*3.1. Combustion Surroundings and Flue Gas Composition*

The pressure gradient in the reactor was investigated under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. Table 2 shows results of basic characteristic analysis of waste sewage sludge. In proximate analysis, waste sludge fuel contains much of volatiles and ashes. The calorific value was 3008 kcal/kg. In element analysis, carbon and hydrogen contents were 28.14 and 4.74, respectively. Chloride content was 530 ppm. In metal analysis, sludge fuel contained much of alkali metals and toxic heavy metals. Figure 2 shows the pressure and temperature profiles in the reactor under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. As shown in Figure 2a, the pressure gradient showed a typical pressure trend of the CFB at each point under both conditions of oxygen with nitrogen and oxygen with carbon dioxide. The decrease in pressure under the oxygen with nitrogen condition was higher than that under the oxygen with carbon dioxide condition. Table 3 shows the gas components of fluidization air. In a previous study, the drop in pressure under the oxygen with carbon dioxide condition with fluidized air, that consisted of carbon dioxide and oxygen, increased as the oxygen rate increased from 21% to 40%, because the kinematic viscosity of the fluidizing injection gas increased as the oxygen rate increased. The kinematic viscosity under the oxygen with nitrogen condition was larger than under the oxygen with carbon dioxide condition. The drop in pressure increased as the kinematic viscosity of fluidizing injection gas increased. As shown in Figure 2b, the temperature gradient in the reactor was uniform as the axis of height under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. In a previous study, the temperature gradient under the oxygen with nitrogen condition was relatively higher than under the 21% oxygen with carbon dioxide condition [9]. As shown in Table 3, the heat capacity of CO2 was much larger than that of N2. It caused the flame temperature of the 21% oxygen with carbon dioxide condition to be lower than that of the oxygen with nitrogen condition since the fuel sludge had much volatile materials. It caused delayed devolatilization and delay in the ignition time of sludge fuel under the 21% of oxygen with carbon dioxide condition because much of the carbon dioxide was utilized, instead of nitrogen, on FBC combustion. However, the temperature gradient under the 23% of oxygen with carbon dioxide condition was higher than that of the 21%

of oxygen with nitrogen condition. It was considered that the 23% of oxygen with carbon dioxide condition caused higher flame temperature than that of oxygen with nitrogen combustion, and devolatilization and total ignition time were fastly declined under the 23% oxygen condition. Table 4 shows the outlet gas temperature under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. Regarding flue gas temperature, the temperature was relatively higher under the oxygen with carbon dioxide condition than that under the oxygen with nitrogen condition. This was due to the fact that H2O and CO2 in the flue gas had larger heat capacity than N2 and O2, as shown in Table 3, and the temperature of the outlet gas, including more H2O and CO2, was larger than it was for typical air conditions. Furthermore, it was indicated that the sludge fuel had much volatiles, and the heat distribution from sludge fuel combustion was reached further along the CFB combustion system under the oxygen with carbon dioxide condition than that under the oxygen with nitrogen condition.


**Table 2.** Results of basic characteristic analysis of waste sewage sludge.

**Table 3.** Physical gas properties of fluid flow utilized as fluidization air.


**Table 4.** Flue gas composition and temperature during the air and oxy-fuel combustion.


**Figure 2.** The pressure and temperature profile from air and oxy-fuel combustion of waste sewage sludge: (**a**) pressure profile (**b**) temperature profile.

