applied thermal engineering - newcastle...

Applied Thermal Engineering 35 (2012), 71-80

1

APPLIED THERMAL ENGINEERING

EVALUATION OF A BIOMASS DRYING PROCESS USING WASTE HEAT FROM

PROCESS INDUSTRIES: A CASE STUDY

Hanning Li, Qun Chen, Xiaohui Zhang, Karen N. Finney*, Vida N. Sharifi and Jim Swithenbank

Sheffield University Waste Incineration Centre (SUWIC), Department of Chemical and Biological

Engineering, University of Sheffield, Sheffield, S1 3JD, UK

*Corresponding author. Tel: +44-114-2227563, Fax: +44-114-2227501, Email: [email protected]

ABSTRACT

Dry biomass provides considerable benefits for combustion, such as increased boiler efficiency,

lower flue gas emissions and improved boiler operations, compared to fuels with high moisture.

Drying is however an energy-intensive pre-treatment. Utilising low-grade, waste heat – of

which large amounts are available from many process industries – could significantly reduce

energy consumption. The integration of a drying process into a power station fuel system was

investigated; the results are presented here. Waste heat from a process industry plant (100 MW

output) was utilised as the heat source for drying. The biomass, pine chips at 60wt% moisture,

was dried and could then be provided as the input fuel for a subsequent 40 MW power plant.

The process consisted of a belt conveyor as the dryer and either flue gases or superheated

steam (generated from the hot cooling water) as the heat source. Flue gas usage would result in

lower capital costs (~€2.5m), but environmental issues, such as pollutant emissions must be

considered. Superheated steam can combine short drying times, good heat recovery and

environmental protection, but would entail greater capital costs (~€3m). A 3-4 year return on

the initial investment was calculated for both technologies, but profitability was sensitive to

fuel price.

Keywords: low-grade waste heat; biomass; belt dryer; process industry.

1. INTRODUCTION

Over the past few decades, the combination of a number of issues has meant that developing

sustainable and renewable energy sources and improving the efficiency of systems using

thermal energy have become increasingly important. The depletion of natural resources, due

to rapid fossil fuel consumption, and environmental issues, like climate change and acid rain,

are just some of these problems.


2

Biomass can be used as a form of renewable energy, for both heat and power generation

through thermo-chemical treatments, such as combustion or gasification. The term ‘biomass’

refers to both energy crops (plants grown to be used as a fuel) and wastes/by-products, such as

forestry residues, sawdust, municipal waste and a range of other agricultural and

commercial/industrial wastes, which can all be utilised for energy production in much the

same way as coal. Biomass is usually combusted on a grate (fixed-/moving-bed) or in a

fluidised-bed boiler. The moisture content of biomass is typically high, often varying between

50 and 63wt%, depending on the season, weather and type of biomass. Typical higher heating

values (the gross calorific value) of biomass fuels are around 15-22 MJ/kg; the energy content of

pine wood, the fuel investigated herein, is reported to be 18.3 MJ/kg [1]. The energy needed for

the evaporation of water from biomass fuels in a combustion boiler cannot be utilised in the

power generation process, since the temperature level of the latent heat is too low, e.g. <110°C at

atmospheric pressure. An initially low level of fuel moisture however could recover much of

the energy used during combustion for water evaporation. It would also be beneficial for

decreasing the dimensions of the boiler and reducing the emissions of unburned solids.

Biomass with lower moisture contents could also minimise or eliminate other combustion

control problems caused by fluctuations in the fuel moisture. Nonetheless, drying is an energy-

intensive process and can easily account for up to 15% of industrial energy utilisation [2].

Consequently, in many industrial drying processes, a large fraction of energy is wasted [3].

Energy management is therefore an essential part of any drying process and energy

conservation can significantly lower the overall operating costs [4].

This paper investigated a biomass drying process using low-grade waste heat as the heat source.

The heat source (100 MW) consisted of either waste flue gases at 250-450°C or hot water at 90°C,

both exiting an industrial process plant. After drying, the lower moisture content pine wood

chip fuel was then supplied to a 40 MW power generator. Two alternative drying systems, flue

gas drying and steam drying with a water pre-heating process, were compared to assess the

differences in energy consumption. A continuous belt dryer with a heat exchanger (if steam

drying was used) was considered as the dryer configuration in both systems. The dryer design

mainly consisted of the determination of various sizing and operational variables. The

evaluation of specific process variables for each design was carried out using economic criteria.

Both the capital and running costs were included in the evaluation and the profitability was

assessed by determining the net present value (NPV).

2. INDUSTRIAL DRYERS FOR BIOMASS DRYING

The dominant combustion technique for biofuels in the 1970s and 1980s was grate firing. This

type of boiler can handle fuels with varying levels of moisture, but ideally, one of 30-40% should

be used [5]. Since the 1970s, fluidised-bed boilers have generally replaced grate-firing as a

combustion technique [6]. Compared with grate firing, fluidised-bed boilers are a more suitable

method of combustion for moist biofuels. The use of fuels with a high moisture content

however decreases the overall energy efficiency of the power plant and reduces the boiler


3

capacity to such an extent that it becomes reasonable to install a dryer in combination with

the boiler. In the 1970s and 1980s, industrial dryers tended to be direct flue gas dryers [7]. Flue

gases were either taken directly from the boiler or generated in a separate flue gas burner. The

most common dryer types are rotary dryers, flash dryers, fluidised-bed dryers and belt dryers.

Typical performance data for these are presented in Table 1, along with other considerations in

Table 2.

Table 1: Typical range of design parameters and performance data for various dryers [7-11].

DRYER TYPE

Rotary Flash Belt

Evaporation Rate (t/h) 3-23 4.8-17 0.5-40

Drying Temperature (°C) 200-600 150-280 30-200

Capacity (t/h) 3-45 4.4-16 -

Feed Moisture at Inlet (%) 45-65 45-65 45-72

Moisture Discharge (%) 10-45 10-45 15-25

Feed Moisture at Outlet (%) - 12 25

Pressure Drop (kPa) 2.5-3.7 7.5 0.5

Optimal Particle Size (mm) 19-50 - -

Maximum Particle Size (mm) 25-125 0.5-50 -

Thermal Requirement (GJ/t-evaporation) 3.0-4.0 2.7-2.8 1.26-2.5

Table 2: Considerations when choosing a dryer [7-10].

