Combustion and environmental performance of clean coal end products

INTERNATIONAL JOURNAL OF ENERGY RESEARCHInt. J. Energy Res. 2007; 31:1237–1250Published online 20 June 2007 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/er.1331

Combustion and environmental performance ofclean coal end products

G. Skodras1,2,3, E. Someus4,*,y, P. Grammelis1, A. Palladas2, P. Amarantos1,P. Basinas2, P. Natas2, M. Prokopidou2, I. Diamantopoulou2,

E. Kakaras1 and G. P. Sakellaropoulos1,2,3

1 Institute for Solid Fuel Technology and Applications/Centre for Research & Technology Hellas,P.O. 1520, 54006, Thessaloniki, Greece

2Chemical Process Engineering Laboratory, Chemical Engineering Department, Aristotle University of Thessaloniki,Thessaloniki, Greece

3Laboratory of Solid Fuels and Environment, Chemical Process Engineering Research Institute, Thessaloniki, Greece43R Environmental Technologies Ltd., H-1222 Budapest, Szechenyi 59, Hungary

SUMMARY

Thermal desorption recycle–reduce–reuse technology (TDT-3R) is based on the low-temperaturecarbonization fuel pre-treatment principles. Clean coal samples were produced according to this methodin an indirectly heated rotary kiln and were examined for their combustion efficiency and environmentalperformance. Raw material included coal and biomass, such as willow and straw. Investigations wererealized via on-site measurements both in the clean coal procurement and combustion facilities and thecharacterization of end products and by-products.Clean coals were proved to be less reactive than raw coal samples, while biomass chars increased the

reactivity of fuel blends during combustion. In the clean coal production and combustion processes, fluegases emissions, such as CO, SO2, NOx, were particularly low, while polychlorinated dibenzo-p-dioxins anddibenzofuran emissions were in an order of magnitude less than anticipated from the EU legislation.Reduced total CO2 emissions are anticipated with the implementation of TDT-3R process compared to theconventional combustion of raw fuels. All ash leachates were accepted by the EPA-TCLP test, since noneof them exceeded the EPA limits for safe disposal. The mercury content of all samples was very low andwas reduced by about 90% after the leaching treatment. The pilot scale combustion tests demonstrated themajor advantage offered by the TDT-3R process, which is the production of clean fuels with much lowerpollutants content. High added value commercial application of the technology is feasible, provided thatthe ash content of raw fuel does not exceed 20% w/w. It is expected that the employment of produced cleancoal originating from high-grade coal in combination with biomass derived materials may result in almostzero emission power plant operation. Copyright # 2007 John Wiley & Sons, Ltd.

KEY WORDS: low-temperature carbonization; TDT-3R; CO2 reduction; heavy metals; dioxins; furans

*Correspondence to: Edward Someus, 3R Environmental Technologies Ltd., H-1222 Budapest, Szechenyi 59, Hungary.yE-mail: [email protected], http://www.terrenum.net/cleancoal

Contract/grant sponsor: European Commission (EC)

Received 18 December 2006Revised 3 January 2007Accepted 7 March 2007Copyright # 2007 John Wiley & Sons, Ltd.

1. INTRODUCTION

In order to achieve sustainable and sound economic development, there is a strong demand forextended clean power production which is affordable by cost, utilizes existing coal-fired plantsand large quantities of renewables, while offering high level of process safety and comprehensivelong-term management of all residual operation streams. Since regional utilization of theavailable biomass in power or combined heat and power production is often not cost effective asstand-alone installations, new patterns need to be developed combining low cost and calorificvalue domestic fuels, such as low-grade coal and biomass. However, the utilization of these fuelsentails the risk of severe environmental impact due to their content in polluting compounds, i.e.N, S, Hg, Cl, etc. Therefore, such combinations are subjected to significant improvements inoverall environmental performances of solid fuel utilization, including improvements ongreenhouse gas emissions (Antal et al., 2003; Braden et al., 2001).

During the past decade, several clean coal technologies have been developed as ‘end of pipe’solutions, such as gasification, the integrated gasification combined-cycle systems for large-scaleuse well over 150MWe net capacity, and the flue gas desulphurization. In order to meet thenew environmental norms, the ‘end-of-pipe’ technologies have increased their complexity,resulting in increased cost. However, it seems that their technical complexity cannot be furtherincreased and therefore that they have reached their ultimate limits, while new environmentalrestrictions and international conventions, such as the Kyoto protocol, require moreefforts (Liao et al., 1998). An alternative option that continuously attracts R&D interest isfuel pre-treatment. Among various available technologies for clean energy production, thelow-temperature carbonization (LTC) fuel pre-treatment technology is looking promising forfuture implementations. The thermal desorption recycle–reduce–reuse technology (TDT-3R) isbased on LTC principles and is essentially a process for producing cleansed fuels from coaland other carbonaceous material, such as biomass (Someus, 2006). Past R&D project(CERTH/ISFTA, 2005) has established the concept’s viability, since it results in a value addedtechnology that converts widely available low-grade domestic fuels to high-grade fuels, whileremoving hazardous air pollutants, such as sulphur, chlorine and mercury, which can beseparately treated or recycled. The integrated application is primarily designed for small-scalepower plants of less than 50MW capacity and cogeneration options, and for medium-scaleplants with less than 300MW power capacity, as well. It is also worth mentioning that accordingto energy calculations, the incorporation of the TDT-3R process in a medium-sized powerplant may bring about a 2% efficiency gain in electricity generation. An electric efficiency of32% in a raw brown coal-fired power plant will be increased to 34% if clean coal is used.These calculations are based on the assumption that the rotary kiln reactor operates with95% efficiency, clean coal consumption equals to 100 t/h, boiler efficiency is 0.92, and 46%of the steam energy is converted to electricity of which one-fifth is needed for various electricauxiliaries.

