Co-Combustion of Coal and Alternative Fuels

Energy utilization of alternative fuels including biofuels is one of the main tasks for development of recoverable sources in the world. The research consists of combustion tests in the large CFB boilers and measuring data inside the combustor. Since 1995, 29 large CFB boilers of different designs and power outputs have been in operation in the Czech Republic as shown in the Table 4. Construction of the boilers, technical documentation, licensing and engineering has been based on foreign experience. Every large power project is always preceded by trial measurements and tests on smaller pilot, trial or if need be model equipment. Due to the great difference in scale, some unexpected measuring equipment behavior or problems must be taken into consideration for cocombustion of coal and alternative fuels.


Introduction
Energy utilization of alternative fuels including biofuels is one of the main tasks for development of recoverable sources in the world.The research consists of combustion tests in the large CFB boilers and measuring data inside the combustor.Since 1995, 29 large CFB boilers of different designs and power outputs have been in operation in the Czech Republic -as shown in the Table 4. Construction of the boilers, technical documentation, licensing and engineering has been based on foreign experience.Every large power project is always preceded by trial measurements and tests on smaller pilot, trial or if need be model equipment.Due to the great difference in scale, some unexpected measuring equipment behavior or problems must be taken into consideration for cocombustion of coal and alternative fuels.
The present research aiming at characterising the co-combustion under-atmospheric fluidized bed conditions by different physical and chemical characteristics has the following objectives:

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Ash formation upon fluidized bed co-combustion.

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Fate of toxic trace metals upon fluidized bed co-combustion.

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Recommendations for suitability of co-combustion in the atmospheric circulated fluidized bed boilers CFB and minimizing the harmful solid and gaseous emissions.

Model research
The model research has been carried out at the Technical University of Dresden.It includes the combustion tests -Table 2. -on experimental pilot equipment -Fig. 1. , 2. with atmospheric circulating fluidized bed for coal and bio-fuels produced from the sewage sludge from WWTP (waste water treatment plant) and biomass, and thermo-analytical study of bio-fuels -Table 1.The modelling had the following aims: -To determine non-uniformity of combustion in the fluidized bed combustor as influencing the composition of flue gases and specification in terms of minor constituents (NO x , chlorine compounds, alkalis, etc.).-Analogically the influence of the size or for that matter the influence of fuel granulometric distribution on the process.-Chemical composition, crystallographic structures, and mechanical properties of combustion solid products (bottom ash, fly ash, deposits).

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Fossil Fuel and the Environment 64 -Analytical establishment of sulphur forms in fuel and combustion solid products, as well as element analysis for fuel and biomass.-To perform leaching tests for combustion solid products.-Detailed study of mineralogical and chemical composition of bottom ash, fly ash, and the solid emission phase in cyclone, heat exchanger and filter.Fig. 3., 4. Table 3. -Balance for volatile elements, Cl, S, Hg, Se, semi-volatile elements, V, Ni, Co, As, and some non-volatile elements, Cr a Sn.Based on these balances to calculate the content of these elements in emissions and compare with the results of balance measurements.

Fig. 1. General view on pilot plant
Laboratory studies were focused on a detailed identification of input raw materials (coal, biofuel, limestone) so that the measurements could be reproducible: 1. Raw material input analysis and dependence of combustion solid residues on raw material input.2. Combustion inaccuracy assessment in actual unit condition (T, gaseous and solid components, velocities, modelling).3. Balance of combustion elements choice, studying mechanisms of deposit formation and composition.4. Verifying a redistribution model for a choice of elements between the fuel and solid by-products.The model research verify if the alternative fuel produced from biomass and sewage sludge may be used as alternative energy source in respect of the EU legislation, and/or its other modifications (with additives, decontamination technologies) for suitable fuel, which would comply with emission limits or the proposed energy process optimizing the preparation of coal/sludge mixture for combustion in the existing power engineering equipment.
The limiting factor for sewage sludge utilization from WWTP (waste water treatment plant) in agriculture is the increased content of risk elements and also the occurrence of organic pollutants -primarily polyaromatic hydrocarbons, PCB (polyaromatic byfenyls) and AOX (adsorbable organic halid).Other alternative fuels have not these limiting conditions.The limiting factor for sludge combustion at incineration plants is water content.With regard to the fact that from 2005 the EU Directives EU expects to ban waste disposal sites with any material with content of organic substances above 10 %, it is apparent that the priority condition for sludge utilization is sludge decontamination or power engineering utilization (Loo & Kopperjan 2008).Results from tests may be evaluated as very good with the prerequisite for utilization, testing of investigated substances in real combustion units.On the basis of carried out laboratory and pilot tests one may expect good results from these real units (equipment with greater output), many of these experiments have been already performed.From the results of experiments and thermoanalytical studies it is clear that 15 % of alternative fuels -biofuels based on sludge and brown coal can be used in the large fluidized bed boilers located in the Czech Republic.The combined combustion will enable to fulfil the Czech Republic's pledge to the European Commission concerning the development of renewable energy resources by 2010.