DRYER TYPE

Rotary Flash Belt Fluidised-Bed

Requires Small Particles none yes none none

Heat Recovery difficult difficult easy easy

Fire Hazard high medium low medium

Air Emission medium high low medium

Steam Use yes none yes yes

2.1 Dryers

As outlined above, there are several different types of dryer that are available. Rotary dryers,

for instance, are the most common type for biomass applications and have low maintenance

costs. Their robust and simple construction combines flexibility with reliability, enabling this

type of dryer to operate under the most arduous conditions. They can handle a vast range of

materials and are less sensitive to particle size, as shown in Table 2. The material moisture

however is hard to control in rotary dryers because of the long lag time [13]. Though their

design does permit the use of the highest possible drying/operating temperature (Table 1) and

they can accept hot flue gases, this poses a considerable fire risk (Table 2); this also means that

they require a lot of space – the most of all dryer types. Most dryers have outlet temperatures

higher than 100°C to prevent the condensation of acids and resins.

Flash dryers are able to dry biomass rapidly, owing to the easy removal of free moisture. Wet

material is mixed with a stream of heated air (or other gas), which conveys it through a drying

duct where high heat and mass transfer rates rapidly dry the product. Flash dryers require


4

smaller biomass particle sizes to suspend and transport the biomass by the fluid stream alone

(Table 2). Gas temperatures tend to be slightly lower than for rotary dryers. Flash dryers are

much more compact than rotary dryers, but have higher installation costs [13]. They also have

high blower power costs in addition to the heat requirements for drying. Flash dryers have a

lower fire risk than rotary dryers due to the shorter retention times and lower operating

temperatures (Table 1 and 2). They can be used to dry most types of biomass.

In belt dryers, the feedstock is spread on a moving perforated conveyor to dry the material in a

continuous process. Fans blow the drying medium through the belt and biomass material.

Belt dryers are versatile and can handle a wide range of materials. They are now frequently

used in low temperature operations (as low as 30°C) to save energy, reduce air emissions and

minimise fire hazards (Tables 1 and 2).

Dryers can also be classified into fixed- and fluidised-bed designs according to the hot air

velocity flowing through the bed. In a fluidised-bed dryer, the hot air flows through the bed at

a velocity sufficient to support the weight of particles in a fluidised state. Bubbles form and

break within the bed and as a result, there is a high volume of gas in contact with the biomass

particles, leading to high heat and mass transfer rates, providing fast evaporation (Table 2).

2.2 Selection of the Dryer

Belt dryers are better suited to take advantage of low-grade and waste heat because they

operate at lower temperatures than rotary dryers (Table 1). Rotary dryers, for example,

typically require inlet temperatures of 260°C, but more optimally operate around 400°C. In

contrast, the inlet temperature of a belt dryer, such as a commercially-available vacuum dryer,

can be as low as 10°C above the ambient temperature, although more typically they operate at

higher temperatures, between 90°C and 200°C. Because of their lower temperature operation,

fire hazards and emissions to the air are lower for belt dryers (Table 2).

Using steam to dry moist fuels has recently attracted much interest for a number of reasons:

the high energy efficiency, low fire hazard and better environmental control. Steam drying is

mostly done using belt feeders or fluidised-beds (Table 2). The superheated steam in the dryer

provides the thermal driving force necessary to evaporate the moisture in the fuel. Generally,

the wet material is mixed with enough superheated steam to dry the material and end with

saturated steam. There are disadvantages to this process however; these include the

requirement of a small particle size to ensure good mixing with the steam, the high capital

costs incurred for a stainless steel pressure vessel and wastewater treatment issues.

In the design of a drying process, fire safety and emission issues should be considered. Fire

safety refers to precautions that are taken to prevent the likelihood of a fire. Fires start when

flammable and/or combustible materials with an adequate supply of oxygen are heated to their

ignition temperature. Biomass generally has an auto-ignition temperature of 260-280°C. In

most cases, air-drying poses a potentially high fire risk, because of the high amount of oxygen

in the air supply and the temperature used. Flue gas dryers can operate at higher temperatures


5

than air dryers, because flue gases contain lower amounts of oxygen [9]. Compared with air or

flue gas dryers, superheated steam drying processes have an even lower fire risk because no

oxygen is present. There are additional fire risks if the dried biomass is heated above its

ignition temperature though [9]. As the high temperatures used in these dryers are a fire

hazard, one effective method is to reduce the operating temperature; a temperature of less than

100°C could significantly minimise the likelihood of a fire.

The exhaust gas from a biomass dryer mostly contains sulphur dioxide, carbon dioxide, carbon

monoxide, hydrocarbons and suspended particulate matter as pollutants. SO2, CO2 and CO can

be removed by absorption processes before the exhaust is released to the atmosphere and

particles can be removed in part by cyclones and wet scrubbers. All types of woody material

contain volatile organics that may be emitted together with the water vapour. The emissions

from biomass drying are greatly affected by the drying temperature, especially when it exceeds

100°C [7]. Below 100°C, emissions are reported to be low [14]. Exhaust gases or unclean

condensates must be treated after the dryer if they contain high concentrations of the above

emissions; this increases the overall drying costs. The drying temperature when using flue

gases should be controlled so that it remains below 100°C to reduce the gas treatment costs,

even though the temperature of flue gas is normally much higher than this. In steam drying,

contaminated condensates include aerobic biological organisms, organic compounds, organic

carbon and non-condensable components, such as CO2, H2, CO, CH4 and C2-C4 compounds,

which require removal from the gas stream before release to the atmosphere for environmental

and health reasons [8].