In this work, the combustion efficiency and the environmental performance of the TDT-3Rprocess are investigated via pilot-scale tests of clean fuel production and of their subsequentcombustion, which were performed in order to demonstrate the operational feasibility of theprocess. Tests included flue gas emissions monitoring, raw fuel and product characterizationand thermogravimetric tests, polychlorinated dibenzo-p-dioxins and dibenzofuran (PCDD/F)and heavy metals analyses, and toxicity tests. Raw material included coal and biomass, such aswillow, straw, and demolition wood.

G. SKODRAS ET AL.1238

Copyright # 2007 John Wiley & Sons, Ltd. Int. J. Energy Res. 2007; 31:1237–1250

DOI: 10.1002/er

https://www.researchgate.net/publication/200736405_The_Art_Science_and_Technology_of_Charcoal_Production?el=1_x_8&enrichId=rgreq-da6a8960-b323-4853-89b4-fd0120666c81&enrichSource=Y292ZXJQYWdlOzIyNzc5MzgwNztBUzoxMDQzMzAwNzgxOTU3MjNAMTQwMTg4NTYzNzg5MQ==

https://www.researchgate.net/publication/204979364_Co-pyrolysis_of_coal_with_hydrogen-rich_gases_1_Coal_pyrolysis_under_coke-oven_gas_and_synthesis_gas?el=1_x_8&enrichId=rgreq-da6a8960-b323-4853-89b4-fd0120666c81&enrichSource=Y292ZXJQYWdlOzIyNzc5MzgwNztBUzoxMDQzMzAwNzgxOTU3MjNAMTQwMTg4NTYzNzg5MQ==

2. EXPERIMENTAL

2.1. Clean fuel production

TDT-3R pilot plant has been built and tested under semi-industrial conditions at Polgardi, inHungary, during 2005. The installation consists of a horizontally arranged rotary kiln, a gas–vapour post-burner, a multi-venturi off-gas treatment scrubber, and a carbon heat exchanger.The kiln is heated externally and raw material is carbonized and decomposed into char and gas–vapours in a reductive environment, under less than 8508C material core temperature andvacuum conditions. Volatile HAPs are removed from the solid feedstock by thermal desorptionand are combusted in the post burner at 8508C, while the carbonized ‘Clean Coal’ end productcan be utilized as an upgraded clean fuel in power plants with capacities up to 300MWe.A detailed description of the process is given elsewhere (CERTH/ISFTA, 2005). In this work,results obtained from tests of four representative raw fuels are presented, specifically of coalfrom two mines, the Rakoczi mine and the Lencshegy mine, of the corresponding clean coals, aswell as of two demolition wood produced chars. Tests included flue gas emissions monitoring,raw fuel and product characterization, and thermogravimetric tests.

2.2. Clean fuel combustion tests

Combustion tests of raw coal, clean fuels produced by the TDT-3R process and blends,including clean coal and raw or clean biomass fuels, were performed in a combustion unit at theTechnical University of Dresden Facilities, Germany. The utilized pilot-scale installationincluded a pulverized coal combustion unit with a down-shot burner and a maximum thermaloutput of approximately 75 kW, Figure 1. The vertical cylindrical combustion chamber (290mmdiameter) had air-cooled walls and was insulated with a refractory lining. The down-shot burnerwas designed as a twist burner with twisted boundary surface air. The primary air–fuel mixtureentered through the central pipe into the combustion chamber, while the twisted secondary airwas preheated to 200–3508C. The main goals were to demonstrate produced clean fuelcombustion that presents minimum pollutants emissions and to obtain useful informationregarding the combustion chamber optimization. Tests included assessing the emissionscharacteristics of the combustion unit, regarding gas pollutants and toxic emissions, and thecharacterization and thermogravimetric analysis of samples collected in order to investigatepyrolysis and combustion behaviour of the specific fuels and their blends.

2.3. Methods and means

The collection, treatment, and proximate analysis of fuel samples were performed according toASTM standard analytical methods. A ThermoFinnigan Flash EA 1112 CHNS analyser wasused for the ultimate analysis. Samples behaviour under pyrolysis and combustion conditionswas also investigated by thermogravimetry in order to obtain the fingerprint of the fuels andtheir blends. Experiments were performed in a non-isothermal thermogravimetric analyser (TAQ 600) where the sample weight loss and the rate of weight loss were continuously recordedunder dynamic conditions, as functions of time or temperature, in the range of 30–10008C.