Diagnostic methods for operating surveillance of large fluidized bed boilers
Once large units are put into operation during co-combustion of alternative fuels and coal, guaranteed-performance tests must be conducted.The aim of the guaranteed-performance tests is to verify design parameters.Guaranteed-performance figures are compared with reality.Apart from basic measurements there are a number of other similar measurements of specific equipment parts that might be initiated because: the manufacturer is interested in using the experience to improve or design new units and the operator is interested in both eliminating problems and improving the economics of the operation process -Table 4. (Čech 2006).This chapter reviews the development of verification methods and presents some equipment for the determination of all important and interesting measuring data.The conclusions might be useful to energy companies and operators that want to verify operation data of fluidized bed boilers, flue gases and air channels.
Diagnostic measurements at a particular unit basically cover:

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The measurement of fluidized bed temperatures, furnace temperatures, flue gases temperatures at ancillary heating surfaces up to the boiler.

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The measurement of flue gas velocity in the furnace chamber and exits of cyclones, in cyclones, at the cyclone exit to second pass as well as in the area of additional boiler surfaces, sampling of flue gas in the boiler.

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Sampling of characteristic solid ash particles including isokinetic sampling to determine solid particle concentrations.

Flue gas elements
Sampling of flue gas elements from the entire boiler can be divided into three groups:  Sampling of flue gases from the bottom part of the fluidized bed.

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Sampling of flue gases from the boiler second pass up to the exit to the chimney.

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Sampling of flue gases from the boiler furnace, cyclones and cyclone link channels.
To monitor the fluidized bed boiler operation process O 2 , CO, CO 2 , NO X , and SO 2 measurements can be taken.Other elements are usually monitored up to the exit from the separator of solid particles in front of the chimney.Fig. 5. illustrates a cooled sampling probe that might be used to take flue gases samples.The probe has an identical construction to that used for temperature measuring.During exhaustion gas is rapidly cooled down (from 800 o C to approx.30 o C in cooled probe) so that there is no reaction with any other flammable waste gases.Gas is then sampled to be analyzed in the mobile laboratory.It is always recommended that a cooled probe is used to take samples from the furnace, cyclone and cyclone linking channels -Fig.6. Sampling of flue gases from the second pass of the boiler occurs at temperatures safely below 800 o C. Thanks to that temperature a larger part of the gaseous sample is not able to oxidize quickly and thus it is possible to use a sampling tube made of stainless steel or sintered corundum, Al 2 O 3 .To set concentration (e.g., SO 2 ), sampling channels must be heated up during the sampling operation so that no reaction with water occurs.The measurment results suggest that there is intensive suppression of NO X formed increases in areas of secondary and tertiary air supply.CO concentration has developed as expected.
The concentration decreases if the secondary air supply is gradual.

Solid particle concentration measurements
To determine the solid particle concentration in air flow, the Czech standard ČSN ISO 9096 needs to be observed.It is gravimetric determination of concentrations based on isokinetic sampling of solid particles from air flow.In the case of fluidized bed chambers the aim is to determine the solid particle concentration in the lower part of the fluidized bed layer and in the boiler furnace.For a bigger or smaller particle separation, the fluidized layer density varies depending on furnace height as well.The density in the lower part is in the range of 500 -800 kg.m -3 , in the upper part of the furnace with the circulating layer the range is 0.1-0.5 kg.m -3 .Pressure in the fluidized layer is always measured for various height levels.The acquired pressure data are continuously monitored by operation measuring instruments.To determine the solid particles concentration in flue gases, the gravimetric method with solid particles isokinetic sampling can be used.Fig. 9. illustrates the measurement unit for solid particle isokinetic sampling.To determine the concentration at the measuring points, a disposable sampling probe is used.Another option would be a sampling probe with cooled support as illustrated in Fig. 10.This probe was developed to measure solid particles through an opening in the membrane wall flag.The sampling device touches the cooled parts only with smaller part to prevent gases from cooling below the dew point during the sampling process.The results of measurement of the solid particle concentration are illustrated in Table 5. for the 100 %, 70 % and 40 % nominal outputs for 15 % biofuel and 90 % lignite.Table 5. Determination of the circulation number at different boiler outputs