In addition to the safety and environmental considerations, the selection of a dryer should also

take into account the water evaporation rate, biomass properties (such as particle size),

operating temperature and the availability of heat resources. Table 2 summarized some

considerations in choosing the dryers. The significant advantages of rotary dryers are that

they are less sensitive to material size, operate at high temperatures to reduce drying time,

have a wide range of evaporation rates and are easy to install. The main drawback is the much

greater fire risk, due to this high operating temperature. Gaseous emissions from this type of

dryer also need to be highly controlled and heat recovery is difficult. Flash dryers on the other

hand are more compact and easier to control, but require a small particle size; reducing the

size of the material may thus be beneficial for drying in this type of system, although this is an

energy-intensive operation, adding further to the overall processing costs. Flash dryers can

also be used in high capacity water removal applications. Belt dryers are used in low

temperature operations with reduced fire risks, fewer gaseous emission and low energy

consumption. The advantages of belt dryers over the other types for biomass drying hence

mean that the feasibility of their application was assessed herein.

2.3 Drying Rate

In the operation of a belt drying process, air or steam that is used as the drying media flows

through the solid bed and comes into contact with the surface of the material (fuel). This

convective drying process removes water from the surface of the material whilst increasing the


6

temperature of the fuel, because the temperature of the drying media stream is higher than

that of the fuel. During the convective drying process, two distinct phases can be distinguished

in the material [15]. During the first stage – the constant drying rate period – the surface water

on the material is removed. In the second phase – the falling drying rate period – internal

diffusion of the water to the surface of the material takes place. These physical phenomena

have been described by various models [15-17]. In the first kind of model, mass and energy

balances are formed to describe the convective mass and heat transfer between the solid

surface (fuel) and the gas stream (drying media). In these models, partial differential equations

have a good physical basis to predict the drying process of a material with appropriate drying

models, given the drying stream conditions and the mass and characteristics of the material.

In general, there are three equations needed to predict the drying properties of wood: those for

the stream temperature, wood temperature and drying rate. In the second type of model, the

drying rate is described by the characteristics of the material, such as porosity, hardness, pore

size and particle size, among others. The drying rate is generally determined by experimental

observations and then developed into an empirical relation. The drying rate curve can be also

used for estimating the residence time of materials in the dryer.

Various studies have investigated fuel drying, many of which have focussed on wood fuels.

Sheikholeslami and Watkinson [18], for example, explored the water evaporation rate from

wood-residue fuels, mainly bark, comparing air and superheated steam drying. The maximum

drying rate was obtained after a short drying time and then the drying rate rapidly decreased.

Comparing the effect of temperature on both the superheated steam-drying and air-drying

techniques, the maximum drying rate was much higher with the superheated steam than with

the relatively dry air at temperatures above approximately 180°C, while the relationship was

reversed below this point. The maximum drying rate represents the initial drying rate, which

identifies the optimal operating conditions. The results indicated that in view of the drying

rate, air-drying is the preferred option to accelerate the drying process at temperatures below

180°C, whilst steam drying will significantly improve drying rates at temperatures above 180°C.

Later, Fyhr and Rasmuson [16] investigated the drying rates of pine and spruce woods at

various particle sizes and operating temperatures in a superheated steam system. They found

that drying pinewood takes less time than spruce wood, mainly due to the internal structure of

the pine being more permeable than the spruce. They also assessed the effects of solid size (L,

the longitudinal particle length, in m, and T, the thickness, also in m) on the total drying

residence time (TDT), which can be estimated by:

0

0

0

L

LTDTTDT 1

1

1T

T= (1)

Gigler, et al. [15] carried out drying experiments for willows chips and simulated drying for the

same chips using airflow. At the beginning of the drying process, convective heat transfer

dominates, leading to a rapid drop in the fuel moisture content. Following this, the diffusion of

water inside the solid becomes significant, slowing down the drying rate. An increased chip

size lengthens the drying time. Most recently, Holmberg [7] studied the drying rates of pine


7

wood bark in a bed of variable height using air as the drying medium. It was found that an

increased drying temperature significantly reduced the drying time and hence accelerated the

drying process. Furthermore, it was concluded that the bed must be deep enough so that the

drying air at the exit has reached its saturation point. Increasing the bed height still decreased

the dryer size but the influence on dimensions was not as significant as for thin beds. Bed

heights between 0.2m and 0.8m are generally selected for conveyor dryers.

3. DESCRIPTION OF DRYING SYSTEM: A CASE STUDY

The UK faces the combined challenges of maintaining secure energy supplies and reducing

carbon dioxide emissions to address climate change. Biomass can provide an alternative

source of energy, replacing coal, to alleviate, at least in part the above issues. Accordingly, a 40

MW power station has been proposed for Sheffield, UK, using waste wood as the fuel source.

Fresh wood chips can have a considerable moisture content, often up to 70%, which would

significantly reduce the thermal efficiency of the combustor if they were used directly.

Reducing the moisture content through drying was thus required to improve efficiency.

As stated above, biomass drying is an energy-intensive process, so utilising waste energy can

make this more efficient and reduce the overall energy demand. In a survey of process

industries near Sheffield, it was found that the waste heat contained in flue gases, with a

temperature of 250-450°C, could be recovered and used as an energy source with the greatest

potential for biomass drying. Around 100 MW of energy can be provided in the form of waste

heat: 60% available thermally as hot water at 90°C, and the rest as hot flue gas. Figure 1 shows a

schematic diagram of the biomass drying process using waste heat from a process industry

plant. The mass flowrate of the 90°C hot water exiting the plant has been estimated to be 737

t/hr. The mass and volumetric flowrates at the flue gas exit are shown in Table 3.

Figure 1: Schematic of the biomass drying process integrated into the power stations.

40% flue gas at 250-450°C

100 MW combustor

fresh biomass

dryer dried biomass

40 MW combustor

60% hot water at 90°C

heat source


8

Table 3: Flowrates of the flue gas exiting the process industry plant at various temperatures.

