**5. 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-1. Straw is another advantageous energetic source and its calorific value ranges from 17.6 to 18 MJ.kg-1, 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.

#### **5.1 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

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 ash. Specific surface area of unburned carbon collected at the test where waste fuel was co-combusted with the coal (297 m2/g) was significantly higher that that of unburned carbon from the combustion test without waste materials (194 m2/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

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-1. Straw is another advantageous energetic source and its calorific value ranges from 17.6 to 18 MJ.kg-1, 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 /

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

undoubtedly of better adsorption properties.

wood waste ratio is 10 :1.

**5.1 Combustion tests** 

**5. Co-combustion of coal and waste wood** 

output flow was fly ash). Mass flows of input and output materials (BA – bottom ash, FA – fly ash, E,s – solid emission particles) and volume of gaseous emissions (VE,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.

Fig. 14. Simplified diagram of the combustion facility

### **5.2 Analyses of emissions**

Emissions from combustion unit were analysed and CO, NOX and SO2 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 NOX, CO, SO2 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

Co-Combustion of Coal and Alternative Fuels 81

Input mass flow of carbon converted to carbon dioxide (CO2): I – 28 kg/h.GW and II-12 kg/h.GW, Table 11. - where the index corresponds to the C carbon in coal - are then calculated giving all the input flows of carbon-converted CO2. For simplicity it is assumed

 **Regime I Regime II Regime III Limits** 

**Input (t/h) Output (t/h)** 

**dust Bark Wood Chips Minp FA BA Mout**

**Input CO2 (t/h) mCO2,out/Qp**

**% SE,g ms,E/Qp**

CO [mg/Nm3] 17.0 19.9 7.50 250 NOx [mg/Nm3] 308 286 150 400 SO2 [mg/Nm3] 294 236 53.81 500 F- [mg/Nm3] - - 0.9 - As [mg/Nm3] 1.29 1.92 1.76 - Se [mg/Nm3] 0.10 0.15 0.13 - Cd [mg/Nm3] 0.19 0.17 0.25 - Pb [mg/Nm3] 0.79 0.85 1.06 -

I 4,743 1,733 - - - - 6,47 3,21 3,210 6,420 II 1,989 0,640 0,033 0,75 3,41 1,36 2,020 3,380 III - - 0,015 - 0,037 0,155 0,20 0,38 0,004 0,390

**Carbon Limestone Sawdust Bark Wood Chips mCO2,inp (t/h.GW)** 

**(kg/h.GW) FA BA ms,E ms,out**

I 45,46 0,90 - - - - 46,36 0,20 II 20,74 0,33 6,43 15,70 - - 43,20 0,16 III - - 3,73 - 21,74 18,01 43,48 0,14 Table 11. Calculation of the incoming flow of inorganic materials for 1 GW of power boilers

I 102 59 29,6 190,6 15,5 0,13 II 39.0 58.8 27,3 125,5 21,8 0,10 III 13,9 0,1 5,5 19,5 28,2 0,018

The results confirm that burning wood emits less CO2 to the atmosphere per unit of energy input than burning brown coal. The data listed in Table 12. show that the minimum content of SO2 emissions (% SE,g) is the combustion of coal with limestone (regime I). Absolute numbers of sulphur contained in the mass emissions (ms, E), however, clearly demonstrate that by burning wood the amount of sulphur getting in the emissions into the atmosphere is

**Output (kg/h)** 

that all the carbon is burned and transferred to the emissions in the form of CO2.

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

**Saw** 

**Coal Lime stone** 

Table 10. Mass flow of inorganic materials

Table 12. Output flows of sulphur (S)

quantity of solid emissions, can be evaluated in Table 10. The summary of calculated values shows a relation between the input (minp) and output (mout) data. The difference between the weights of the input current minp and output current mout 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 mout.


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


Table 8. Proximate analysis of lignite and wood wastes (related to undried samples) for regimes I, II, III.

quantity of solid emissions, can be evaluated in Table 10. The summary of calculated values shows a relation between the input (minp) and output (mout) data. The difference between the weights of the input current minp and output current mout 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

**Regime Input Output** 

mBA = 3,250 kg. hr-1 mFA = 3,170 kg. hr-1, mE,s = 0.42 kg. hr-1 VE,g = 201,130 Nm3.hr-1

mBA = 2,020 kg. hr-1 mFA = 1,360 kg. hr-1 mE,s = 0.57 kg. hr-1 VE,g = 234,830 Nm3.hr-1

mBA = 5 kg. hr-1 mFA = 396 kg. hr-1 mE,s = 0.16 kg. hr-1 VE,g = 205,340 Nm3.hr-1

Lignite (C): mC = 25,920 kg.hr-1 (W = 14.7 %, A = 18.3 %)