Co-combustion of coal and solid waste fuels
Substitution of conventional fossil fuels (like bituminous coal or lignite) by low-carbon fuels for the energetic use is an efficient and cost-effective means of meeting the Kyoto Protocol establishing greenhouse emission targets for each of the participating developed countries (related to their 1990 emission levels).Considerable reductions of CO 2 emissions can be achieved by combustion of waste; therefore combustion of waste materials of various origins (industrial, agricultural etc.) or their co-combustion with fossil fuels in fluidized bed boilers became a legitimate alternative to conventional coal combustion.Another reason why particular attention is paid to energetic utilization of wastes is also elimination of waste and minimizing costs of waste deposition.(Loo & Kopperjan 2008).
But there are still challenges to be solved such as behaviour of the mineral matter during the wastes' combustion.Although the elemental behaviour during coal combustion has been studied and described in detail, the works dealing with redistribution of elements during waste combustion are quite rare, nevertheless, the conclusions described in these works are rather analogous -the application of the results obtained for the coal combustion on the combustion of wastes is not possible since the character of these materials is quite different.(Bartoňová at al., 2008).Another problem is that even if the waste materials differ from one another in their characteristics and content of toxic elements, most works only focus on wood and bark combustion.
This chapter intends to shed more light on the spectrum of alternative fuels used for energy production focusing on the evaluation of the effect of co-combustion of waste fuel and coal on the environment.In the circulating fluidized bed power station in Tisová -350 t/h -Table 4., the waste alternative fuel (WF) containing plastics (1-20 %), fabric and carpets (45-75 %), rubber (5-15 %), paper (1-10 %) and wood (1-10 %) was co-combusted with the coal and the limestone.The samples of coal, limestone, bottom ash and fly ash were collected at regular time intervals and unburned carbon particles were separated from bottom ash by hand.Analysis of major, minor and trace elements was performed by X-ray fluorescence spectrometry (SPECTRO XEPOS) and mineral analysis was carried out using X-ray diffraction analysis (BRUKER D8 ADVANCE).Ash content of the samples was determined at 815˚C.The distribution of macro pores was determined by means of mercury porozimetry (Micromeritics -AUTOPORE IV); SORPTOMATIC 1990 (Thermo Finnigan) equipment was used for the determination of specific surface area and mezopore-size distribution.Scanning electron microscope micrographs were taken by SEM PHILIPS XL -30.

Mineral analyses
The X-ray diffraction patterns were obtained for the samples of unburned carbon (UC), bottom ash (BA) and fly ash (FA) Fig. 11.With the aid of elemental analyses of unburned carbon and ash samples the major mineral phases were established in the diffraction patterns and are marked with abbreviations explained in the figure caption.Coal has already been given in indicating the dominant occurrence of quartz and kaolinite.Diffuse area observed in the unburned carbon diffraction pattern (approximately from 25˚ to 31˚) corresponds with semi-crystalline carbon phases.Somewhat lower crystallinity (broadened peaks) is evident also in case of magnetite and calcium hydroxide.Conversely, high-degree crystalline levels are represented e.g. by sharp peaks of quartz,

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Co-Combustion of Coal and Alternative Fuels 75 lime or anatase.The comparison of the diffraction patterns revealed nearly the same mineral composition obtained for both unburned carbons -the dominant mineral phase in both samples was quartz and minor occurrence of anatase was identified as well.The both bottom ashes showed the similar mineral composition as well -it was lime there that was the most abundant mineral phase and also minor amount of quartz, anhydrite and anatase was identified in these samples.The similar mineral composition was obtained also for both fly ashes where quartz was the most dominant mineral and where the occurrence of lime, anhydrite, anatase and calcite was of minor significance.Hence, it can be concluded that the addition of solid waste fuel to coal during the combustion did not change the mineral composition of both unburned carbon and the ash samples.(Bartoňová at al., 2009).