TEMPERATURE (°C) FLOWRATE

250 300 350 400 450

Mass (kg/s) 179.71 146.24 123.12 106.19 93.26

Volumetric (Nm3/s) 139.29 113.35 95.43 82.31 72.29

Volumetric (× 105 m

3/h) 9.61 8.56 7.84 7.30 6.89

To estimate the usage of the waste heat required for biomass drying, two process

configurations were proposed and compared. The first was the direct use of the flue gas as the

heat source, without the need for a heat exchanger. The temperature of the inlet gas flow in

this first drying option was 250-450°C. Figure 2 shows a flow diagram for this adiabatic drying

process. The second configuration utilises the flue gas to raise the hot water to superheated

steam at a desired temperature via a heat exchanger. The generated steam can then be used as

the energy source for drying. Figure 3 outlines this drying process and the associated pre-

heating. The inlet superheated steam in the second configuration was around 150-180°C at 1-2

bars. The drying temperature is usually 20°C lower than the temperature of the heat source.

These waste thermal energy sources were then evaluated for supplying heat to dry the biomass.

The capital and operation costs, as well as the profitability of using a conveyor-belt dryer were

subsequently estimated to provide information for the industrial design and construction of

this drying process.

Figure 2: Flow diagram of the adiabatic drying process using waste flue gases as the heat source.

Figure 3: Flow diagram of the adiabatic drying process using superheated pressurised steam generated from the hot cooling water as the heat source, heated by the waste flue gases in a

pre-heater.

Direct use of the flue gases for drying can mitigate the need for a heat exchanger, as shown.

The problem with direct drying though is commonly believed to be contamination of the

biomass, but this should not be an issue, since the biomass is to be combusted. The use of the

flue gas at a high temperature (250-450°C) could increase the drying rate, meaning that a dryer

with smaller dimensions could be used. These higher drying temperatures however can cause

problems, such as increasing both the amount of gaseous emissions and the fire risk, as

DRYER flue gas feed

wood feedstock

wetted flue gas

dried wood

recycled steam

DRYER

steam

wood

saturated steam

dried wood

PRE-HEATER 90°C water

flue gas feed


9

identified above [7,9,14]. Thus the design of the dryer must take into consideration that the

temperature of flue gas at the outlet of the dryer should be less than 100°C.

The use of steam as the drying medium, however, represents a more valuable energy source,

because by allowing the steam to expand in a turbine, it would be possible to also recover

mechanical work (electricity) from the steam [8]. Furthermore, the construction of the

associated heat exchanger for this dryer arrangement would increase the capital costs. The

flue gas or steam demand for drying depends on the type of dryer and many drying parameters.

The waste flue gas from the industrial plant could provide sufficient heat for two alternative

drying approaches.

Industrial conveyor-belt dryers are the most popular type for removing moisture from

agricultural products. As discussed in the review of industrial dryers above (Section 2), belt

dryers are able to use low-temperature heat sources to achieve biomass drying. Moreover, the

utilisation of a belt dryer will reduce the fire risk and minimise emissions, compared to other

dryers if the exit temperature of the flue gas is designed to be below 100°C in the case of direct

drying. The interior of this type of dryer is illustrated in Figure 4.

Figure 4: Side view of a continuous cross-flow dryer.

4. ESTIMATION OF FLUE GAS USAGE FOR BIOMASS DRYING

The mass flowrates of flue gas in the two process configurations were determined according to

the heat and mass balances for the heating medium and the biomass streams in a steady-state

adiabatic process. Firstly, the dryer capacity was estimated and then the flue gas usage and

superheated steam requirements were calculated for the different drying configurations.

4.1 Capacity of the Dryer

The solid biomass – white pine wood chips – had an initial moisture content of around 50-

60wt%, as shown in Table 4. The dried fuel was to be the input fuel for a 40 MW power plant.

The heating value of this fuel was 16.66 MJ/kg on a dry basis. The mass flowrate of the dry

biomass was calculated from the power input requirement and biomass heating value. The

evaporation rates of water from the biomass were then evaluated (Table 4).

OUT

wood at a lower moisture content and specific

temperature flue gas in

flue gas out IN

wood at a high moisture content and specific

temperature


10

Table 4: Evaporation rates of water from the solid biomass.

FUEL MOISTURE CONTENT (wt%) Initial, Final

EVAPORATION RATE kg/s t/h

60, 10 3.3339 12.0019

60, 20 3.0005 10.8017

60, 30 2.5718 9.2586

50, 10 2.1337 7.6812

50, 20 1.8003 6.4810

50, 30 1.3716 4.9379

4.2 Estimation of the Flue Gas Requirement

The first drying option involved the direct use of the flue gas. The amount of flue gas required

was estimated based on the heating rate of the flue gas that can evaporate water from the

biomass. It is common practice to assume that within the interior of a dryer, the drying stream

follows an adiabatic process. The enthalpy of the flue gas at the entrance of the dryer (Hf,in, in

kJ/kg-dry flue gas) was equal to that at the outlet of the dryer (Hf,out):

outf,inf, HH = (2)

The enthalpy of the flue gas with a specific water content could thus be estimated using the

humidity data for air containing water:

flatentffwater,pair,pf HumHT)HumCC(H ×+×+= (3)

where Cp,air is the specific heat of the air, Cp,water is the specific heat of the water vapour, Humf is

the humidity of the flue gas, Tf is the flue gas temperature and Hlatent is the latent heat of water.

The humidity of the flue gas can be calculated in terms of the saturation pressure:

sat

sat

air,r

water,r

fPP

P

M

MHum

ϕ−

ϕ×= (4)

where Mr is the molecular weight or mass (in this case of the water and the air), φ is the relative

moisture, P is the pressure and Psat is the saturated pressure (both in mmHg). This saturation

pressure can be estimated with the Antoine equation:

TC

BA)(Plog sat10

+−= (5)

where the parameters A, B, and C are constants, selected according to temperature, either

above or below 100°C.

During the drying process, a known amount of water was removed from the solid biomass over

a certain period of time – Wevap, the water evaporation rate (kg-water/s). A corresponding

amount of flue gas must therefore have removed the water vapour from the dryer at the

specified humidity. The flowrate of the flue gas balanced the mass of water evaporated from

the material in the dryer. The mass flowrate of the flue gas (Gf in kg/s) was then determined


11

according to the water removal rate at a given humidity change (the difference in humidity

between the inlet and outlet, Humf,in - Humf,out) in the flue gas:

out,fin,f

evap

fHumHum

WG

−= (6)

The mass flowrate of the flue gas determined from Equation 6 was then used to evaluate the

availability of the supplied flue gas. Figure 5 shows the volumetric flowrate of the flue gas at

varying flue gas temperatures for different final fuel moisture contents. The maximum

flowrate to obtain the required amount of flue gas was 2.3×105 m3/hr, which was sufficient to

dry the biomass. Figure 5 also demonstrates that a higher flue gas temperature could reduce

the loading of flue gas in the dryer.