Limestone (L) mL = 2,630 kg.hr-1 (W = 0.45%, LOI = 34.1 %)

Lignite (C): mC = 11,840 kg.hr-1 (W = 16.4 %, A = 16.8 %) Limestone (L): mL = 970 kg.hr-1 (W = 0.45 %, LOI = 34.1 %) Sawdust (S): mS = 5,220 kg.hr-1 (W = 28.1 %, A = 0.64 %)

Tree-bark (B): mB = 5,620 kg.hr-1

Wood (W): mW = 14,905 kg.hr-1 (W = 12.8 %, A = 0.25 %) Sawdust (S): mS = 3,114 kg. hr-1 (W = 29.7 %, A = 0.47 %)

Wood chips (Ch): mCh = 14,870 kg. hr-1

**Regime I Regime II Regime III** 

C (%) 47.84 47.77 33.58 34.43 39.87 32.76 33.02 H (%) 3.98 4.05 4.14 4.26 4.91 3.92 4.03 N (%) 0.88 1.12 0.11 0.50 0.12 0.14 0.27 Stotal (%) 0.59 0.84 < 0.01 < 0.01 < 0.01 < 0.01 < 0.01 O (% ) 13.71 13.06 33.43 29.3 42.03 33.03 34.79 Wa (% ) 14.7 16.41 28.12 26.70 12.83 29.71 26.84 Aa (% ) 18.3 16.8 0.64 4.8 0.25 0.47 1.04

Table 8. Proximate analysis of lignite and wood wastes (related to undried samples) for

**Lignite Lignite Sawdust Tree-bark Wood Sawdust Wood chips** 

(W = 26.7 %, A = 4.8 %)

(W = 26.8 %, A = 1.04 %)

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

LOI value in limestone is thought to be CO2 released during the combustion

is greater than the output current mout.

I

II

III

regimes I, II, III.

Input mass flow of carbon converted to carbon dioxide (CO2): I – 28 kg/h.GW and II-12 kg/h.GW, Table 11. - where the index corresponds to the C carbon in coal - are then calculated giving all the input flows of carbon-converted CO2. For simplicity it is assumed that all the carbon is burned and transferred to the emissions in the form of CO2.


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


Table 10. Mass flow of inorganic materials


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


Table 12. Output flows of sulphur (S)

The results confirm that burning wood emits less CO2 to the atmosphere per unit of energy input than burning brown coal. The data listed in Table 12. show that the minimum content of SO2 emissions (% SE,g) is the combustion of coal with limestone (regime I). Absolute numbers of sulphur contained in the mass emissions (ms, E), however, clearly demonstrate that by burning wood the amount of sulphur getting in the emissions into the atmosphere is

Co-Combustion of Coal and Alternative Fuels 83

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

PCB (summary of 6 kongerens) [mg/kg] dry residue 0.2 0.3

zoomed 57 x zoomed 500 x zoomed 1000 x

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 MWt - Table 4. The mixture of hard energy

content Ar = 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 Qi

17 MJ.kg-1, water ratio wr = 14,5 %, ash content Ar = 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

r = 19 MJ.kg-1, water ratio Wr = 7,5 %, ash

r =

indicator – sample from the CSPO Limit value Rated value Benzen [mg/kg] dry residue 0.1 0.135 BTEX [mg/kg] dry residue 10 5.46 EOX (Cl) [mg/kg] dry residue 10 11.7 NEL [mg/kg] dry residue 200 4,840 ΣPAU (15) [mg/kg] dry residue 10 103

TOC [%] 20 25.3 Tetrachlorethen [mg/kg] dry residue 0.5 < 0.030 Trichlorethen [mg/kg] dry residue 1 0.233

range of 7 – 10 MJ.kg-1. Fig.15. shows the sewage sludge structure.

Table 13. Organic polutants in sewage sludge

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

coal and the coal sludge of average efficiency Qi

**6.1 The combustion test description** 

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:


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).

### **6. 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 harmful 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

about 10 times smaller than that of burning coal. This parameter is much more favourable

 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

 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)

 Mass flow of CO2 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

 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

In conclusion - the results described above unambiguously suggest that the waste wood combustion produces lower amount of environmentally-hazardous pollutants than fossil

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 harmful 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

for burning wood. The most significant results are summarized below:

/hr.GW when wood wastes were combusted.

/hr.GW released when wood wastes were combusted.

obtained for wood wastes combustion (0.01 kg /hr.GW).

**6. Co-combustion of coal and sewage sludge** 

fuel combustion, even if combusted with Ca-bearing additives. (Klika 2010).

will not be as significant).



Table 13. Organic polutants in sewage sludge

zoomed 57 x zoomed 500 x zoomed 1000 x

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