An investigation into heat recovery from the surface of a cyclone dust collector attached to a downdraft biomass gasifier

The use of waste heat recovered from a biomass gasifier was ex-amined by attaching a thermoelectric generator system TEG to the surface of a catalytic reactor used for cleaning the syngas

Research Paper

An investigation into heat recovery from the surface of a cyclone dust

collector attached to a downdraft biomass gasifier

aInstitute of Technology, University of Fort Hare, P/Bag X1314, Alice, 5700, South Africa

bPhysics Department, University of Fort Hare, P/Bag X1314, Alice, 5700, South Africa

H I G H L I G H T S

• At a temperature of 450 °C–500 °C, hot syngas is regarded as a good heat carrier

• A significant quantity of energy (665893.07 kcal) is lost via the surface of the cyclone

• The surface temperature 150 °C–220 °C was within the low waste heat recovery temperature

A R T I C L E I N F O

Article history:

Received 12 September 2015

Accepted 5 January 2016

Available online 15 January 2016

Keywords:

Waste heat

Downdraft gasifier

Producer gas

Cyclone dust collector

Temperature

A B S T R A C T The gas leaving the reactor of a downdraft biomass gasifier contains large quantities of heat energy; this

is due to the fact that the gas passes through a hot bed of charcoal before leaving the reactor This heat

is normally wasted in the gas scrubber/cooler that cools it from between 400 °C–500 °C to ambient tem-perature (around 25 °C) The waste heat stream under consideration is the raw syngas that emanates from a gasification process in a downdraft gasifier situated at Melani Village, Eastern Cape This loss of heat is undesirable as it impacts on the thermal efficiency of the system This study investigates the fea-sibility of heat recovery from the surface of the cyclone dust collector prior to entering the gas scrubber

It was shown that there was a downward decrease in temperature along the length of the cyclone It is found that the total quantity of heat contained in the gas was 665893.07 kcal, which could indicate the viability of recovering heat from the cyclone

© 2016 Elsevier Ltd All rights reserved

1 Introduction

Biomass gasification is a thermochemical process that involves

the production of gaseous fuel from carbonaceous feedstock A wide

range of carbonaceous fuels have been used for the purpose of

gas-ification, such as pine wood, eucalyptus wood, rice husk, wheat straw,

corn cob, sugarcane bagasse, corn stalk, poplar, hazelnut shell,

switchgrass, olive husk, coconut shell and many others[1,2] Biomass

conversion process provides a more versatile application for the

gaseous fuel, thus increasing the efficiency of energy utilization of

biomass Gasification when compared to combustion achieves a

better and more efficient energy production[3]

Gasification is made possible by the use of a controlled amount

of air, oxygen, steam or mixtures of two or more of these The choice

of the gasifying agent used determines the heating value of the

gaseous product For instance, air gasification results in a low to

medium heating value of gas (4–7 MJ/Nm3) while oxygen or steam

gasification result in medium heating value of 10–14 MJ/Nm3[4]

Variation in the ratio of the gasifying agent to the carbonaceous feed-stock, impacts on the quality of the final gaseous product However, other factors particularly reactor temperature also contribute to the quality and quantity of gaseous fuel produced

The quality of syngas produced via gasification is crucial as the presence of impurities and undesirable products such as particu-late char, tar, nitrogen oxides, and sulfur dioxides can interfere with the downstream application of the syngas These undesirable prod-ucts are traceable to carbonaceous feedstock composition and incomplete gasification, which stems from variation in operating and design parameters Some end user applications of the gaseous fuel require a more intense gas cleaning and conditioning tech-nique Some of these uses range from heat and power application such as integrated gasification combined cycle (IGCC) to produc-tion of synthesis fuels such as methanol and ethanol[5] Development and commercialization of biomass gasification unit

is still hindered by inconsistency in quality of syngas produced The presence of impurities such as particulate matter and tar can cause operational problems such as fouling, clogging, obstruction of pipes and filters, reduction in heat exchange efficiency[6] Particulate matter mostly constitutes of inorganic compound, which include alkali metals (potassium and sodium); alkaline earth metals (calcium); silica (SiO2)

* Corresponding author Tel.: +27833433195.