Figure 5: Flue gas flowrates at varying flue gas temperatures for drying pine wood chips, where the initial moisture content of the fuel was 60wt%.

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

150 250 350 450 550

Flue gas temperature (oC)

Flu

e gas

volu

me

flow

rate

(x105

M3/h

)

20

Final MC

(wt%-wet)

30

10

4.3 Estimation of the Superheated Steam Requirement

Superheated steam was the alternative option for drying the biomass. In the absence of other

gas species in the flue gas, water vapour is a major component in the heating medium. The

mass flowrate of the steam in the dryer was estimated based on an adiabatic process, as for the

flue gas case above, limited by saturated steam at a given temperature. In the steam drying

process, the heating source was steam that can be partially generated by using the hot water at

90°C, but an integrated pre-heating process using the high temperature flue gas can be used as

an additional source of heat, as shown in Figure 3. Thus, the flowrate of the flue gas also

needed to be evaluated in the thermal balance to determine the energy required to raise the

90°C hot water to the desired superheated steam temperature of 140-180°C.

Figure 6 shows the variation in the mass flowrate of steam with steam temperature at different

final moisture contents. As expected, an increased steam temperature results in a reduction in

the flowrate of the steam required. For the same final fuel moisture content, a fuel with an

initially high moisture content required a higher flowrate of steam, and thus, for the same

initial moisture, a fuel with a higher final moisture content required a lower steam flowrate.

Flue Gas Temperature (°C)

10 wt%

20 wt%

30 wt%

Flue Gas Volumetric Flowrate

(105 m

3/h

r)

final moisture content


12

The steam recycle ratio (R) had no effect on the required flow rates since the recycled steam

was mixed with the generated steam before entering the dryer. However, varying the recycle

ratio significantly affected the operation of the pre-heater, because an increased recycle ratio

reduced the generation of steam, consequently lowering the requirement of flue-gas usage, as

shown in Figure 7. It is interesting to note that at a high recycle ratio, i.e. R=0.75 (75%), flue gas

usage is negligibly affected by the flue gas temperature for steam generation in the pre-heater

and steam temperature in the dryer. Figures 6 and 7 also demonstrate that the maximum

flowrate of flue gas was about 2.4 × 105 m3/hr and the maximum flowrate of the 90°C hot water

was 12 t/hr. The maximum amount of waste low-grade energy in terms of the available flue

gases (up to 9.6 x 105 m3/hr) and 90°C hot water (737 t/hr) that can be supplied would be

sufficient to generate enough steam at a temperature of 140-180°C.

Figure 6: Steam flowrates at different steam temperatures for drying wood, at an initial moisture content of 60wt% and a flue gas temperature of 250°C.

2.00

4.00

6.00

8.00

10.00

12.00

14.00

120 140 160 180 200

Steam temperature (oC)

Ste

am

flo

w r

ate

(t/h

)

Final MC

(wt%-wet)

10

20

30

Figure 7: Flue gas flowrates required for generating steam at various flue gas temperatures and for different steam recycle ratios, where the steam temperature was 140°C, the initial

fuel moisture content was 60wt% and the final moisture content was 10wt%.

0.00

0.50

1.00

1.50

2.00

2.50

3.00

200 250 300 350 400 450 500

o

G(1

05xM

3/h

)

R=0

R=0.5

R=0.75

final moisture content

10 wt% 20 wt% 30 wt%

Steam Temperature (°C)

Steam Flowrate (t/hr)

Flue Gas Temperature (°C)

Flue Gas Volumetric Flowrate

(105 m

3/h

r)

R=50%

R=75%

steam recycle ratio

R=0%


13

5. THE COST OF DRYING

The overall costs of biomass drying consist of both the capital costs (Costcapital) and the running

costs (Costrun). The capital cost is generally considered to primarily be composed of the

equipment costs (Costeq):

∑= eqcapital CostGCost (7)

where G is the Lang factor, determined to be 1.6, which included 0.1 for electricity, 0.1 for

instrumentation, 0.05 for lagging, 0.15 for civil work and 0.2 for installation. Equipment costs

are usually correlated with the capacity factor using the following relationship:

bkYCosteq = (8)

where k is the proportionality factor, Y is the capacity parameter and exponent b is typically

within the range of 0.4-0.8, as demonstrated below [19]. In drying systems, the conveyors and

heat exchangers are the main pieces of equipment. While the capacity factor of each of these is

different, it is directly or indirectly dependent on the mass flow of air or steam. Equations 9

through 11 outline the equipment cost functions of individual items:

belt dryer: Costeq = 2700Y Y is cross-sectional area (9)

heat exchanger: Costeq = 660Y0.7 Y is heat transfer area (10)

cover: Costeq = 1200Y0.5 Y is cover area (11)

In calculating the cost of the belt dryer, the capacity parameter is affected by the belt cross-

sectional area. This can be determined based on the fuel mass flowrate and the residence time

of the wood. The mass flowrate (Mwood) was derived from the data in the previous section. The

residence time (τwood) was considered to be the drying time of the pinewood, which was based

on published date from both Fyhr and Rasmuson [16] and Holmberg and Ahtila [20]. As the

total amount of the wet fuel on the belt was known, calculated by:

Mwood x (1+MC) x τwood) (12)

where MC is the initial fuel moisture content. The required belt area can be estimated based on

the unit area loading (Wload) of the wet fuel. Kiranoudis and Markatos [21] recommended a

maximum unit area loading of 50 kg/m2 on a wet basis. A unit area loading of 30 kg/m2 was

used here and thus the effective area of drying (Aeff) was estimated by:

load

woodwood

effW

MC)(1 MA

τ×+×= (13)