Chemical analyses
By means of X-ray fluorescence spectrometry the contents of major, minor and trace elements were determined in coal (C), unburned carbon (UC), bottom ash (BA), fly ash (FA) and waste alternative fuel (WF).These results as well as the ash contents in these materials are given in Table 6.The porosity of the coal and bottom ash is rather low, whereas unburned carbon shows highly-developed system of ruptures, pores and cavities leading to high porosity of this material.That is why unburned carbon is being studied in relation to its adsorption properties.

Surface morphology and pore-size distribution
The morphology of coal, waste fuel, unburned carbon and bottom ash grains was studied using scanning electron microscopy with the secondary-electron beam method.The surface structure of coal and waste fuel was determinated and the texture of a typical grain of unburned carbon and bottom ash is shown in Fig. 12, 13.A general view (with magnification of 50x) and a surface detail (with magnification of 1500x) are shown for each material studied.The surface texture shown in Fig. 12., 13. indicates that the porosity of unburned carbon collected at waste fuel co-combustion with coal is much better developed than that of unburned carbon when pure coal without waste fuel was combusted.But some caution is needed in such conclusions due to somewhat low representativity of one studied grain towards the average unburned carbon sample.Therefore pore-size distribution and specific surface area measurements were conducted in order to prevent misinterpretation when comparing adsorption properties of unburned carbon collected during pure coal combustion and during co-combustion of coal and waste fuel.Specific surface area of unburned carbon collected at pure coal combustion was 194 m 2 /g, whereas during co-combustion of the same coal with waste fuel the specific surface area of unburned carbon reached 297 m 2 /g, which is significantly higher value.This work was focused on the comparison of minor and trace elements behaviour during the co-combustion of coal and waste alternative fuels with the previous results regarding the combustion of the same pure coal in the same power station but without the added waste fuel.Elemental behaviour exactly in the combustion chamber did not change noticeably when waste alternative fuel was co-combusted with the coal.Even the most abundant elements in waste alternative fuel (related to coal) -Zn, Cl and Br -showed nearly the same behaviour.This observation can be explained through similar high volatility of these elements both in the coal and in the waste materials.(Bartoňová at al., 2009).Comparison of elemental contents in bottom ash and fly ash was performed to describe further behaviour of elements when leaving the combustion chamber.It was established that when waste fuel was co-combusted with coal, a slight shift towards the higher enrichment of most elements in fly ash (vs.bottom ash) was observed.This trend is the most significant in case of Zn, Cl and Br which are the very elements that were the most abundant in waste fuel (when compared to coal).Therefore it can be concluded that the elements showing high concentrations in waste fuel tend to concentrate in fly Specific surface area of unburned carbon collected at the test where waste fuel was co-combusted with the coal (297 m 2 /g) was significantly higher that that of unburned carbon from the combustion test without waste materials (194 m 2 /g).Comparison of pore-size distribution curves obtained for both unburned carbons revealed that unburned carbon collected during coal and wastes combustion contains larger amount of small pores, whereas macropores are more abundant in the unburned carbon form coal combustion without the waste alternative fuel.The unburned carbon collected at the co-combustion of the coal and wastes is undoubtedly of better adsorption properties.

Co-combustion of coal and waste wood
Biomass represents a lot of various materials, either waste materials or special energetic plants.Fuels based on wood biomass (sawdust, shavings, chips, tree-bark) can be used also for the production of high-quality biofuels, such as wooden briquettes and pellets, or can be co-combusted with coal.(Bartoňová at al., 2008).Average ash content of wood is about 1 -2 % and calorific value ranges from 11 to 18 MJ.kg - .Straw is another advantageous energetic source and its calorific value ranges from 17.6 to 18 MJ.kg - , ash content is about 5.3 -7.1 % and is often used e.g. in Sweden, Denmark or USA.(Loo & Kopperjan 2008).The disadvantages of this material are its huge volume and heat-exchanger fouling problems.
There are also other biomass materials used for the energetic utilization -various agricultural residues (green wastes, hull, shells, pruning, rice straw, rape residues, corncobs and stems, sugar cane trash, cassava rhizome) as well as growing energetic plants.(Winter & Hofbauer, 1997).This chapter mainly evaluates the environmental impact of fluidized bed combustion of different fossil and biomass fuels.Particular attention was paid to the comparison in the release of environmentally most significant molecular species -amount of solid coal combustion products and their leaching behaviour or emissions of sulphur and carbon dioxide.For this work the samples from circulating fluidized bed power station in Štětí -Table 4. -were collected.In this power station coal combustion and co-combustion of coal / wastes tests were performed in circulating fluidized bed boiler at 870°C.Simplified diagram of the combustion facility is given in Fig. 14.In this power station usually lignite is co-combusted with the wood waste (coming from the cellulose production).Usual lignite / wood waste ratio is 10 :1.