E-mail address:nnwokolo@ufh.ac.za (N Nwokolo).

http://dx.doi.org/10.1016/j.applthermaleng.2016.01.014

Contents lists available atScienceDirect Applied Thermal Engineering

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / a p t h e r m e n g

In addition elements such as arsenic, selenium, antimony, zinc, and

lead are also present in particulate matter, but in trace amounts[7–9]

According to internal combustion engine, manufacturer’s

particu-late matter content in the syngas should be lower than 50 mg/Nm3

so as to guarantee long-life failure-free operation[10]

Secondly, tar is a complex mixture of condensable

hydrocar-bons, comprising single-ring to 5-ring aromatic compounds and other

oxygen-containing organic molecules The application of the syngas

determines the tar tolerance limit; for instance, in compressors, the

allowable limit is 500 mg/Nm3with 100 mg/Nm3for internal

com-bustion systems, and 5 mg/Nm3for direct-fired industrial gas turbines

[11] The severe operational problems induced by the presence of

impurities in syngas necessitate the need for syngas cleaning before

its end use application

Synthesis gas stream produced from gasification can be cleaned

through a number of methods or techniques Some of these methods

can only remove one contaminant while some others can remove

more than one contaminant in a single process such as wet

scrub-ber The cleaning techniques can be classified based on the process

temperature range: Hot gas cleanup (HGC) and cold gas cleanup

(CGC) Hot gas cleanup refers to the cleaning that occurring within

a temperature range of 400 °C to 1300 °C On the other hand, cold

gas cleanup are cleanings that occur near ambient condition Cold

gas cleaning makes use of water sprays such as wet scrubbers

Al-though cold gas cleaning technologies are effective, they still suffer

from energy inefficiencies and also generate waste water[5]

The aim of this study is to investigate and ascertain the

possi-bility of recovering heat energy at the cyclone prior to the gas

entering the wet scrubber where it loses the inherent heat energy

through cooling The temperature of the syngas is cooled at the wet

scrubber from about 400–500 °C to ambient temperature before

reaching the gas engine As a result, a significant amount of energy

in the form of heat, which could be harnessed for other heating

pur-poses, is lost Harnessing the heat energy at the cyclone before the

wet scrubber will improve the thermal efficiency as well as the

overall efficiency of the gasification system The overall aim is to

integrate a heat exchanger around the cyclone so as to recover the

waste heat from the gas and convert it to usable heat But before

the heat exchanger integration, there is a need for baseline study

so as to determine the possible amount of energy that could be

re-covered from the surface of the cyclone

The use of waste heat recovered from a biomass gasifier was

ex-amined by attaching a thermoelectric generator system (TEG) to the

surface of a catalytic reactor used for cleaning the syngas In

addi-tion, the electrical properties of the thermoelectric generator along

side with the efficiency of the gasification system were studied The

measured surface temperature (473 °C–633 °C) of the catalytic

reactor was high enough to serve as a heat source to the hot

junc-tion of the TEG The power output and power density of the TEG

was found to be approximately 2.9 W–6.1 W and 91.5 W/m2–

193.1 W/m2, respectively More also, a cold gas efficiency (CGE) of

76.26% was obtained[12]

Pavlas et al.[10]evaluated the utilization of waste heat using a

heat pump from a biomass gasification unit integrated with an

ex-isting boiler The integration of a heat pump was so as to utilize the

low grade heat more effectively and efficiently The study

con-cluded that a significant energy savings can be achieved through

the use of heat pump A combined heat and power system using

gas from gasification of biomass was analyzed to determine the effect

of using a different fuel than was originally designed on the

ther-modynamic characteristics of the system The efficiency of heat and

electricity generated was found to depend on the type of system

An overall efficiency of 67% taking into account the gas generator

efficiency was obtained[13]