#### *3.2. The Behavior of Ash and Heavy Metals*

#### (1) Metal Composition of Bottom and Fly Ash

Sewage sludge consists of many volatiles, metal elements, and ash fractions. During sewage sludge combustion, metal elements undergo a series of reaction mechanisms, such as volatilization, nucleation, condensation, and coagulation for particulate formation [2,17]. These particulate formation mechanisms in bottom ash and fly ash under the oxygen with carbon dioxide condition were affected by different conditions in the combustion environment. Figure 3a shows the comparative concentration of alkali metals in the bottom ash and fly ash under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. The species of aluminum, calcium, and potassium were distributed mainly in the bottom and fly ash from sludge fuel combustion under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. The metal components tended to be associated with particle formation mechanisms by causing the growth of fly ash from sludge fuel combustion. In sludge fuel combustion, agglomeration and fouling are highly involved with calcium and alkali compounds, such as the chloride and sulfate species, which block the distribution of fluidizing air and heat exchange. Aluminum compounds are concerned with Cl-related corrosion, which arises mainly from the boiler of the sludge fuel combustion process. The Cl-related corrosion problem of Al compounds creates a serious problem for boilers and doubles the repair and operation costs of the sludge fuel combustion process. [18,19]. In bottom ash, the concentrations of alkali metals, such as aluminum, calcium, and potassium, under the oxygen with carbon dioxide condition were less than that under the oxygen with nitrogen condition. In fly ash, these metals showed a similar trend to that regarding the bottom ash under the oxygen with carbon dioxide condition. As shown, the concentrations of these metals in fly ash and bottom ash under the oxygen with carbon dioxide condition were lower than they were under the oxygen with nitrogen condition. It was indicated that the oxy-fuel combustion mitigated agglomeration, fouling, and corrosion problems from sludge fuel combustion, and was economically beneficial in terms of long-term operation of a sludge fuel combustion facility. In addition, oxy-fuel combustion for sewage sludge could contribute to a shorter ignition time of the metals and moderate particle growth in the bottom ash, which would temper de-fluidization by agglomeration, fouling, and corrosion in the CFB boiler. Figure 3b shows the comparative concentrations of heavy metals in the bottom and fly ashes under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. Chrome, nickel, copper and zinc were portioned mostly in the bottom and fly ashes from sludge combustion under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. Copper and zinc compounds in bottom ash and fly ash from sludge fuel combustion were less under the oxygen with carbon dioxide condition than that under the oxygen with nitrogen condition. In the oxygen with carbon dioxide condition, the gas circumstances were significantly different than under the oxygen with nitrogen condition. Lots of CO2 under oxygen with the carbon dioxide condition had larger heat capacity than N2 under the oxygen with nitrogen condition. Accordingly, CO2 tended to take more heat in the condition of oxygen with carbon dioxide than the condition of oxygen with nitrogen condition, which caused decrease in combustion flame temperature. Eventually, a lower adiabatic temperature for oxy-fuel combustion occurred and caused a delay in ignition time, which delayed particle growth and formation, which was the pathway for the nucleation, vaporization, condensation, and coagulation mechanisms of those metals. However, an ignition delay would be rapidly declined over 23% of oxygen injection with decreased carbon dioxide rate for oxy-fuel combustion. Due to decreased carbon dioxide injection rate, the adiabatic flame temperature of this condition would be higher than it was for the air and 21% of oxygen injection rate for oxy-fuel combustion. As shown figure, the concentrations of alkali and heavy metals showed different trends for air and oxy-fuel combustion. The concentration of alkali metals, such as aluminum, calcium, and potassium in the fly ash under this condition were lower than they were under air combustion. Regarding heavy metals, the concentrations of zinc and copper compounds in the fly ash under this condition were lower than they were from

air combustion. Cr and Ni compounds in both ashes showed a different trend from the previously mentioned metals, with greater increase in ranges of oxy-fuel combustion than in air combustion. It was indicated that particle size distribution of fly ash should be changed along the air and oxy-fuel conditions because the different combustion surroundings and adiabatic flame temperatures likely affected the particle formation mechanisms from each metal compound in air and oxy-fuel conditions. Based on the test results, it was determined that oxy-fuel combustion was more efficient than air combustion in terms of heat recovery and economical operation by mitigating agglomeration, fouling, and corrosion problems from sewage sludge combustion.

(2) Particle Size Distribution and Mass Fraction of Metals

As sewage sludge combustion generates a large amount of fly ash, an understanding of particle size distribution of fly ash is important because it mainly consists of alkali and heavy metals. It was hypothesized that particle size distribution would be affected by different surroundings during air and oxy-fuel combustion. Figure 4 shows the particle size distribution in fly ash from air and oxy-fuel combustion of sewage sludge. Particle size distribution from air combustion was mainly accumulated by coarse particles over 2.5 μm in size, whereas from oxy-fuel combustion, it showed each accumulation mode in fine particles below 1 μm in size, super-micron particles ranging in size from 1 μm to 2.5 μm, and coarse particles over 2.5 μm in size. Generally, fine particle formation was followed by a series of reactions of metal elements, such as nucleation, condensation, and coagulation. The sub-micron mode was depicted by each formation process, including direct vaporization of volatile metals and chemical reactions of refractory metal oxides. Refractory metal oxides (MOX), in particular, could be reduced to sub-oxides (MOX−1) by the reaction shown in Equation (1).