The equipment cost of the conveyor was then calculated according to Equation 13 and the cost

function outlined in Equation 9. The cost of the cover was evaluated according to the cost

function in Equation 11, based on the area that covers the conveyor belt. The length and width

of the cover were slightly larger than the belt. The height of the cover was 6 m, which is


14

commonly used in industrial belt dryer applications. For the heat exchanger, the heat transfer

area (Aheat ex) was determined by:

)TT(h

QA

watf

heat ex−×

= (14)

where Q is the thermal flow rate, h is the heat transfer coefficient and Tf and Twat are the

temperature of the flue gas and the water respectively. The heat exchanger was used to convert

the hot water at 90°C into steam at temperatures of 140-180°C. During this process, the heat

transfer rate consists of three stages: (i) the water temperature rises from 90°C to 100°C, (ii) the

water evaporates to steam at 100°C, and (iii) the steam temperature increases to the desired

temperature. Tf is the average value of the inlet and outlet flue gas temperatures for the heat

exchanger. Because evaporation of liquid water to steam at 100°C is energy intensive, Twat was

set to 100°C. The capital cost of the heat exchanger was then evaluated according to the cost

function in Equation 10.

Figure 8 shows the variation in capital costs with the final fuel moisture content at different

flue gas temperatures. As expected, leaving the material at a higher final moisture level and/or

using a higher operating temperature can reduce the capital costs. As the operation of the

steam dryer is likely to cause corrosion problems, stainless steel can be partially used for

equipment construction to minimise this issue. In the heat exchanger, for example, the tubes

and dryer are constructed from stainless steel and the shell is constructed from carbon steel.

The costs derived from Equations 9 and 10 are based on equipment constructed from carbon

steel; for stainless steel, corrections factors of 2.2 and 2.9 need to be applied here as multipliers

for the heat exchanger and the dryer respectively [19]. Figure 9 shows the variation in capital

costs with different final fuel moisture contents for various steam conditions and equipment

materials. The capital costs will be significantly increased if stainless steel is used.

Figure 8: Variation in the capital costs with final fuel moisture at an initial moisture content of 1.5 kg-water/kg-fuel for different flue gas temperatures.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 0.2 0.4 0.6

Ca

pit

al

cost

(M

illio

n €

) 90 °C

110 °C

Capital Cost (millions €)

Final Fuel Moisture Content (kg-water/kg-fuel)

90°C

110°C

flue gas temperature


15

Figure 9: Variation in the capital costs with final fuel moisture at an initial moisture content of 1.5 kg-water/kg-fuel under different steam conditions and equipment materials.

0

1

2

3

4

5

6

7

8

9

10

0 0.2 0.4 0.6

Ca

pit

al

cost

(M

illi

on

€)

140 °C C-steel

160 °C C-steel

140 °C SS316

160 °C SS316

As stated above, the overall costs of drying consist of both the capital and running costs.

Running costs encompass all the costs associated with the operation of the dryer. The most

significant are for the use of heat and electricity, as well as the maintenance costs, which are

dependent on the annual operating time of the dryer and the price of energy. Maintenance

costs are usually estimated as a percentage of direct capital costs; typically, values range from

2% to 11%, averaging around 5-6% [19]. Personnel costs and insurance are also often included in

the running costs.

6. PROFITABILITY

Based on the cumulative cash flow, the profitability was evaluated in terms of payback time,

which is generally the main concern for investors. It is sometimes taken as the time from the

commencement of the project to the recovery of the initial capital investment. When

measuring profitability, the net present value (NPV) is used, which is a measure of the net cash

benefit generated by a project and is utilized herein to evaluate the profitability of the designed

drying processes. The NPV was calculated by:

capital

kt

0tt

maint Cost)1(

CostC NPV −

+

−= ∑

=

=i

(15)

where t is an individual/specific year, k is the total number of years, Ct is the cash benefit in t

years, Costmain is maintenance costs and i is the interest rate. The maintenance costs are

generally around 5% of the capital costs. Expressions for calculating Costmain and Ct are as

follows:

Capital Cost (millions €)


140°C carbon steel

160°C carbon steel

140°C stainless 316

140°C stainless 316


16

Costmain = 0.05 Costcapital (16)

Ct = (Csave-Q•Cf) • τop (17)

where Csave is the saved fuel per MWh, Cf is the price of energy stored in the flue gas (€0.5/GWh

here) and τop is the total number of operating hours in year ‘t’ (in this case, 8400 hours). Since

the heating rate for the water evaporation is Q (kJ/s), the total flue gas costs in this case would

be Q•Cf.

When the water content in the biomass is reduced, i.e. from an initial ratio of 1.5 down to 0.1 kg-

water/kg-fuel, the calorific value of the biomass will increase. This increased energy content

will be beneficial in saving energy during the operation of the boiler to evaporate the same

amount of water as removed from the fuel during drying. The saved fuel and thus the saved

energy in the boiler (Csave) can be converted into a positive cash flow, as follows:

fuellatentevapsave CHWC ××= (18)

where Cfuel is the price of fuel. Wevap x Hlatent represents the total energy required to evaporate

the desired amount of water in one hour, where Wevap can be found in Table 4. Cfuel generally

depends on the type of fuel, time and other parameters but is considered here to be the same

price as the biomass fuel used in the drying-boiler integrated process. The fuel price – how

much the dried pine wood biomass can be sold for – is generally in the range of €6-20/MWh;

here, €14/MWh was used for the calculations.

Figure 10 shows the variation in the NPV when the dryer is operating at a temperature of 90°C,

using flue gas as the heat source – the first configuration described in Section 3. The finial

moisture levels for the two cases considered here were 0.1 and 0.3 kg-water/kg-fuel. A return on

investment should be achieved after 3 years of operation for the higher final moisture content

and about 4 years for the lower moisture level. Figure 11 compares the 10-year NPV at different

final fuel moisture levels. At an operating temperature of 110°C, a profit of €3.6m can be

achieved after 10 years with the fuel moisture as low as 0.1 kg-water/kg-fuel. Increased fuel

moisture contents lower the profit. At an operating temperature of 90°C however, the most

profitable value is found a fuel moisture content of 0.3 kg-water/kg-fuel.