Combustion tests
Three combustion tests were performed -Regime I, II and III.In Regime I lignite and limestone were combusted (in weight ratio of lignite/limestone = 10:1).In regime II lignite, limestone, sawdust and tree-bark were combusted in coal/wood waste ratio of 1:1.76.In regime III wood, sawdust and wood chips were combusted in ratio of 1:0.21:1.(This combustion test was rather unusual because no bottom ash was created and the only solid output flow was fly ash).Mass flows of input and output materials (BA -bottom ash, FAfly ash, E,s -solid emission particles) and volume of gaseous emissions (V E,g ) are summarized in Table 8.The ash and water contents in these materials are given in Table 8. as well.Mass flows relate to undried samples.Proximate and ultimate analyses of input and output materials are given in Table 7.

Analyses of emissions
Emissions from combustion unit were analysed and CO, NO X and SO 2 were determined in flue gas, while As, Se, Cd, Hg and Pb were determined in solid particles captured on the filter in flue gas stream.The results of emissions analysis are given in Table 9.In the boiler mantle there are four holes into the combustion chamber.Using these holes as progressive sliping thermocouples to measure temperatures in the fluidized bed at different levels through the holes, where the proble was plugged, three samples of gaseous emissions and ash were collected directly from the fluidized bed.The sliping probe measured temperatures in the fluidized bed at the inlets.All combustion regimes were sampled from storage tanks of fuel and all four sections of the electrostatic precipitator.Furthermore, there was continuous measurement of emissions of NO X , CO, SO 2 in the flue gases (see Fig. 5.).The balance of fuel and combustible waste, the mass flow, moisture content and ash, as well as the mass flow bed ash (BA) and fly ash (FA), the volume of gaseous emissions (VE, g), the quantity of solid emissions, can be evaluated in Table 10.The summary of calculated values shows a relation between the input (m inp ) and output (m out ) data.The difference between the weights of the input current m inp and output current m out under regime III can be explained by the fluid in the boiler not "running" the whole regime III -cleaning ash from coal combustion and therefore part of the ash has gone into the output stream and so its weight is greater than the output current m out .The results confirm that burning wood emits less CO 2 to the atmosphere per unit of energy input than burning brown coal.The data listed in Table 12.show that the minimum content of SO 2 emissions (% S E ,g) is the combustion of coal with limestone (regime I).Absolute numbers of sulphur contained in the mass emissions (m s, E ), however, clearly demonstrate that by burning wood the amount of sulphur getting in the emissions into the atmosphere is about 10 times smaller than that of burning coal.This parameter is much more favourable for burning wood.The most significant results are summarized below:

Regime
 Mass balance calculations suggest that mass flow of inorganic matter produced per 1 GW of boiler output has dropped from 28 kg /hr.GW for lignite combustion to 0.7 kg /hr.GW when wood wastes were combusted.

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This observation is a source of many advantages relating to ash land-filling e.g.decreasing the amount of ashes produced during the combustion process will consequently result in decreased amount of toxic leachates, above all sulphates, and also the increase of pH (due to high amount of Ca-bearing minerals present in coal ash) will not be as significant). Mass flow of CO 2 produced during the combustion was related to 1 GW boiler output.0.20 kg /hr.GW was obtained for lignite combustion and it has dropped to 0.14 kg /hr.GW released when wood wastes were combusted. Sulphur emissions were also recalculated to 1 GW boiler output -sulphur emission flow calculated for lignite combustion (0.13 kg /hr.GW) was considerably higher than that obtained for wood wastes combustion (0.01 kg /hr.GW).
In conclusion -the results described above unambiguously suggest that the waste wood combustion produces lower amount of environmentally-hazardous pollutants than fossil fuel combustion, even if combusted with Ca-bearing additives.(Klika 2010).