Duan et al.[14]developed a comprehensive model using ASPEN

Plus for the energy assessment of an integrated coal gasification

combined with a blast furnace slag waste heat recovery system Blast furnace is a by-product of an iron making process that is dis-charged at a high temperature of about 1500 °C–1600 °C The optimal temperature required to simulate the gasification reaction as well

as recover the blast furnace heat was found to be 800 °C Guangul

et al.[15]compared the temperature profiles for gasification of oil palm frond using high temperature air and unheated air The tem-perature profile along the height of the gasifier as well as the producer gas temperature was reported The peak of the temper-ature was observed at the combustion and reduction zone as expected

Furthermore, a dedicated ASPEN Plus model was used by Fran-cois et al.[16], to predict the mass and energy balance (including pollutant emissions) of a combined heat and power (CHP) biomass gasification plant from biomass dryer to gasifier, gas cleaning and

IC engine A total of 10.3 MW of electricity and 13.3 MW of heat were produced from the biomass CHP plant while utilizing about 34.4 MW of anhydrous wood[16] Damartzis et al.[17]assessed a small CHP biomass gasification system consisting of a fluidized bed reactor, a gas cleaning system and internal combustion engine for power generation Most studies on CHP application are based on model development and predictions, but limited study are avail-able on the experimental application of CHP

2 Gasification mechanism

The major chemical reactions that occur during gasification are summarized as shown inTable 1 The heat that supports the reac-tion is either provided by partial oxidareac-tion of the gasified materials

or is externally supplied

These reactions are made possible because of the high operat-ing temperature of reactors used for gasification Regardless of the type of reactor used, gasification process involves four basic steps, namely drying, pyrolysis, oxidation and reduction Each of this process corresponds to the different zones found in a reactor Re-actors also referred as gasifiers are majorly classified into three, fixed bed, fluidized bed and entrained flow gasifiers Fluidized bed and entrained flow are mostly used for large scale or industrial appli-cation while fixed bed (conventional type) is used for small scale applications[4]

This conventional type of gasifier consists of a bed of solid fuel that moves down slowly during the gasification process In fixed bed gasifier, feedstocks are fed into the gasifier system through the top and the oxidizing agent either goes in the same direction or oppo-site direction with the feedstock They are characterized by long residence time, low ash carry over, high carbon conversion and low gas velocity[19] Fixed bed is further classified into downdraft, updraft and cross draft gasifier Each differs in the flow direction

of feedstock and gasifying agent

2.1 Description of gasifier system

The Johansson biomass gasifier under study is of a downdraft type, it offers the advantage of producing a tar free gas, which

Table 1

Basic gasification reactions [18]

C + H 2 O ↔ CO + H 2 131.3 kJ/mol Water gas primary

C + 2H 2 O ↔ CO 2 + 2H 2 90.2 kJ/mol Water gas secondary

2CO + H 2 O ↔ CO 2 + H 2 −41.2 kJ/mol Water gas shift

CH 4 + H 2 O ↔ CO + 3H 2 −206.2 kJ/mol Steam reforming

CH 4 + CO 2 ↔ 2CO + 2H 2 247.4 kJ/mol Dry reforming

makes it suitable for engine application This system comprises of

many components that include the reactor where the solid fuel is

fed into and subsequently gasified The other components are

collectively known as the purification unit where the syngas is

cleaned of impurities such as carbon particles and as well cooled

down to meet the gas engine quality requirement Finally the gas

is then used to drive the generator, which generates the

electrici-ty A typical composition of the gases produced in this system is

shown inTable 2

The system component is depicted inFig 1 The cooling down

of the syngas occurs at the scrubber, where water is sprayed over

a scrubbing medium consisting of a low resistance, but porous large

surface area This scrubbing media usually consist of a coarse or even

graded charcoal The water used in the scrubber is recycled through

an ambient pond over a long period of time[20,21]

