waste heat recovery in a coffee roasting plant

Case study

Waste heat recovery in a coffee roasting plant

Michele De Monte *, Elio Padoano, Dario Pozzetto

Department of Energetics, Via A. Valerio 10, 34127 Trieste, Italy

Received 3 September 2002; accepted 6 February 2003

Abstract

The paper presents the possibility of introducing, in the event of substitution of an old plant, the recovery

of heat produced during the roasting process of coffee.During the analysis, thermo and fluid dynamic operating parameters of the present plant were defined

also with the support of an experimental measuring campaign. Energy recovery possibilities were, then,

evaluated and a possible plant solution was examined taking into consideration its economic feasibility.

The case study is also interesting because the methodology used for the analysis can be generally applied

to production plants, which have hot air exhaust emissions. Waste heat recovery, actually, is an important

topic not only for its economic benefits, but also for its environmental outcomes and resource saving.

� 2003 Elsevier Science Ltd. All rights reserved.

Keywords: Energy balance; Food industry; Waste heat recovery; Case study

1. Introduction

Coffee roasting process is characterized by a heating process, from ambient temperature to alevel greater than 200 �C, of raw coffee beans. The heating process is usually performed in aroasting drum by means of a hot air stream, which comes from a burner fed with natural gas(drum roasting). The most critical parameter during the process is the temperature, which affectsroasting degree and aroma. In drum roasting the process can take 15–20 min at 200 �C. Roastingmust not be ‘‘pushed’’ too hard to increase aroma since there is a risk of decreasing it given thevolatility of many of the aromatic components [1]. Another problem of drum roasting is due tothe carbonization of the roasted chaff which creates volatile products that deteriorate coffee

*Corresponding author. Tel.: +39-40-558-3259; fax: +39-40-558-3812.

E-mail address: [email protected] (M. De Monte).

1359-4311/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S1359-4311(03)00033-4

Applied Thermal Engineering 23 (2003) 1033–1044www.elsevier.com/locate/apthermeng

mail to: [email protected]

quality and originate oil and char deposits on the cylinder walls and on the beans. A possibletechnological solution is represented by spouted bed roasters which make it possible high tem-perature/short time roasting processes [2]. After such phase, a cooling process is performed to stopreactions and changes primed into the bean by the above phase.Exhaust gases exiting from roasting drums and cooling basins contain PM, N2, CO2, steam, O2

and a mixture of other 700 volatile compounds (VOC like ketones, aldehydes, pyrroles, furans,pyridines, etc., nitrogen and sulphur compounds). Such emissions can be strongly reduced, inaccordance with US and European legislation on air quality and on industrial plant emissions, bythe use of cyclones for PM and thermal oxidizers for volatile compounds. In particular as far asthe VOCs were concerned, the insertion of a post-combustion stage at about 500–600 �C for theroasting and cooling exhaust gases was foreseen as a possible solution [3]. Such option was thendischarged because the existing plant, that presented already a high level of exploitation, wasconsidered not able to satisfy the growing demand of company�s target market. Therefore, acomplete substitution of old units was considered as a better solution than a partial refurbishing.In the occasion of such substitution, the amount of thermal energy needed for the process andhigh values of temperatures measured at the stack suggested to evaluate the possibility of in-cluding a waste heat recovery system. The analysis and the proposed solutions are reported in thefollowing sections.

2. Roasting process and plant description

Roasting is the process which gives aroma and flavour to the coffee and the final result dependsgreatly from bean quality and from roasting degree. The process is characterized by a heatingprocess of raw coffee to temperatures higher than 200 �C, that is sufficiently fast to allow waterelimination from beans.In detail, the process is composed by the following phases [1]:

• A drying phase, that is an endothermic process which covers the first half of the whole proce-dure, to eliminate completely moisture.