Co-combustion of coal and sewage sludge
The sewage sludge is a heterogenous mixture of organic elements (both live and lifeless microorganizm cells) and incorganic elements.The organic part of the sewage sludge is mainly represented by the proteins, sugars and lipids.The inorganic part susteins mainly of the compounds of silicon, ferrum, calcium and phosphorus.Morover the sludge consist of a wide range of substances as well -heavy metals, persistent organic elements PCB, PCDD/F, PAU etc. and other organic harmful elements.The Table 13.illustrates the summary of the organic polutants in the sewage sludge dry residues taken from the Central Sewage Plant of Ostrava (CSPO) and it is evident that almost all limits of the monitored polutants are exceeded.Such high values prevent the sewage sludge from being used for agricultural purposes and land reclamation -necessitating the usage of both the underground and exterior storage.The biggest problem in this case is the high content of the polyaromatic hydrocarbons that is ten times higher than the allowed limit.It is probably because of the industrial waste-water disposal.The value of TOC (Total Organic Carbon) that does not fit can rather be considered a useful than limiting factor.The energetical content of the sewage sludge is based on the chemical energy of the organic components that are capable of oxidation.To be able to describe the sewage sludge as fuel a material that converts its primary energy into the thermic energy -the condition of being flamable must be met.To make the combustion process balanced it is necessary to achieve fuel efficiencies from dry sludge residues and other heat distributed to the furnace, making it possible to use the water vaporization heat contained in fuel, the heat needed for the superheating of the water vapours in the waste gases and the heat needed for the waste gases heating.The important criterion of keeping the combustion process balanced is thus the water ratio in the sludge.Thus a problem exists because water ratio of the mechanically drained sludge is high (cca.60 -80 %) for the relatively low fuel efficiency and therefore the sludge cannot be combusted by itself.The most important energy characteristic of each single fuel is its efficiency.The dry residue efficiency of the anaerobicly stabilized sewage sludge is in the range of 7 -10 MJ.kg -1 .Fig. 15.shows the sewage sludge structure. indicator

The combustion test description
The combustion test with the mechanically drained digested sewage sludge (the water proportion in the sludge was approx.63 %) was carried out at circulating fluidized bed power station in Třinec with an output of 130 MW t -Table 4. The mixture of hard energy coal and the coal sludge of average efficiency Q i r = 19 MJ.kg -1 , water ratio W r = 7,5 %, ash content A r = 30 % was combusted at the fluid boiler.During the combustion test the fuel was distributed to the boiler in the ratio: 11 % weight -sewage sludge from the Central Sewage Plant of Ostrava, 28 % weight -energy coal and 61 % weight -coal sludge.During the additional combustion of the sludge the mixture characteristics changed as follows: heating value Q i r = 17 MJ.kg - , water ratio w r = 14,5 %, ash content A r = 28 %.Based on the fact that the total heating value of the fuel mixture thus dropped by cca 2 MJ/kg during the additional combustion, the volume of the mixture must be enlarged by approx.0,65 kg.s -1 to maintain the constant boiler output.However the total coal consumption does not raise and this fact is important.The description of the combusted fuel is illustrated in Table 14., 15., and 16.
The glory-angle of the mixture was rapidly changed for the worse, compared to the hard coal.The chain feeders of the crude fuel worked reliably and had no failures.Thanks to the mixture passing through the chain feeder the big pieces of the sludge were crushed.The combustion test showed that 15 % of the sludge content in the mixture was the cut-off amount able to pass the swing-hammer crasher.The moisture was of a fundamental importance concerning the allowable amount of the sludge in the mixture.During the combustion test the sludge moisture was approx.65 % compared to the hard coal moisture of 7, 5 %.The higher moisture makes the temperature drop behind the crasher and it results in sealing the crasher with the mixture of the wet mud.If we focus on the operational efficiency of the boiler under the condition of the additional combustion of the sludge it it is advisable to monitor unwanted states like rust formation caused by high and low temperataure and silting the heat transfer surfaces and abration.In the case of boilers with the fluid bed and the additive desulphurisation, the marks of the chlorine desulphurisation.This reduction in SO 2 content in the waste gases is obtainable only under the condition of additional combustion of the sewage sludge.The additive gets into the sewage sludge during the process of the sludge hygienisation by the lime dosing at the sewage plant.Furthermore it is hydrated on Ca(OH) 2 by the sludge humidity.The lime hydrate is rid of CaO while entering the fluidized bed boiler.The CaO then reacts with the SO 2 and the CaSO 4 .The amount of the additives in the sludge lowers the lime stone consumption as the primary source (Szeliga 2008).The analysis of heavy metals and microelements in the combusted fuels (the energy coal, the coal sludge, the sewage sludge) were carried out in the laboratories of The Technical University of Ostrava.The evaluation was made for the single coal combustion and then for the mixture of coal and sludge.The redistribution of heavy metals and microelements during the additional combustion of the sewage sludge to the combustion hard residues and the emissions are a matter of further research.The combustion test proves there are further opportunities for additional fuel combustion in the fluidized bed boilers.The advantages of this kind of sewage sludge usage are mainly in the reliable decomposition and oxidation of the organic harmful elements and significant sludge volume reduction.Another suitable way of using the sludge is to reduce its humidity, which improves fuel efficiency, transport and manipulation.The disadvantage of the thermic usage of the sewage sludge is higher concentration of heavy metals and microelements entering the combustion equipment.