A significant amount of energy in the form of heat is lost at the

scrubber during the cooling of the syngas to room temperature

Usually the gas is cooled down to improve the volumetric

efficien-cy of the engine, but at the same time it impacts on the overall

thermal efficiency of the system Therefore, this study seeks to

in-vestigate the quantity of heat that could be harnessed from the body

of the cyclone based on surface temperature measurement

2.2 Cyclone separator

The cyclone is the first purification unit for the syngas after the

gas exits the reactor and before entering the scrubber The main

purpose of the cyclone is to remove the fine carbon particles that

exit the reactor with the gas Generally, cyclone is less prone to

explosion; hence, it offers a better advantage when compared to

fabric filters in high temperature application The schematic flow diagram of a cyclone is shown inFig 2 [22]

As the raw gas exits the gasifier it enters the cyclone in a tan-gential manner The tantan-gential entry results in a spiral flow of gas beginning at the cylindrical part of the cyclone to the conical part

At the conical section, the clean gas reverses and exits in a straight stream through the vortex finder, whereas the particulates collide with the outer wall and fall to the bottom (collection chamber) About 80% of these particulates are removed when operating at full power and this is equivalent to 4 g/Nm3.Table 3shows the main dimen-sions of the cyclone

The removal of particulates present in the syngas at the cyclone

is enhanced by centrifugal force The cyclone performance is usually rated in terms of particle cut diameter or cut size and is repre-sented mathematically as follows:

NV

p

i 50

1

9 2

=⎡

⎣

⎤

⎦

μ

π ρp

(1)

Table 2

Average gas composition of Johansson biomass gasifier system.

Composition (%) 22.3–24.3 22.3–22.5 10.7–9.8 1.90–2.10 42.9–41.5

Barrel

Cone Inlet Duct

Fig 2 Schematic and pictorial view of the cyclone[22]

μ= Gas viscosity (kg/ms)

W= Width of inlet duct (m)

N= Number of turns inside the cyclone

Vi= Gas inlet velocity (m/s)

ρp=Particle density kg m( 3)

This formula is predicted both for general cyclone and high

ef-ficiency cyclone, and it represents the particle size that can be

separated at 50% efficiency

3 Temperature measurements

For the temperature measurement both contact

(thermo-couples) and non contact (infrared camera) temperature measuring

technique was used The setup for the temperature measurement

is shown inFig 3, it comprises of thermocouples, CR1000 data logger,

external power supply and some gas sensors, but the gas sensors

were not used for the purpose of this study Type k

thermo-couples were used because of its wide operating temperature range

(−270 °C to 1260 °C) It has a measuring accuracy of ±2 2 °C The two

thermocouples fitted on the body of the cyclone were 50 cm apart

Four thermocouples were used in all; the third thermocouple

was inserted at the reduction zone of the gasifier and the fourth

to the inlet duct of the cyclone The entire measurement was

con-ducted outside at the location of the biomass gasification system

Effect of ambient temperature was not considered in the surface

tem-perature measurement The surface temtem-perature of the cyclone was

as well measured with FLIR thermaCAM (infrared camera) with a

temperature range of−20 °C to 250 °C and an accuracy of ± °2 C FLIR

thermaCAM (infrared camera) is a non contact instrument that can

visualize the temperature distribution of a surface

The gasifier was loaded with chunks of pine wood sourced from

a nearby sawmill The pine wood chips varied in sizes owing to the

fact that they were off cuts Before the ignition of the gasifier two

thermocouples were fitted at two different heights on the surface

of the cyclone This was done so as to determine if there are

tem-perature variations between the bottom and top part of the cyclone

One thermocouple was also inserted into the inlet duct of the cyclone through a drilled hole The hot bed temperature of the gasifier was also monitored by a thermocouple that was inserted at the lower zone of the gasifier This monitored the temperature of the gas leaving the reduction zone of the gasifier before making its way to the cyclone All the thermocouples were connected to the chan-nels of a CR1000 data logger The data logger was powered with a

12 V external power supply The initial temperature at the inlet duct and surface of the cyclone were noted prior to igniting the gasifier The total quantity of heat that could be recovered from the syngas was as well determined using equation2

Where

Q is the heat content in kcal

V is the flow rate of the substance in m3/hr ρis density of the flue gas in kg/m3