• The roasting phase, that primes a set of pyrolysis reactions which cause the transformation offorerunner compounds to hundreds final compounds characterizing coffee aroma and flavour.Chemical reactions in the beans are also characterized by an high release of CO2 (5–12 l/kg ofcoffee [1]), produce a bean color changing (beans become brown) and cause a growing of beanvolumes (which increase of about 40–60% depending from the roasting level) and a related de-creasing of coffee specific weight. The primed process is exothermic in its first step, for temper-atures starting from 160 to 190 �C; then it becomes endothermic with the release of volatilecompounds and, at the end, it is another time exothermic in the temperature range between210 and 220 �C (very close to coffee fire point). Final roasting temperature influences not onlyquality and quantity of aroma compounds, but also the correct ratio between bitter and acidflavour.

• The cooling phase, that puts down rapidly temperature to ambient level using a cold air forcedflow. The speed of temperature decreasing is another important element influencing final coffeeflavour: many aroma compounds, actually, can leave the roasted bean if it is left at high tem-

1034 M. De Monte et al. / Applied Thermal Engineering 23 (2003) 1033–1044

perature. If the bean is rapidly cooled, instead, its inner pressure decreases quickly and its poresclose, imprisoning a greater amount of aroma compounds.

The roasting plant considered in our study is composed by four roasting drums, which arecontrolled by a central station. The roasting units are of two different types, with the main dif-ference related to production capability and thermal energy absorption. Before this system, coreof the whole production process, there is a set of systems useful to select coffee beans, to composethe correct mixture and to eliminate foreign materials. After this system, instead, structures forweighting coffee, eliminating metal bodies (metal detectors) and packaging it are present.From the central control station it is possible to control, for each machine, the following pa-

rameters:

• pressure and temperature of the prime burner for heat generation;• coffee temperature inside roasting drums, during the whole process, and into drum hopper;• temperature of exhaust gases exiting from drums;• temperature of coffee into cooling basins;• weight of coffee charged into drums or discharged from cooling basins.

The common elements composing each roasting machine of the plant (Fig. 1) are:

• a drum hopper;• a burner for heat generation;• a cylindrical roasting drum with horizontal axis of rotation;• a cooling basin with forced air flow;

Fig. 1. Schematic of presently installed roasting machines.

M. De Monte et al. / Applied Thermal Engineering 23 (2003) 1033–1044 1035

• some cyclones for dust separation;• a fire extinguishing system using water.

The considered machines perform a discontinuous process: their time cycle can be chosen in therange between 400 and 800 s, depending on the wanted roasting level. For this reason the con-sidered plant has more than one unit to guarantee a sufficient continuity to the following processphases.

3. Experimental measures

In order to define a system with lower emission values and with the possibility of energy re-covery, it was necessary, in particular, to examine the temperature courses and flow rates ofexhaust gases, exiting from the roasting and cooling stacks of the existing plant. Such informationwas important to define plant running conditions from a thermal point of view.So, temperature course and air flows were measured at the stacks for more than one working

cycle, both for the roasting and for the cooling phases. The first parameter, temperature, waseasily measured. The second one, mass flow rate (or volume flow rate), was, instead, indirectlyevaluated using a Pitot tube and Bernoulli�s theorem relations.

3.1. Experimental measures for the coffee cooling phase

Temperature course and air flow rate were measured, at first, for one of the cooling stacks,because its geometric parameters were available and it could offer a straight run of gases beforethe measure point.Before the beginning of such measures, in accordance with [4], a set of measure points into the

stack was identified and the speed profile, for each point, was defined to detect which of themcould be used to measure the mean speed of exhaust gases during a cycle.The results of such measure campaign are presented, for the cooling basin, as follows:

• Fig. 2 refers to temperatures; in particular it shows the temperature course at the exit of thecooling basin (Fig. 2(a)) and the temperature course at the stack (Fig. 2(b)) during a coolingcycle for coffee with a normal roasting level.

• Fig. 3 refers to volume flow rates; in particular it shows mean volume flow rate of exhaust gasesduring three consecutive cooling cycles (Fig. 3(a)) and normalized flow rate course of exhaustgases during a cycle, evaluated using simultaneous measures of temperature and flow rate (Fig.3(b)).