Contained ash in fuels and ash samples collected during combustion tests were undertakentoxicity tests in order to estimate the potential environmental impact of their disposal. Thestandard toxicity characteristic leaching procedure (TCLP) method according to EPA Method

COMBUSTION AND ENVIRONMENTAL PERFORMANCE OF CLEAN COAL END PRODUCTS 1239


DOI: 10.1002/er

1311 was applied to the solid samples to determine the potential leachability of their traceelements (U.S. Environmental Protection Agency, 1992). The chemical composition of ashleachates was determined by a Perkin-Elmer Inductively Coupled Plasma–Atomic EmissionSpectroscopy spectrophotometer, while the determination of total contained mercury required adifferent extraction protocol to be followed, because of its high volatility. Therefore, itsquantification was made using a cold vapour atomic absorption spectrometer (AAS,LabAnalyzer 254, Mercury Instruments, GmBh) (U.S. Environmental Protection Agency,1994). The toxicity of the liquid samples collected from the TCLP leaching test was evaluatedusing the bioluminescence bacteria Vibrio fischeri (Microtox test). The tests were performedusing the Microtox 500 analyser, while the toxic effect of the leachates on V. fischeri wasevaluated following the 82% screening test protocol (Microbics Corporation, 1992).

During clean fuel production and combustion tests, conventional flue gas emissions, such asCO, CO2, SO2, NOx, were measured on-line with a MADURGA-40T plus flue gas analyser, whilesamples for PCDD/Fs analyses were collected by a TECORA isokinetic sampling device. Theanalysis of the samples for the determinations of the PCDD/F content of the flue gases was madein high-resolution GC/MS and the toxicity equivalency factor of all the isomers was estimated.

3. RESULTS AND DISCUSSION

3.1. Clean fuel production

The average quality characteristics of the samples are given in Table I. According to the results,clean coals exhibit reduced moisture content, especially the Rakoczi mine ones, while biomass

Figure 1. Flow diagram of the combustion unit.



DOI: 10.1002/er

chars moisture is quite low. Volatile matter also decreased during clean fuel production, whilefixed carbon is increased with biomass chars presenting very high values up to 74.6%. Ashcontent is generally increased. Ultimate analysis illustrates that carbon content of clean fuelsincrease and hydrogen, nitrogen, and sulphur content decrease. The calorific value of the cleancoal samples varied from about 14.7 to 17.5MJ kg�1, while the heating value of biomass charswas around 29.3MJkg�1.

The main combustion characteristics established from the thermogravimetric analysisof the samples are summarized in Table II. The TG (weight loss) and DTG (weight loss rate)curves of all raw fuels and chars are given in Figure 2(a)–(f). The shape of the TGA/DTGcurves indicates that combustion in air proceeds in a different way for the various samples.Lower combustion rates and total conversions are observed for the Lencshegy chars comparedto the Rakoczi ones. DTG curves indicate also that the maximum peaks of the Rakoczi raw

Table I. Proximate and ultimate analysis of the raw and clean fuel samples.

Sample Rakoczi mine Lencshegy mine Demolition wood

Analysis Raw coal Clean coal Raw coal Clean coal Char 1 Char 2

Proximate analysis (% w/w as received)Moisture 14.50 2.00 8.00 2.00 6.70 6.10Volatiles 21.60 17.5 38.20 20.28 13.95 20.54Fixed carbon 19.86 21.23 10.21 35.02 74.59 70.15Ash 44.04 59.25 43.59 42.70 4.76 3.21

Ultimate analysis (% w/w dry)C 32.95 37.31 42.81 44.37 78.79 83.19H 2.90 1.28 3.23 1.42 2.07 2.15N 0.87 0.83 1.18 1.02 0.25 0.43S 0.90 0.69 4.22 2.15 0.00 0.00Cl (ppm) 150.50 93.70 84.30 86.30 135.10 111.30Hg (mg g�1) 0.22 0.22 0.32 0.04 0.03 0.04O (by subtr.) 10.87 0.02 1.18 7.47 13.79 10.81Heating value (MJ kg�1) 15.6 14.7 19.8 17.5 30.1 31.7

Table II. Combustion characteristics of the fuels.