Cpis the specific heat of the substance in kcal/kg °C

ΔT is the temperature difference in °C

4 Temperature results and discussion

In assessing the potential of recovering heat from any system, one of the parameters of significance is temperature The magni-tude of the temperature difference between the heat source and heat sink determines the quality of heat to be recovered Recover-ing heat from the cyclone section of the Johansson biomass gasification system will improve the system from a standalone power system to a combined heat and power system Combined heat and power systems based on gasification are valuable to sawmills and wood processing industry In this study the heat source is the hot syngas stream and the aim is to recover the heat from the surface

of the cyclone prior to the gas entering the scrubber.Fig 4 pres-ents the inlet gas temperature profile and cyclone surface temperature profile

Prior to starting of the gasifier system the temperature of the gas entering the cyclone (Tin) and cyclone surface temperatures (TSL and TSU) were 18.79 °C, 22.59 °C and 21.31 °C, respectively After the ignition of the gasifier the temperature of the gas entering the cyclone was the first to show an increase while the two surface tem-peratures followed after 5 minutes A maximum temperature of 608.8 °C was obtained from the syngas stream as it exits the

Table 3

Main dimensions of the cyclone.

Infrared Camera

Thermocouple Wires

External Power supply

Data Logger

0 50 100 150 200 250 300 350 400 450 500

Time (min)

reduction zone of the gasifier Guangul et al.[15]obtained a similar

temperature profile for gas outlet temperature, the temperature

in-creased from about 50 °C to a maximum of 600 °C In addition, the

obtained gas temperature of 608.8 °C did not differ much from the

temperature range (623 °C–700 °C) obtained by Balas et al.[3] This

was the temperature range within which Balas et al.[3]obtained

the individual gas components: H2, CO2, CH4, N2, CO

As the gas stream approached the cyclone inlet duct, a

de-crease in temperature was observed This is as a result of the

utilization of some of the sensible heat of the syngas in heating the

air entering the gasifier through an internal heat exchanger Hence,

there is no waste of heat in this regard There were fluctuations in

the temperature profiles, particularly the temperature of the gas

en-tering the cyclone within the first 120 mins of operation Similarly

the gas outlet temperature profile obtained by Guangul et al.[15]

showed some fluctuation as it was increasing This is also in

agree-ment with the different zone temperature profile reported by

Mamphweli and Meyer[23] Afterwards, some stability was

re-corded as shown inFig 5

The percentage difference between the temperature of the gas

entering the cyclone and the surface temperature of the cyclone

showed that above 65% of heat in the gas is transferred to the wall

of the cyclone This implies that about 65% of the energy entering

the cyclone is currently lost and this is waste heat available for

con-version to useful energy FromFig 5, the gas temperature was

observed within a temperature range of 450 °C–500 °C while the

two surface temperatures ranged from about 150 °C–220 °C

Comparing this result with that reported by Ma et al.[12]in

which the temperature of the gasifier outlet is about 350 °C–

500 °C and surface temperature of the catalytic reactor is

approximately 200 °C–360 °C The two gas temperatures

com-pared very closely The difference is that the heat recovery in Ma

et al.[12]study occurred at the catalytic reactor while in the current

study, the heat recovery is intended to take place at the cyclone

However, there are some similarities in terms of the position of the

heat recovery unit, in both cases heat is recovered from the gas before

entering the wet scrubber For maximum heat recovery to occur,

the position of the heat recovery unit is important as well as the

choice of the heat recovery equipment

In addition, the surface temperature at the upper part of the

cyclone was found to be higher than the surface temperature at the

lower part of the cyclone This indicates that there was a

down-ward decrease of temperature along the surface of the cyclone as

shown inFig 6

The temperature gradient along the length of the cyclone shown

inFig 6was obtained using a FLIR thermal camera As observed from Fig 6, the highest obtained temperature was around 200 °C, which compares closely to the surface temperature result (Fig 5) ob-tained using thermocouple.Fig 6shows that a larger part of the decrease in temperature occurred at the lowest part of the cyclone, which is closer to the collection chamber of the particulates This major decrease is represented between 59 cm and 97 cm, which cor-responds to the lowest part of the cyclone The decrease in temperature could be attributed to more deposit of particulates at the lower part of cyclone, thus, inhibiting the ease of heat trans-fer Secondly, because the gas enters from the top of the cyclone consequently, the upper part gets heated up first The actual thermal image (Thermogram) is presented inFig 7

50

100

150

200

250

300

350

400

450

500

550

600

Time (min)

40 60 80 100 120 140 160 180 200 220

Length (cm)

Fig 6 Temperature gradient along the length of the cyclone.