Figure 12 shows the NPV for biomass drying using superheated steam – the second

configuration described in Section 3 – at an operating temperature of 150°C; different steam

recycle ratios are compared. As shown, 3-4 years of operation is expected to achieve a return

on investment, as with the first configuration. Figure 13 shows the NPV after 10 years for

different final fuel moisture contents at an operating temperature of 150°C; various steam

recycle ratios were assessed. In general, the NPV decreases slowly as the final fuel moisture

increases until it reaches levels of around 0.25 kg-water/kg-fuel, after which the profitability

rapidly declines with increased final moisture. This indicates that drier biomass would yield a

higher profit.


17

Figure 10: Cumulative cash flow for biomass drying using flue gas at an operating temperature of 90°C and drying biomass from an initial moisture of 1.5 kg-water/kg-fuel to

0.1 and 0.3 kg-water/kg-fuel.

Figure 11: Variation in the net present value after 10 years with final moisture (initial moisture, 1.5 kg-water/kg-fuel).

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0 0.2 0.4 0.6

NP

V (

Mil

lio

n €

)

90 °C

110 °C

The results calculated and plotted in Figures 10 to 13 are all based on a fuel price of €14/MWh.

As expected, profitability is very sensitive to the selling price of the fuel. Figure 14 outlines the

effect of fuel price on the NPV after 10 years of operation. Selling the biomass fuel at a higher

price obviously results in better profitability. Figure 14 also demonstrates that the fuel price

needs to be greater than €8/MWh in order to see a return on investment after 10 years.

Furthermore, the results indicate that drying fuels with a high moisture content will be

beneficial.

Net Present Value (millions €)

Duration of Operation (yrs)

90°C, 0.1 kg-water/kg-fuel

90°C, 0.3 kg-water/kg-fuel



90°C

100°C

drying temperature


18

Figure 12: Cumulative cash flow for biomass drying using superheated steam at an operating temperature of 150°C and drying biomass from an initial moisture content of 1.5

kg-water/kg-fuel to 0.1 kg-water/kg-fuel.

Figure 13: Variation in the net present value after 10 years with final moisture, where the initial moisture is 1.5 kg-water/kg-fuel and the steam temperature is 150°C.

2.0

2.5

3.0

3.5

4.0

0 0.1 0.2 0.3 0.4

NP

V (

Mil

lio

n €

)

R= 0%

R= 50%

R=75%

The sensitivity of the UK market to the Euro exchange rate is highlighted in Figure 15, which

demonstrates the effects of the exchange rate on the NPV of the system. The fuel price and

NPV both are calculated based on the current exchange rate and the error bars (+6.7% and

−7.9%) represent the fluctuations in the exchange rate over the last two years, which varied

between €1.06 and €1.22 to the pound. The results indicate that variation in the UK Sterling to

Euro exchange rate only significantly impacts the NPV when fuel prices are high.


Duration of Operation (yrs)

steam recycle ratio

0% 75%

Final Fuel Moisture Content (kg-water/kg-solid)


steam recycle ratio

R = 0%

R = 50%

R = 75%


19

Figure 14: Variation in the net present value with fuel price after 10 years, for initial and final moisture contents of 1.5 and 0.1 kg-water/kg-fuel and a steam temperature of 150°C).

-2

-1

0

1

2

3

4

5

6

7

0 2 4 6 8 10 12 14 16 18 20

NP

V (

Mil

lio

n €

)

R=0%

R=75%

Figure 15: Variation in the net present value with fuel price after 10 years, showing the sensitivity of the UK market to the Euro exchange rate over the past last two years (initial and final moisture contents of 1.5 and 0.1 kg-water/kg-solid, steam recycle ratio=0, and a

steam temperature of 150°C); the exchange rate is based on data from [22].

7. CONCLUSIONS

This paper has studied the integration of a drying process into a power generation plant using

two different forms of waste energy from the process industries. The potential thermal energy

sources for biomass drying were both low-grade heat – either in the form of the flue gas from

the process or hot cooling water that could be used to form superheated steam. The dried


Fuel Price (€/MWh)

steam recycle ratio

Fuel Price (£/MWh)

Net Present Value (millions £)

R = 0%

R = 75%


20

biomass could then be provided as the fuel input for a subsequent power station. A belt

conveyor was the chosen dryer. According to the result herein, sufficient heat is contained in

both the waste flue gases and the hot water exiting from the industrial process plant to be the

heat source for biomass drying – in this case, white pine wood chips. The moisture levels can

be reduced from 1.5 to 0.1-0.3 kg-water/kg-fuel, which is satisfactory for this to then be used as a

fuel for combustion in the latter energy generation process, at a higher efficiency.

By using flue gases as the heat source for drying, the capital costs would be in the region of €2.5

million. Although a higher flue gas temperature would reduce the capital costs, environmental

issues may then become a problem, such as increased emissions. Using superheated steam as

the drying medium however would mean that the capital costs would greater – about €3

million. To protect the equipment from corrosion, many components can be constructed from

stainless steel, though this will double the equipment costs. In the selection of either the flue

gas or superheated steam, the use of the flue gases would result in lower capital costs. Even

though superheated steam is a good option in terms of short drying times, good heat recovery

and environmental protection, the high capital costs associated with this dryer configuration

is a considerable issue, particularly when stainless steel is used for some of the equipment

components. Overall, for both the flue gas and steam drying configurations, 3-4 years of

operation is expected to give a return on the initial investment at a fuel price of €14/MWh.

However, profitability was found to be very sensitive to the biomass fuel-selling price. It was

calculated that this needs to be higher than €8/MWh to achieve a return on the investment

after 10 years of operation.

ACKNOWLEDGEMENTS

The authors would like to thank the UK Engineering and Physical Sciences Research Council

(EPSRC Thermal Management of Industrial Process Consortium) and our industrial partners

for their financial and technical support for this research programme.