SampleInitial combustiontemperature (8C)

Max combustionrate (min�1 10�2)

Temperature at maxcombustion rate (8C)

Combustiontime (min)

Total conv.(% w/w)

Rakoczi raw 254 6.2 487 31.34 42.1Rakoczi cleancoal

372 5.6 521 31.97 53.5

Lencshegyraw

300 6.2 541 37.43 63.6

Lencshegyclean coal

398 5.9 552 38.53 53.8

Demolitionwood char 1

343 8.3 527 33.85 93.0

Demolitionwood char 2

312 8.6 518 34.36 88.2



DOI: 10.1002/er

fuels and chars do not deviate significantly and the burn process is completed in almost thesame time. It is also worth noting that the combustion of the chars is completed at lowertime compared to the pure fuels, Table II. For all the wood chars DTG curves indicatethat combustion takes place between 350 and 6008C. This temperature region accountsfor the 96–98% of the total weight loss. It should be noted that the obtained DTG curvesfor the wood chars are quite different from the ones of the Rakoczi mine and resemble more to

200 400 600 800 100040

50

60

70

80

90

100

Temperature (°C)

Wei

gh

t (%

)

0

1

2

3

4

5

6

7

200 400 600 800 100050

60

70

80

90

100

Temperature (°C)

Wei

gh

t (%

)

0

1

2

3

4

5

6

Der

. Wei

gh

t (%

/min

)D

er. W

eig

ht

(%/m

in)

Der

. Wei

gh

t (%

/min

)

Der

. Wei

gh

t (%

/min

)D

er. W

eig

ht

(%/m

in)

Der

. Wei

gh

t (%

/min

)

200 400 600 800 100040

50

60

70

80

90

100

Temperature (°C)

200 400 600 800 1000

Temperature (°C)

Wei

gh

t (%

)W

eig

ht

(%)

0

1

2

3

4

5

6

200 400 600 800 1000

40

30

50

60

70

80

90

100

Temperature (°C)

200 400 600 800 1000

Temperature (°C)

Wei

gh

t (%

)W

eig

ht

(%)

0

1

2

3

4

5

6

7

(a) (b)

(c) (d)

(e) (f)

0

10

20

30

40

50

60

70

80

90

100

0

1

2

3

4

5

6

7

8

9

0

10

20

30

40

50

60

70

80

90

100

0

1

2

3

4

5

6

7

8

9

Figure 2. Non-isothermal combustion tests in the TGA of: (a) Rakoczi Raw sample 1; (b) Rakoczi CCSample 1; (c) Lencshegy Raw Sample 1; (d) Lencshegy CC Sample 1; (e) Demolition wood char 1; and

(f) Demolition wood char 2 (medium: Air, heating rate: 208Cmin).



DOI: 10.1002/er

the corresponding curves of the Lencshegy samples. The combustion of the biomass charspresent two clear peaks. The first is considered to correspond to the volatiles evolution andcombustion. The second correspond to the remaining volatiles release and combustion and tothe combustion of the produced char. In general, results revealed that produced clean coal wasless reactive than the raw coal. Combustion tests exhibited higher initial combustiontemperature and lower total conversion in the case of clean coal, which may be mostlyattributed to the reduction of the volatile matter and increment of ash content during the cleancoal preparation stage.

Conventional flue gas emissions, such as CO, CO2, SO2, NOx, were measured in two samplingpoints at the TDT-3R pilot plant, in Polgardi. Sampling point 1 was at the chimney, andsampling point 2 was before the flue gas scrubber. In Figure 3(a) and (b), the values and thevariations of the emissions versus the time are shown, during the pyrolysis test of Rakoczi rawcoal. All values remained almost constant within a time interval of 1.5 h. The temperature atthat position was around 508C. CO2 content of the flue gases varied between 6.2 and 6.5%. Theoxygen content at the chimney was 13.80%, while the humidity was very high, more than 91%.The temperature before the scrubber was around 110–1208C. Gaseous pollutants values were alittle higher than those after the scrubber, while increased variations were observed. The CO2

content was also higher, varying between 7 and 7.5%. All pollutants measured values werebelow the limits set by the legislation (2000/76/EC Directive). European Union accepts CO andNOx emissions in the order of magnitude of 50 and 200 mgm�3, respectively, for such units,while the average emissions of the specific pollutants in the pyrolysis plant were 20 and89 mgm�3, respectively.

A major aspect of environmental concern is the CO2 emissions derived from fuel combustionor thermal treatment. Kyoto protocol is in force and all Annex I countries are obliged tomeet their greenhouse gases emissions targets. Implementations of technologies thatcan provide measurable results of reduced CO2 emissions are potential options for the powerutilities. The TDT-3R process can offer the benefit of clean fuels utilization for energyproduction. However, the process itself produces considerable CO2 amount. In order toevaluate this quantity, basic mass balance calculations based on on-site measurement valueshave been performed. Taking into account the geometry of the chimney and the averagevelocity of the flue gases at the sampling point, average CO2 concentration at thesampling point, and the feed rate of the utilized raw coal, produced CO2 is calculated as0.54 kg kg�1 of feed. Such value is considered as reasonable in the case of coal thermal

0

40

80

120

160

200

0 20 40 60 80 100 0 2010 4030 6050 70

Time (min) Time (min)

CO

, NO

, NO

x (m

g/N

m3,

dry

, 6%

O2)

35

45

55

65

75

85

Tga

s (°

C)

CO NO O2 O2NOx Tgas Excess air: 2.85 - 3.11

CO2: 6.28 - 6.54%

(a) (b)

0

40

80

120

160

CO

, NO

, NO

x (m

g/N

m3,

dry

, 6%

O2)

80

100

120

140

160

Tga

s (°C

)

CO NO NOx Tgas Excess air: 2.48 - 2.94 CO2: 7.00 - 7.53%

Figure 3. Emitted pollutants in the flue gas at: (a) position 1 (chimney) and (b) position 2 (scrubber inlet).