A run without roasted coffee was also carried out to quantify air temperature increase during itsflowing into the cooling system. Such value was equal to 10 �C, while ambient temperature duringmeasures was 1 �C.From Fig. 2(a) and (b) it is possible to see a fast temperature raise during the first cooling

phase, with a maximum at 50 s after the start and a drum discharging time of 30 s. After themaximum, the temperature decreases with a low gradient. At the end of the process, after 200 s,


the fan forcing the air flow stops. At this time, the basin temperature is lower than the stack�s one.This is due to the fact that the stack gives back the thermal energy stored during the previous timeinterval.Fig. 3(a) shows a maximum volume flow rate of about 2.6 m3/s (more than 9200 m3/h) equal to

about 2.2 Nm3/s (from Fig. 3(b)). Starting from Fig. 3(b), then, it is possible to assess the massflow rate of exhaust gases produced during each cooling cycle: such value, mean of a set of values,is equal to 446.9 kg/cycle (about 345.6 Nm3/cycle). In each cycle about 112 kg of coffee areprocessed, so the cooling process causes the emission of about 4 kg of exhaust gases for each kg ofcooled coffee.

Fig. 2. Cooling basin temperature courses: at the exit (a) and at the stack (b).

Fig. 3. Cooling basin volume flow rates: at operation conditions during tree cycles (a) and at normal conditions during

a single cycle (b).


Eventually, to define the energy balance for the cooling process, the course of thermal powerduring each cycle was evaluated (with respect to the 0 �C temperature) and it is presented, for asingle cycle and together with its mean value, in Fig. 4.Starting from the mean thermal power of exhaust gases, equal to about 210 kW, it is possible to

evaluate, using the mean flow during the same period, the mean equivalent temperature valueduring the same time. Such value is equal to about 66 �C.Using these data, it was also possible to carry out the energy balance of each cooling cycle (Fig.

5(a)). Such balance is done at the following conditions:

• coffee input at 220 �C and output at 30 �C;• coffee specific heat of 1674 kJ/kg �C [5];

Fig. 4. Thermal power during each cooling cycle.

Fig. 5. Energy balance for the cooling phase (a) and for the roasting phase (b) with respect to old roasting plant units.


• coffee mass of 111.5 kg/cycle;• air input at 14 �C.

At the end of such analysis, exhaust gases of the cooling process can be characterized, during aperiod of 200 s for each cooling cycle, by the following features:

• dust into the exiting exhaust gases;• a mean thermal power of about 210 kW (assessed with respect to 0 �C) for 200 s/cycle;• a maximum flow rate of about 2.6 m3/s (greater than 9200 m3/h �7000 Nm3/h) for 200 s/cycle;• a maximum temperature value of 105 �C;• a mean temperature value of 66 �C during the cycle;• range of exhaust gases temperature from 30 to 100 �C during the cycle;• a discontinuous emission of heat.

The low mean temperature value, the discontinuous emission of heat and the great range oftemperature during the process are the main reasons that do not make viable a recovery ofthermal energy produced in this process.

3.2. Experimental measures for the coffee roasting phase

A detailed analysis was also developed for the roasting process. Such analysis was devel-oped for both roasting machine types because of their different features. In particular, the twomore recent units are characterized by double production capacity with respect to the old ones,by a partial recirculation of the exhaust gases, by high temperatures at the stack and by alower emission of volatile compounds, burned at temperatures greater than 550 �C by the burnerbecause of the blow-by loop. The two older units, instead, are characterized by no exhaustgases blow-by, by high levels of volatile compounds emissions and by lower temperatures at thestack.Using all information collected during the measure campaign for the first type roasting machine

(with recirculation), it was possible to define the energy balance for each roasting process cycle(Fig. 5(b)).The analysis allowed defining also the characteristics of the exhaust gases exiting from the

roasting plant units. Such results are showed in Table 1 for the two types of roasting machinespresent in the plant.Such measures showed significant differences between the two types. In particular the first type

of machine has low emission values and high exhaust gases temperature at the stack, conditionthat can suggest a possible recovery of thermal energy.For this and for their production capacity, machines with features comparable to the first type

of roasting unit were considered to be suitable for substituting the presently used ones. Thesubstitution option was considered not only for the oldest units, but also for the more recent ones.Such fact mainly depended on a greater wear of these machines with respect to design conditions,due to the overloading of roasting drums to answer to product requests.


