue gas has been successfully implemented to dry sewage sludge in
Jiangyin, China (Ma et al., 2012). The systems uses two serial rotary driers, the flow rate of the flue gas
is about 150,000 m3/h and its temperature is 170°C. This way, about 100 tons of sewage sludge with a
water content of 78% are dried to below 30% daily. A wide range of drying technologies for sewage
sludge is described in literature with rotary dryers and belt dryers being the most prominent (Chen et
al., 2002; Gruter et al., 1990; Kasakura et al., 1993; Kragting, 2002; Lowe, 1995; Stasta et al., 2006;
Trenkwalder, 2010; Water Environment Federation, 2004).
Conclusions and Recommendations
Even though the clinker production line studied already reuses state-of-the-art waste heat recovery
technologies, the energy deriving from hot flue is not utilised and could also be recovered for the final
drying of dewatered faecal sludge before it is being used as fuel. In older production lines, waste heat
might also be recovered from the clinker cooler or the kiln exit. However, many cement production
lines already dispose of a heat-recovery and -distributing system using thermal oil.
8 │ Waste heat recovery for faecal sludge drying
Even when waste heat and space for drying infrastructure are available on spot, the economic and
ecologic feasibility of FS drying has to be evaluated carefully. Transportation costs are an important
consideration in FS management, as the associated water weight is very costly to transport. The
economic feasibility of waste heat recovery for FS drying will therefore also be determined by the
location and logistics of transporting FS from household level on-site sanitation systems to pre-drying
and drying facilities, in addition to the energy potential of the waste heat.
It might be a promising solution that the drying of the FS is done by a company which is formally
detached from the heat producing plant, with the FS drying company tapping the existing thermal oil
system. The coupling between the different actors is limited to a heat exchanger and the handling of
the faecal sludge can be done outside the cement factory’s ground. However, since the successful
operation of the drying facility is closely linked to the cement plant and possibly involves big
infrastructure investments, only long-term contracts will guarantee an effective and sustainable
operation of the heat recovery system (Stehlik, 2007).
The combination of the two waste sources, FS and waste heat, can create a financially attractive
alternative to fossil fuel in industries. This valorisation process will enhance the motivation for FS
collection and proper disposal and will help to alleviate financial pressure of households.
Waste heat recovery for sludge drying │ 9
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Waste heat recovery for sludge drying │ 11
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