DOI: 10.1002/er

treatment. The produced clean fuels contain higher carbon contents that result in higheroverall combustion efficiencies. Higher efficiencies in turn result in lower CO2 emissions,since less fuel is required in order to produce the same amount of heat or power. Therefore,the total amount of emitted CO2 is lower than the one originating from raw fuelscombustion. Moreover, when biomass chars are utilized in combustion, possibly as secondaryfuels in coal–biomass fuel mixtures, the benefit regarding CO2 emissions is much greater,since it is commonly accepted that biomass is CO2 neutral fuel. Consequently, the TDT-3Rprocess offers an even greater advantage since it may produce clean fuels from both coal andbiomass raw fuels.

In Table III the PCDD/F emissions during the pyrolysis process as well as the PCDD/Fcontent of the water collected at the scrubber and the corresponding toxicity equivalence valuesare shown. PCDD/F emissions at the chimney were 0.01 ngNm�3 an order of magnitude lessthan the emissions limit that the EU legislation accepts (2000/76/EC Directive). WastewaterPCDD/F content was much lower than this of the flue gases. The water had 0.02 ng l�1 while thelimit is 0.3mg l�1.

Scrubber wastewater analysis for heavy metal composition has been performedfocusing on elements of environmental interest as described in related EU directive (2000/76/EC Directive), Table IV. Results show low concentrations for most metals, including As, Cr, Pb,Ti, and Hg. However, in some cases the heavy metal concentration exceeds the abovementioned legislation limits. More specific, Cu and Zn concentrations were measuredas 1.80 and 2.88mg l�1, while the respective limits are somewhat lower (0.5 and 1.5mg l�1).Moreover, Cd and Ni concentrations were found to be much higher than the acceptedvalues; Cd concentration was 1.07mg l�1 and Ni content was 6.5mg l�1, and the respectivelimits are 0.05 and 0.5mg l�1. Therefore, unless such results can be attributed to specificparameters of unit operation that may be improved, wastewater treatment is necessary prior toits disposal.

Table III. Fuel blends in pilot plant combustion tests and corresponding emissions.

Fuel/blend Total PCDD/F PCDD/F I-TEQ Emission limits

Rakoczi RM 0.050 (ngNm�3) 0.01 (ngNm�3) 0.1 I-TEQ ngNm�3

Wastewater from scrubber 1.380 ng l�1 0.024 ng l�1 0.3mg l�1

Table IV. Heavy metal concentrations in scrubber wastewater.

Element Concentration (ppm) EU limits (ppm)

As 50.001 0.15Cd 1.07 0.05Cr 0.29 0.5Cu 1.80 0.5Ni 6.50 0.5Pb 0.037 0.2Ti 0.001 }Zn 2.88 1.5Hg 0.006 0.03



DOI: 10.1002/er

3.2. Fuel combustion

Utilized fuels and fuel mixtures for pilot-scale tests are presented in Table V. It must be notedthat during combustion tests of the first and the last fuels slagging and fouling problems in thecombustion chamber occurred, resulting in emissions monitoring interruption.

The average quality characteristics of the selected fuels and blends are given in Table VI. Allsamples present very low moisture content, while their ash content was rather high, varyingfrom 50.53% w/w (dry basis) for the Markushegy raw coal up to 67.04% w/w for the Rakocziclean coal. Mercury content in raw fuels ranges from 0.08 to 0.3 mg g�1. Generally, mercury is atrace element contained in coal whose percentage varies with coal rank (0.01–3.3 mg g�1)(Hancai et al., 2004). The low contents that have been measured in our samples are in agreementwith the above observation, since all the raw fuels could be characterized as low rank coals.

The main combustion characteristics of the fuels tested, the total weight loss and thecontribution of the temperature regions in the weight loss are given in Tables VII and VIII.Combustion of raw fuels starts at about 2008C, while two degradation steps occur during theconversion of Markushegy clean coal and its blends. Furthermore, raw fuels presented the

Table V. Fuel blends used in the combustion tests.

Fuel mixture Test code

Fuel 1 Markushegy raw coalFuel 2 CMF Markushegy clean coalþDW (3:1 blend) Test AFuel 3 Rakoczi clean coal Test BFuel 4 Rakoczi clean coalþ straw char (3:1 blend) Test CFuel 5 Rakoczi raw coal dry Test DFuel 6 Rakoczi clean coalþ willow char (3:1 blend) Test EFuel 7 Markushegy clean coal

Table VI. Proximate and ultimate analysis of the raw fuels.