4. Foreseen features of the new coffee roasting units

In order to choose between different market available models, information concerning themwas requested to know in detail their running conditions. Design technical data were not con-sidered a sufficiently reliable source to guarantee the final coffee quality. Therefore, some test runs,done with different available models, were carried out and the one that guaranteed the requestedcoffee quality, the target emission reduction and an investment cost in line with the foreseenbudget had the schematic showed in Fig. 6.For such new unit, the energy balance was carried out for the cooling (Fig. 7(a)) and the

roasting phases (Fig. 7(b)), also with the use of available plant rating.

Table 1

Characteristics of exhaust gases exiting from the roasting plant units

Parameter First type roasting unit Second type roasting unit Coffee cooling system

Exhaust gases blow-by Yes No –

Dust into exhaust gases Yes Yes Yes

VOC emission Low High Present but not high

Mean thermal power About 454 kW

continuous

About 127 kW

continuous

About 210 kW for

200 s/cycle

Flow-rate at running

condition

About 6209 m3/h

continuous

About 3892 m3/h

continuous

>9200 m3/h

discontinuous

Maximum temperature

value

>600 �C during drum

discharge

About 150 �C About 105 �C

Mean temperature value >550 �C About 133 �C About 66 �C for 200 s/cycle

Exhaust gases range

temperature

540 fi 580 �C (range of

40 �C)115 fi 150 �C (range of

35 �C)30 fi 100 �C (range of

70 �C)Emission of heat Continuous Continuous Discontinuous

(for 200 s/cycle)

Fig. 6. Schematic of the new roasting machine.


Temperature and flow rate courses were then evaluated, for supposed running conditions, toallow the correct design of the recovery system (Table 2).Analyzing such data, a waste heat recovery from cooling exhaust gases was not judged prof-

itable (discontinuous flow rate and low medium temperature value). The thermal energy recoverywas instead taken into consideration for the roasting exhaust gases. The idea was to fix tem-perature of exhaust gases at the stack equal to 250 �C and to exploit the range between 500 and250 �C for factory needs.Such idea made available a thermal power of about 180 kW from each roasting machine, for a

total exploitable thermal power of 720 kW for the whole plant.

Fig. 7. Energy balance for the cooling phase (a) and for the roasting phase (b) with respect to new proposed roasting

units.

Table 2

Foreseen characteristics of exhaust gases exiting from the roasting plant units

Parameter New roasting unit New cooling system

Exhaust gases blow-by Yes –

Dust into exhaust gases Yes Yes

VOC emission Low Present but not high

Mean thermal power About 350 kW continuous About 433 kW for 240 s/cycle

Flow rate at running condition About 4,800 m3/h continuous About 18,000 m3/h discontinuous

Mean temperature value About 500 �C (supposed) About 79 �C for 240 s/cycle

Emission of heat Continuous (during the whole cycle) Discontinuous (for 240 s/cycle)