Crude
Co-combustion of coal and sewage is possible only if there is appropriate content of heavy me tals in the sewage sludge entering the combustion process.The monitoring and analyses of heavy metals in the sewage sludge are nessesary.

Findings
The most important findings from the research can be summarized as follows: 1. Stability of combustion depends on two factors: a) regular and uniform feed regulation of the fuel mixture, b) perfect homogenization of the fuel mixture.Otherwise, pulsation in the furnace can occur.2. Experience with the combustion of sewage sludge showed that the highly volatile matter contents significantly affect the overall combustion process.Care must be taken to achieve complete combustion of the volatiles to ensure higher combustion efficiency and low emissions of CO, hydrocarbons and PAH (polyaromatic hydrocarbons).3.During devolatilization the biomass undergoes a thermal decomposition with subsequent release of the volatiles and formation of tar and char.The results show that the quantities of char and gas formed depend on the type of carbonised material.Furthermore, increasing the pyrolysis temperature leads to a decrease in the quantity of char formed and an increase in the quantity of volatiles.Analyses of the compositions of the volatiles from straw and stover as well as from wood chips and sewage sludge show that CO, H 2 , CO 2 and CH 4 are the main gaseous components.High moisture contents have been found to increase the devolatilisation time.For dry residues, in addition to the expected immediate ignition and the highly volatile matter contents, the volatiles consist mainly of combustibles -CO, H 2 and C x H Y .4. The composition of the ashes from sewage sludge, coal, peat and wood influences melting point.It is known that the Na 2 O contents of the residues are low and comparable to those of sewage sludge, wood, peat and coal.The K 2 O content of the fuel ashes on their melting points is well demonstrated.5. Combination of low flow rate and high temperature causes the particles, which are coated with fuel ash, to contact each other and form weak physical bonds or to agglomerate.The formation of these weak bonds or agglomeration is due to the surface of the particles having a low eutectic point or ash softening temperature.This low value is caused by the high alkali content, specifically sodium and potassium compounds, formed during combustion of the boiler fuel.The agglomerated particles, subjected to high temperatures, then begin to sinter or stick together through bond densification thereby forming a strong physical and chemical bond.6. Agglomeration begins when part of the fuel ash melts and causes adhesion of bed particles.Beginning of agglomeration in the fluidized bed is often indicated by occurrence of temperature differences in the bed and the presence of large fluctuation of bed pressure.When the feeding of the fuel continues it eventually leads to a defluidization of the whole bed.7. To rate the propensity of fuels against fouling, the alkali index has been developed.This index relates the mass of alkali metal oxides K 2 O + Na 2 O produced with ash to the GJ of energy generated thermally and may be used for biomass feedstock.Above 0,17 kg alkali/GJ fouling is likely and above 0,34 kg/GJ fouling is virtually certain to occur.8. Ash deposition from biomass fuels which contain certain chemicals can also create corrosion and erosion of metals.Two most abundant inorganic elements are Si and K that form silicates with a low melting point.The combustion leads to the condensation of molten silicates which are likely to cause fouling and corrosion.Analyses showed that corrosive reactions occur between chemical compounds in the ash particles and the elements in the metal on un-cooled samples at gas temperatures near 650 o C. 9. Solutions for the problems resulting from the low melting points of the ash are: use of additives, use of alternative bed materials in the case of fluidized bed combustion and blending of biofuels with coals, lignite.10.There are three routes of formation of NO X during coal combustion, namely: thermal, prompt and fuel -NO x .Biomass has high contents of volatile matter and low contents of fixed carbon, so that the effect of char on formation of NO X and N 2 O may be significant.However, the catalytic effect of the ash could be important for residues which have high CaO contents.11.Concentrations of heavy metals are in compliance with environmental directive of EU2000/76/EG (including cancerous harmful components and benzopyren +Cd+Co+Cr+As).Combustion of alternative fuels and coal has no significant influence to leaching and Ph factor.