Analysis Fuel 1 Fuel 2 Fuel 3 Fuel 4 Fuel 5 Fuel 6 Fuel 7

Proximate analysis (% w/w as received)Moisture 11.4 1.2 0.9 1.3 4.7 1.4 0.7Volatiles 37.43 22.99 28.12 19.02 26.92 18.57 28.62Fixed carbon 6.40 27.87 4.54 22.34 16.38 26.67 16.01Ash 44.77 47.94 66.44 57.34 52.00 53.36 54.67

Ultimate analysis (% w/w dry basis)C 31.02 40.33 22.12 31.43 29.57 43.32 30.67H 2.81 1.85 1.31 1.63 2.50 1.71 1.44N 0.57 0.81 0.56 0.80 0.66 0.50 0.38S 1.97 0.76 0.37 0.33 0.99 0.16 1.75Cl (ppm) 207.8 678.5 127.1 71.1 165.9 513.6 195.6Hg (mg g�1) 0.3 0.23 0.27 0.21 0.196 0.076 0.12Ash 50.53 48.52 67.04 58.10 54.56 54.12 55.06O (by subtraction) 13.1 7.73 8.60 7.71 11.72 0.19 10.7Gross HV (MJkg�1) 14.9 16.6 9.5 13.2 13.9 17.4 12.8Net HV (MJkg�1) 14.3 16.2 9.2 12.8 13.3 17.0 12.5



DOI: 10.1002/er

highest total conversion followed by the clean coal blends, while clean coals presented the lowestweight loss.

CO, CO2, SO2, NOx emissions were measured during the combustion tests. Two samplingpoints were selected on the flue gas duct of the combustion unit for these measurements.Sampling point 1 was at the exit of the burner, and sampling point 2 was just after the cyclonefor the fly ash collection. In Table IX, the average values of the measurements at both samplingpoints for the combustion tests A, B, C, and D are shown. During test E, a sampling trainproblem occurred and measuring was not possible.

In Table X the PCDD/F emissions as well as the corresponding toxicity equivalence valuesare presented. It is evident that in all flue gases measurements the sum of all isomers was either

Table VII. Combustion characteristics of the fuels.

SampleInitial combustiontemperature (8C)

Max. combustionrate (min�1 10�2)

Temperature at max.combustion rate (8C)

Combustiontime (min)

Fuel 1 214 7.8 532 35.90Fuel 2 325 7.8 490 33.62Fuel 3 342 4.8 487 35.21Fuel 4 300 4.7 495 40.95Fuel 5 251 6.5 486 30.80Fuel 6 293 8.4 419 44.09Fuel 7 320 6.0 527 37.28

Table VIII. Total weight loss during combustion of the fuels.

Sample Total conversion (% w/w) Combustion time ratio*

Fuel 1 72.9 0.814Fuel 2 70.5 0.762Fuel 3 36.2 0.799Fuel 4 45.7 0.929Fuel 5 60.4 0.699Fuel 6 74.7 1.000Fuel 7 50.8 0.845

Table IX. Flue gas emissions.

Testcode Position

Tgas

(oC)CO (mgNm�3

dry, 6% O2)NO (mgNm�3

dry, 6% O2)NOx (mgNm�3

dry, 6% O2)O2

(%)Excessair

CO2

(%)

A 1 417.26 158.34 270.37 413.63 3.43 1.18 1.22 15.79 15.97A 2 182.96 101.54 285.20 436.31 8.90 1.65 1.95 11.12 11.40B 1 372.71 455.24 254.31 389.06 6.53 1.44 1.52 13.05 13.29B 2 157.46 293.25 341.20 521.98 10.99 1.73 2.60 7.99 10.34C 1 360.24 451.65 280.76 429.52 6.97 1.34 1.46 12.36 13.74C 2 164.83 403.71 294.47 450.50 10.05 1.82 2.11 9.69 10.26D 1 392.18 245.43 276.49 422.99 4.23 1.21 1.29 15.12 15.38D 2 165.02 193.60 290.59 444.56 9.71 1.82 1.91 10.07 10.08



DOI: 10.1002/er

zero or was well below the emissions limits enforced by the EU legislation (2000/76/ECDirective). Specifically, the relevant limit is 0.1 I-TEQngNm�3 and the higher value measuredduring the combustion tests was 0.018 I-TEQngNm�3 corresponding to the Rakoczi clean coal/straw char blend, almost one order of magnitude lower. This indicates that although thefavouring PCDD/F formation chlorine content of the raw fuel was rather high, the relativelylow carbon fly ash content prevented toxics existence in the flue gases.

In Figure 4(a) and (b), the homologue pattern distribution of PCDD and PCDF is shown,respectively, for the fuel blend Rakoczi clean coal/straw char. In that case, the lower (tetra- andpenta-) chlorinated isomers prevailed over the higher ones, especially in the case of furans. Thisis a typical distribution for coal co-combustion processes.