<50.0°C

220.2°C

<50.0°C

235.3°C

Upper/cylindrical part

Lower/conical part

Lowest part

Fig 7 Thermal image of the cyclone.

Table 4

Waste heat temperature classifications and some source examples [24] Type Temperature range Example source

High 1650 °C–649 °C Hydrogen plant, fume incinerator Medium 650 °C–230 °C Gas turbine exhaust, catalytic crackers Low 232 °C and lower Welding machines, annealing furnaces

The two images shown inFig 7were taken within an interval

of one minute so the time difference was assumed to be

negligi-ble.Fig 7represents the thermal energy that is radiated from the

surface of the cyclone It provided the approximate surface

tem-perature of the cyclone The temtem-perature of the gas entering the

cyclone falls within the medium temperature range while the surface

temperature as detected from the thermal image is within low

tem-perature range for waste heat recovery Hence a heat recovery unit

such as cold water jacket that can transform low level heat into

val-uable use has to be integrated Basically there are three classifications

of waste heat temperature (as shown inTable 4) for various waste

heat recovery opportunities

One basic advantage of the medium and low temperature range

is, in its compatibility with heat exchanger materials, which is a good

motivation for the present study It was also essential to quantify

the total heat that could be recovered from the syngas stream Hence,

the total quantity of heat contained in the syngas was determined

using equation2and parameters presented inTable 5 This was found

to be 665893.07 kcal

The volumetric flow rate of the syngas was assumed to be

300 Nm3/h based on the specifications of the gasification system

A total gas density of 0.9734 kg/m3was determined from the molar

mass, mass percentage composition of gases presented inTable 5

The specific heat capacity (at constant pressure) presented inTable 5

was converted to kcal/kg °C for consistency in units The initial

tem-perature of 18.79 °C was subtracted from the maximum temtem-perature

attained by the gas entering the cyclone to obtain the

tempera-ture difference The estimated quantity of heat would inform on the

best heat recovery method to adopt Some typical heat recovery

methods include water heating, combustion air preheating, steam

generation, feed water preheating and transfer to a low

tempera-ture process[24]

5 Conclusion

The present investigation was conducted so as to determine a

practical and economic method of utilizing the heat of the syngas

that is otherwise lost through the surface of the cyclone and in the

scrubber This paper presented the preliminary results, which include

the temperature profiles of the gas entering the cyclone and cyclone

surface temperature The gas temperature was found to be within

a temperature range of 450 °C–500 °C while the two surface

tem-peratures ranged from about 150 °C to 220 °C At a temperature of

450 °C–500 °C, the hot syngas is regarded as a good heat carrier A

greater percentage of this heat can be recovered at the cyclone and

used for water heating This will be achieved with integration of

cold water jacket around the cyclone The study has established that

a significant quantity of energy (665893.07 kcal) is lost through the

surface of the cyclone Hence, there is a huge potential of waste heat

recovery for the Johansson biomass gasification system The biomass

gasification system is located close to a saw mill industry, hence the

waste heat can also be channeled for timber drying and steaming

Finally, to justify the need for waste heat recovery at any

applica-tion, there should be some valuable use for the heat recovered and

the cost of recovering should not outweigh the heat recovered

Acknowledgement

The authors would like to acknowledge ESKOM, South African Clean Energy Solutions limited and Govan Mbeki Research and De-velopment Centre at the University of Fort Hare for funding

Nomenclature

Abbreviation

IGCC Integrated gasification combined cycle HGC Hot gas cleanup

CGC Cold gas cleanup TEG Thermoelectric generator system

CGE Cold gas efficiency

Chemical formula

SiO2 Silica

CO2 Carbon dioxide

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