NOMENCLATURE

A constant in Antoine equation [-] Aeff effective area of drying [m2] Aheat ex heat transfer area of heat exchanger [m2] b exponent [-] B constant in Antoine equation [-] C constant in Antoine equation [-] Cp specific heat [kJ/kg-K] Cf price of flue gas [€/MWh] Cfuel price of fuel [€/MWh] Csave price of saved fuel [€/MWh] Ct cash benefit [€] Costcapital capital cost [€]


21

Costeq equipment cost [€] Costmain maintenance costs [€] Costrun running cost [€] G Lang factor [-] Gf mass flow rate of flue gas [kg/s] Hf enthalpy of flue gas [kJ/kg] Hlatent latent heat of water [kJ/kg] Hum humidity [kg-water/kg-air] h heat transfer coefficient [W/m2K] i interest rate [%] k total number of years [yrs] k proportionality factor [-] L longitudinal particle length [m] Mr molecular weight/mass [-] Mwood dry mass flow of biomass [kg/s] P pressure [mmHg] Psat saturated pressure [mmHg] Q thermal flow rate [W] R steam recycle ratio [-] t time [s] or [yr] T temperature [K] or [°C] T thickness [m] Twat Temperature of water [K] Wevap evaporation rate of water [kg/s] Wload unit loading of wood on the belt [kg/m2] Y capacity parameter [-] Y area [m2]

Greek symbols:

φ relative humidity [-] τ drying time or operating time [s or hr/year] τop total operating hours in one year [hour] τwood residence time of wood in the dryer [s]

Subscripts:

air air f flue gas in inlet, initial out outlet, final vapour vapour water water

REFERENCES

[1] C. Ryu, Y.B. Yang, A. Khor, N.E. Yates, V.N. Sharifi, J. Swithenbank, Effect of fuel properties on biomass combustion: Part I. Experiments – fuel type, equivalence ratio and particle size, Fuel. 85 (2006) 1039-1046.

[2] K.J. Chua, A.S. Mujumdar, M.N.A. Hawlader, S.K. Chou, J.C. Ho, Batch drying of banana pieces – effect of stepwise change in drying air temperature on drying kinetics and product color, Food Res. Int. 34 (2001) 721-731.


22

[3] H. Ogura, T. Yamamoto, Y. Otsubo, H. Ishida, H. Kage, A.S. Mujumdar, A control strategy for chemical heat pump dryer, Dry. Technol. 23 (2005) 1189-1203.

[4] J.C. Ho, S.K. Chou, A.S. Mujumdar, M.N.A. Hawlader, K.J. Chua. An optimisation framework for drying of heat sensitive products, Appl. Therm. Eng. 21 (2001) 1779-1798.

[5] R. Wimmerstedt, Drying of peat and biofuels, in: A.S. Mujumdar (Ed.) Handbook of Industrial Drying, Marcel-Decker, New York, 1995, pp. 809-859.

[6] M. Huhtinen, A. Hotta, Combustion of bark, in: J. Gullichsen, C.-J. Fogelholm (Eds.), Chemical Pulping Book 6, Papermaking Science and Technology, Finland, 1999, pp. 205-305.

[7] H. Holmberg, Biofuel Drying as a Concept to Improve the Energy Efficiency of an Industrial CHP Plant, PhD Thesis, Helsinki University of Technology, Finland, 2007.

[8] D.M. Bruce, M.S. Sinclair, Thermal Drying of Wet Fuels: Opportunities and Technology, EPRI Report (TR-107109 4269-01), 1996.

[9] W.A. Amos, Report on Biomass Drying Technology, NREL Contract No. DE-AC36-83CH10093, National Renewable Energy Laboratory, 1998.

[10] L. Barré, M. Bilodeau, Drying residuals at low temperature with the Dry-Rex dryer, Pulp Pap.-Canada 100 (1999) 132-138.

[11] H.C. van Deventer, Industrial Superheated Steam Drying, TNO report R 2004/239, 2004.

[12] L. Fagernäs, J. Brammer, C. Wilén, M. Lauer, F. Verhoeff, Drying of biomass for second generation synfuel production, Biomass Bioenerg. 34 ( 2010) 1267-1277.

[13] R.W. Fredrikson, Utilisation of wood waste as fuel for rotary and flash tube wood dryer operation, Biomass Fuel Drying Conference Proceedings, University of Minnesota, Office of Special Programs, Wisconsin, 1984, pp. 1-16.

[14] J.-P. Spets, P. Ahtila, Reduction of organic emissions by using a multistage drying system for wood-based biomass, Dry. Technol. 22 (2004) 541-561.

[15] J.K. Gigler, W.K.P. van Loon, M.M. Vissers, G.P.A. Bot, Forced convective drying of willow chips, Biomass Bioenerg. 19 (2000) 259-270.

[16] C. Fyhr, A. Rasmuson, Some aspects of the modelling of wood chips drying in superheated steam, Int. J. Heat Mass Tran. 40 (1997) 2825-2842.

[17] Z. Tang, S. Cenkowski, W.E. Muir, Modelling the superheated-steam drying of a fixed bed of brewers’ spent grain, Biosystems Eng. 87 (2004) 67-77.

[18] R. Sheikholeslami, A.P. Watkinson, Rate of evaporation of water into superheated steam and humidified air, Int. J. Heat Mass Tran. 35 (1992) 1743-1751.

[19] D. Brennan, Process Industry Economics, Institution of Chemical Engineers, UK, 1998.

[20] H. Holmberg, P. Ahtila, Optimization of the bark drying process in combined heat and power production of pulp and paper mill, in: A.F. Odilio, T.M. Eikevik, I. Strommen, (Eds.), Proceedings of the 3rd Nordic Drying Conference, Karlstad, Sweden, 2005.

[21] C.T. Kiranoudis, N.C. Markatos, Pareto design of conveyor-belt dryers, J. Food Eng. 46 (2000) 145-155.

[22] ADVFN PLC., UK Pound Sterling vs Euro (GBPEUR) Exchange Rate Charts, (2011). Online, available: advfn.com/p.php?pid=qkchart&symbol=FX%5EGBPEUR (accessed 01/09/2011)

applied thermal engineering - newcastle...

Documents