Bottom and fly ash samples were collected and concentrations of the various metalcompounds in their TCLP leachates are presented in Table XI. Cr and Cu compoundconcentrations were not traced to the aquatic phase, while Sn was only found in fly ash leachatethat corresponds to Rakoczi clean coal combustion. Regarding the bottom ash leachates, Cd,Co, Cr, Cu, and Sn were not traced in any of the liquid samples. In general, all ash leachatespassed the EPA-TCLP test, since none of them exceeds the EPA limits for its safe disposal.

Before all TCLP leaching tests, the pH values of the liquid samples were adjusted to 7� 0.2 inorder to ensure that the toxicity is only due to the substances that are present in the samples andtest bacteria are not affected by the pH. The results were obtained by measuring the toxic effectsof the TCLP leachates on the V. fischeri within exposure times of 5, 15, 30 min, according to the

Table X. Fuel blends in combustion tests and corresponding emissions.

Fuel/blendFuelcode

Content(% w/w)

Total PCDD/F(ngNm�3)

PCDD/F I-TEQ(ngNm�3)

CMF Markushegy clean coal/DW (A) 75/25 0.124 0Rakoczi clean coal (B) 100 0 0Rakoczi clean coal/straw char (C) 75/25 1.897 0.018Rakoczi raw coal dry (D) 100 0.213 0.007Rakoczi clean coal/willow char (E) 75/25 0 0Rakoczi RM (mine) (F) 100 0.050 0.001

0.00

0.05

(a) (b)

0.10

0.15

0.20

0.25

TCDD

PeCDD

HeCDD

HpCDD

OCDDPC

DD

Co

nce

ntr

atio

n, n

g/N

m3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

TCDF

PeCDF

HeCDF

HpCDF

OCDFPC

DF

Co

nce

ntr

atio

n, n

g/N

m3

Figure 4. (a) PCDD and (b) PCDF homologue pattern for Rakoczi clean coal/straw char (3:1 blend) fuel.



DOI: 10.1002/er

45 and 82% screening test protocols. Screening test protocols, with pH adjusted at 7� 0.2, aregiven in Figures 5 and 6, where FA1–5 and BA1–5 represent samples from tests A–E,respectively. The toxic effect of the fly ash leachates FA1, FA2, and FA5 on V. fischeri isrelatively high in both protocols, while the liquid sample FA3 presented a sufficient ecologicalquality, as its effect on the bacteria did not exceed the 20%. Concerning the bottom ashleachates, Figures 5 and 6 reveal that their toxic effect on the microorganisms is relatively low,up to 30% for the 45% screening test protocol and up to 40% for the 82% screening testprotocol, for almost all samples. These values were exceeded in the case of the liquid sampleBA2, which corresponds to Rakoczi clean coal that caused the highest toxic effect on V. fischeriin both protocols. The results of the Microtox toxicity tests were compared with the chemical

Table XI. Chemical analysis of the fly and bottom ash leachates.

Values of fly/bottom ash leachates

mg l�1 Test 1 Test 2 Test 3 Test 4 Test 5 EPA max

As 0/0 0.02/0 0/0.02 0/0 0.01/0.018 5Cd 0/0 0/0 0.003/0 0.004/0 0.001/0 1Co 0.12/0 0.1/0 0/0 0.12/0 0.1/0 5Cr 0/0 0/0 0/0 0/0 0/0Cu 0/0 0/0 0/0 0/0 0/0 15Hg 0.0007/0.00007 0.00075/0.00023 0.00045/0.00028 0.0004/0.0002 0.00035/0.00028 0.2Mg 0.4/0 48/24 74/46 59/29 53/23Mn 0/0 0/0.38 1.9/0.68 1.5/0.68 1.3/1.1Ni 0.121/0.068 0.049/0.113 0.16/0.062 0.216/0.06 0.144/0.045 5n

Pb 0.015/0 0.105/0.01 0.01/0.13 0.013/0 0.06/0 5Sn 0/0 0/0 0.128/0 0/0 0/0Zn 0.01/0 0/0.31 1.65/0.14 1.75/0.58 0.71/0.68 25

n100�drinking water maximum, EPA-TCLP maximum has not been established.

FA1 FA2 FA3 FA4 FA50

20

40

60

80

100

(a) (b)

% e

ffect

TCLP leachatesFA1 FA2 FA3 FA4 FA5

TCLP leachates

5 min15 min30 min

5 min15 min30 min

0

20

40

60

80

100

% e

ffect

Figure 5. Toxicity effect on V. fischeri of fly ash TCLP leachates using: (a) the 45% Screening TestProtocol and (b) the 82% Screening Test Protocol.



DOI: 10.1002/er

analysis of the corresponding leachates and no correlation was found between theconcentrations of the metal species and the toxic effect caused on the test organisms. However,for the fly ash leachates, which showed higher toxic effects than the bottom ash leachates on thebacteria, the measured concentrations of heavy metals were also higher than those measured inthe liquid bottom ash samples. In addition, the high toxic effect of the leachate BA2 could beattributed to the high Ni content in the sample.