Conclusion
Since 1996 29 large fluidized bed boilers with desulphurization ability during combustion process have been launched in the Czech Republic.The differences in design and various concepts of these units have helped collect a lot of valuable data and gain a great deal of experience.The opportunity had not existed before these units were constructed.
Because the boilers for co-combustion of coal and alternative fuels in the Czech Republic were developed from know-how of foreign suppliers, it was not possible to get familiarized www.intechopen.com Co-Combustion of Coal and Alternative Fuels 89 with the technical parameters until they started operation.The first operation hours of the most of these boilers were affected by the typical characteristics of Czech coal.Highly abrasive ash matter, high humidity, clay impurity of the fuel and higher content of other elements in the raw fuel (stone, wood, metal) made it necessary to modify fuel feed channels, crushers, separating plants and fuel intake to fluid channels.Many times before, these problems resulted in total unit reconstruction or even replacement.Frequent fuel supply discharges led to reduction of durability of heavy linings of the combustion chamber, especially cyclone bricking and chutes under the cyclones.Some problems were caused by ash extraction from fluidized layer, its cooling down, granulometrics finishing and further manipulation.Other problems occured in sintering fluidized particles when combustion temperatures were well below 900 o C. In spite of this mass sintering happens in various parts of the boiler.Last but not least, there is a trend to reduce desulphurization costs if the molar ratio Ca/S is in the range of / from 2.5 to 3, which means higher operation costs compared to wet tailings.
A quite new area of fluidized bed boilers is the combined combustion of coal and alternative fuels or the co-combustion of assorted fuels from renewable sources.Despite some slowdown in the expansion of activities in energetics, there are further projects in the area of applied research focused on operation process optimization, efficiency improvements and operation costs minimization.These are the areas where the information obtained from measurement results in various boiler types can be used.

Fig. 5 .
Fig. 5. Probe for flue gas sampling from fluid layer Fig. 6. illustrates a sampling probe used in the detailed net measuring of O 2 concentration in the boiler combustion chamber with a steam output 125 t/h.(15 % biofuels and 90 % lignite coal).

Fig. 6 .
Fig. 6.Probe for flue gas sampling from furnace of boiler A grid method of measuring O 2 , CO and NO X concentration was used.Measurements were taken using instrumentation openings in the middle of the side walls of the combustion

Fig. 8 .
Fig. 8. Average CO and NO X concentration distribution along the height of the combustion chamber of the boiler at 60 % nominal output in Power station Tisová.

Fig
Fig. 9. Diagram of measurement unit for solid particle isokinetic sampling.

Fig. 15 .
Fig. 15.The structure of the sewage sludge from CSPO

Fig. 16 .
Fig. 16.The scheme of the distribution of the fuel to the CFB boiler

Table 1 .
The analysis of the fuel mixture Fig. 2. CFB boiler 300 kW

Table 4 .
Newly-built fluid boilers with circulating fluid layer in the Czech Republic

Table 7 .
Mass and volume flows of input and output materials

Regime I Regime II Regime III Lignite Lignite Sawdust Tree-bark Wood Sawdust Wood chips
Input mass flow of carbon converted to carbon dioxide (CO 2 ): I -28 kg/h.GW and II-12 kg/h.GW, Table11.-where the index corresponds to the C carbon in coal -are then calculated giving all the input flows of carbon-converted CO 2 .For simplicity it is assumed that all the carbon is burned and transferred to the emissions in the form of CO 2 .

Table 9 .
Analysis of emissions for regimes I, II , III and emission limits

Table 11 .
Calculation of the incoming flow of inorganic materials for 1 GW of power boilers

Table 12 .
Output flows of sulphur (S)

Table 13 .
Organic polutants in sewage sludge

Table 14 .
The fuel characteristics in crude and waterless form

Table 17 .
The content of the combustible carbon in the combustion products corresponds with the fine hard coal combustion.

Table 17 .
The boiler efficiency η k and the combustible matter content in the ash, where C LP indicates the combustible matter content in the bedding ash and C UP indicates the combustible matter content in the ash