5. Technical feasibility and economic assessment of an air conditioning system for the factorybuildings

Thermal recovery from roasting exhaust gases was confirmed by the possibility of exploitingduring the whole year such energy for the air conditioning of the factory buildings. In effect, thecompany�s production cycles did not require an investigation of other solutions of heat exploi-tation, which could be carried out in other sectors of the food industry [6]. To guarantee accuraterunning conditions, greater efficiency and lower dimension for the heat pump, a regenerator usingoverheated water at 130 �C was chosen. Such regenerator had also to guarantee the recoverywithout interfering with the exhaust gases flow exiting from the stack. It was not possible, ac-tually, to forecast an intake fan into the stack to not risk a change of the pressure course in theroasting drum that would influence final coffee quality.The air conditioning plant, whose overheated section is presented in the schematic of Fig. 8, is

composed by a distribution line which transfers overheated water to winter (heat exchanger) andsummer (heat pump) users. Such recovery system has the advantage of allowing a significant heatrecovery, but has also some disadvantages related to the need of an heat sink to dissipate the

Fig. 8. Schematic of the overheated section of the air conditioning plant.


excess of heat and to the roasting process stop for regenerator maintenance or fail, solvable with aregenerator by-pass.The economic assessment of the proposed alternative was carried out using common indexes

like Pay-back period and IRR.In order to assess the economic profitability of the project, capital costs, running costs and

avoided costs were considered. Such elements are showed in Table 3. An economic life of 15 yearsand a discount rate of 10% were also assumed.The following economic profitability indexes were obtained:

• Pay-back 13 years;• IRR 5.74%.

The results showed that the energy recovery was not economic for both hot and cold pro-duction. To increase the profitability of the initiative, it was contemplated the economic effect ofan appeal to possible public financing, for plant solutions that perform a reduction of emissionsand environmental impacts [7,8]. Such public financing had to cover at least 25% of the plantcapital cost in order to guarantee the following performances for the investment:

• Pay-back 9.8 years;• IRR 10.58%.

Due to the low economic performance of these two alternatives, the possibility of recoveringthermal energy without using the heat pump (namely, to recover thermal energy only for winteruses) was eventually considered. The meaningful reduction of investment cost available with suchnew alternative had immediate effects on capital costs. In fact the initiative became interestingeven without public financing, as showed by the following results:

Table 3

Costs considered for the evaluation of the economic profitability of the plant

Cost type Cost element Unit Amount

With heat pump Without heat pump

Capital costs Overheated distribution pipeline € 76,900 76,900

Overheated water-hot water

exchanger

€ 7800 7800

Heat pump € 130,200 0

Design cost € 6200 6200

Total € 221,100 90,900

Annual running costs Maintenance costs (2% of

investment)

€/year 4422 1818

Electric energy €/year 5784 2410

Nitrogen gas cylinder rent €/year 52 0

Total €/year 10,258 4228

Annual avoided costs Thermal energy €/year 16,165 16,165

Electric energy €/year 11,104 0

Total €/year 27,269 16,165


• Pay-back 7.6 years;• IRR 15.65%.

6. Conclusions

The work presented in this paper had the aim of evaluating the possibility of a waste heatrecovery in a food industry. A major issue of the analyzed company is strict process conditions inorder to guarantee a very high quality of final product.The energy balance of the roasting process confirmed the feasibility of heat recuperation from a

high temperature source. The factory analysis, instead, showed that the only use for the recoveredheat was for the air conditioning of buildings because of scarce energy needs for other plantprocesses.A first possible plant solution for waste heat recovery was also examined both from a technical

and from an economic point of view. Such analysis showed the weight that plant capital costs andpossible local public financing for energy saving can play on total investment profitability.Taking into consideration all these issues, a recovery plant designed only for a seasonable

exploitation of waste heat (e.g. for winter heating) could be a very interesting and profitablesolution because of a lower investment cost.A future development of this work could be the possibility of using a small size adsorption

refrigerator in the plant. Such systems, which can use a low level running temperature, could allowa greater heat recovery (higher performances), a higher availability, zero noise and vibrationvalues, the use of materials not dangerous for the environment and especially the possibility ofproducing both heat and cold. Such system could simplify the plant configuration and reducemaintenance (absence of moving parts) [9]. The consequent reduction of running costs, if ac-companied with a competitive initial cost, could make this investment alternative profitable.

References

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waste heat recovery in a coffee roasting plant

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