$$\rm{MO}\_{\rm{X}} + \rm{CO} \leftrightarrow \rm{MO}\_{\rm{X}-1} + \rm{CO}\_{2} \tag{1}$$

**Figure 4.** Particle size distribution in fly ash from air and oxy-fuel combustion of sewage sludge.

Sub-oxide metals were de-volatilized easily because of their low melting point, and then rapidly re-oxidized to gas phase to form fine particles, by a series of particle formation mechanisms, such as nucleation, volatilization, and condensation. The formation mechanism of a fine particle was dependent on fuel type, combustion temperature, and residence time. On the other hand, super-micron particles were formed from the following: (1) coalescence of the included mineral, which was not volatilized as a sub-oxide metal; (2) fine fragmentation of the excluded mineral; and (3) the solid to particle mechanism of inherent refractory metals that had high melting points. Coarse particles were formed from non-volatile mineral inclusions and char fragmentation. Figure 5 shows the mass fractions of the alkali and heavy metals mainly distributed in the fly ash from the combustion under the oxygen and nitrogen condition compared to the distribution under the oxygen and carbon dioxide condition. Aluminum, calcium and potassium were largely contained in the particle size between 1 μm and 2.5 μm under the conditions of oxygen with nitrogen and oxygen with carbon dioxide. In general, the size of the particles was directly generated from the metals inherent to sludge combustion. These metals were inherently refractory metals in fuel, and the particles were generated by the mechanisms of solid to particle, including those of inherently refractory metals. Fine particles were also generated by particle formation mechanisms, such as vaporization, nucleation, and condensation. The aluminum, calcium and potassium showed significant accumulation

trend between 1 μm and 2.5 μm from the combustion of oxygen with nitrogen and oxygen with carbon dioxide. Generally, potassium tended to be much volatilized, which generated fine particles below 1 μm by particle generation mechanisms, such as vaporization and condensation. On the other hand, potassium also formed super-micron particles, larger than 1 μm. It was considered that metal vapor would spread on ash materials by chemical mechanisms, combine to aluminum and calcium, and finally cause conglomeration of particles. Under the oxygen with carbon dioxide combustion, the portion of aluminum, calcium and potassium were better conglomerated as fine particles below 1 μm than under oxygen with nitrogen combustion. The portions of aluminum, calcium and potassium under the oxygen with carbon dioxide condition, between 1 μm and 2.5 μm in size, were a bit larger than those under the oxygen with nitrogen condition, whereas the portion of aluminum, calcium and potassium under the oxygen with nitrogen condition, over 2.5 μm, were larger than those under the oxygen with carbon dioxide condition. This was explained by the fact that the ignition time delay under the oxygen with carbon dioxide condition was more rapid than that under the oxygen with nitrogen condition, and fine particle formation from aluminum, calcium and potassium was elevated by physical and chemical reactions. Fine particle formation from heavy metals, such as chrome, nickel, copper, and zinc, was conducted more intensively under the oxygen with carbon dioxide condition than that under the oxygen with nitrogen condition. It was considered that the fine particle formation from the oxygen with carbon dioxide condition was more intensive than that from the oxygen with nitrogen condition. The fine particles could be formed by the chemical reaction of the metal oxides. In general, the metal oxides could be changed into sub-oxide compounds when the original metal oxide reacted with carbon monoxide. The sub-oxide compounds were easily volatilized because of low melting temperature. Finally, because of the large amount of carbon dioxide, the vapor of the compounds could be more intensively re-oxidized to generate fine particles under the oxygen with carbon dioxide condition than those under the oxygen with nitrogen condition. Fine particle formation from zinc and copper would occur by volatilization and condensation, because the melting temperature of the compounds was lower than the adiabatic flame temperature under both conditions. On the other hand, the melting temperature of chrome and nickel was much higher. Finally, chrome and nickel mainly formulated in coarse particles, over 2.5 μm, under the oxygen and nitrogen condition.

**Figure 5.** Mass fractions of alkali and heavy metals from air and oxy-fuel combustion: (**a**) alkali metals, (**b**) trace metals.