4. CONCLUSIONS

A series of experimentations focused on the combustion and environmental performance ofclean coals produced by the TDT-3R process were accomplished. In the lab scale investigations,clean fuels were found to be less reactive than the raw samples. The overall combustionbehaviour of both clean and raw fuels was the same, while clean fuels exhibited much highercarbon content and lower heteroatoms and toxic pollutants. Biomass addition in the fuel blendfavoured the reactivity increase compared to the exclusive coal use. In this sense, biomass inputin the clean fuels production may compensate for the low calorific content of coal chars, offeringat the same time significant environmental benefits in terms of CO2 reduction.

In the pilot-scale combustion tests, severe slagging and fouling problems were observedwhen 100% clean coal was employed. Operational problems were mainly attributed to thehigh ash content of the selected raw coal. Such high ash content (33–45% w/w) of the rawfuel has proven to be a limiting factor for the TDT-3R technology. During both cleanfuel production and combustion tests at the pilot-scale facilities, all gaseous pollutantsvalues were well below European and international legislation limits. Toxic emissions weregenerally very limited. However, heavy metal analysis of the wastewater in the clean fuelproduction revealed that wastewater treatment is probably necessary prior to its disposal. Allash leachates passed the EPA-TCLP test, since none of them exceeded the EPA limits for safe

BA1 BA2 BA3 BA4 BA50

20

40

60

80

100

(a) (b)

% e

ffect

TCLP leachatesBA1 BA2 BA3 BA4 BA5

TCLP leachates

5 min15 min30 min

5 min15 min30 min

0

20

40

60

80

100

% e

ffect

Figure 6. Toxicity effect on V. fischeri of bottom ash TCLP leachates using: (a) the 45% Screening TestProtocol and (b) the 82% Screening Test Protocol.



DOI: 10.1002/er

disposal. However, high toxicity effects have been observed in almost all samples. Therefore,safe disposal measures of produced ashes should be maintained even when clean fuels areutilized in energy production. In all collected samples, PCDD/F concentrations and theirrespective toxicity equivalent factor was either zero or it was much lower than the EU legislativeemission limits.

Conclusively, the major advantage of the TDT-3R process is the production of fuels withmuch lower pollutants content. The ‘3R’ recycle–reduce–reuse integrated environment controltechnology provides preventive pre-treatment of low-grade solid fuels, such as brown coal andcontaminated solid fuels to achieve high-grade fuels. It is safer, faster, better, and less costlyprocess in comparison to the ‘end-of-the-pipe’ post-treatment solutions. Its industrial-scaleapplication is considered feasible for power generation up to 300MWe, provided that theash content of the raw solid fuel does not exceed 20% w/w. Future research should befocused on the optimization of the low carbonation operating conditions aiming to furtherimprove the quality of the produced clean coals at minimum pollutants content. The 3Rtechnology could be applied as vital component of an integrated strategy towards the near zeroemission power plant targets, combining technologies for environmentally sustainable andeconomical solid fuel power generation, including but not limited to the decrease or evenremoval of greenhouse gases.

ACKNOWLEDGEMENTS

The authors thank the EC for the financial support of this work through the NNE5/2001/363 project,European Union CORDIS database: http://cordis.europa.eu/search/index.cfm?fuseaction=proj.simple-document&PJ RCN=5487554.

REFERENCES

Antal MJ, Gronli Jr M. 2003. The art, science and technology of charcoal production. Industrial and EngineeringChemistry Research 42:1619–1640.

Braden JB, Kolstad CD, Woock RA, Machado JA. 2001. Is coal desulphurisation worthwhile? Evidence from themarket. Energy Policy 29:217–225.

CERTH/ISFTA, Moving towards zero-emission plants. Proceedings of the International Symposium. Leptokarya,Greece, 20–22 June 2005.

Directive 2000/76/EC of the European Parliament and of the Council of the 4 December 2000 on the Incineration ofWaste. Official Journal of the European Communities.

Hancai Z, Feng J, Jia G. 2004. Removal of elemental mercury from coal combustion flue gas by chloride-impregnatedactivated carbon. Fuel 83:143–146.

Liao H, Li B, Zhang B. 1998. Pyrolysis of coal with hydrogen-rich gases. 2. Desulphurisation and denitrogenation in coalpyrolysis under coke-oven gas and synthesis gas. Fuel 77:1643–1646.

Microbics Corporation. 1992. Microtox Manual. Azur Environmental: Carlsbad, CA.Someus Ed. 2006. The ‘3’R Anthracite clean coal technology: economical conversion of browncoal to anthracite type

clean coal by low temperature carbonization pre-treatment process. The Journal of Thermal Science 3: 55–69. Originalscientific paper, UDC: 662.641.66:662.612, BIBLID: 0354-9836.

U.S. Environmental Protection Agency. 1992. Method 1311}Toxicity Characteristic Leaching Procedure (TCLP). 35.U.S. Environmental Protection Agency. 1994. Method 7471A}Mercury in Solid or Semisolid Waste Manual

Cold-Vapour Atomic Absorption Spectroscopy. 7.



DOI: 10.1002/er

Documents

Combustion and environmental performance of clean coal end products