| Sludge, as a product of wastewater treatment, contains organic compounds, precipitates and heavy metals. Incineration has been considered as a promising method to treat sludge. Due to high moisture content and complex composition, sludge is usually combusted with coal. Oxy-fuel combustion technology has been considered as a promising technology for CO2 capture. Co-combustion of sludge and coal under O2/CO2 atmosphere could obtain dual benefit for sludge treatment and CO2 emission control. In this paper, experiments of co-combustion characteristics and migration characteristics of heavy metals with sludge and coal as fuels were carried out in a thermo-gravimetric analyzer and a fixed-bed reactor. The simulation on heavy metals speciation during co-combustion with sludge and coal was also performed using HSC Chemistry. The main conclusions are presented as follows:1) Co-combustion characteristics of sludge and coal in O2/CO2 atmosphere were investigated, the effects of mass ratio of sludge to coal (Xs), heating rate and incineration atmosphere on co-combustion characteristics of sludge and coal were analyzed. The obtained results indicated that the ignition temperature of the mixtures of sludge and coal decreases obviously with increasing mass ratio of sludge and coal, and the maximum weight loss rate of the test samples increases with the increasing of heating rate at 21%O2/79CO2 atmosphere. The burnout temperature of the mixture decreases from 708.3℃ to 599.8℃ while oxygen concentration increases from 21% to 50% at Xs=0.50, and the burnout time of mixture of sludge and coal is also shortened significantly. The co-combustion characteristics parameters of sludge and coal under 21%02/79%N2 atmosphere fall in those between 21%02/79%CO2 and 30%O2/70%CO2 atmospheres.2) The effects of mass ratio of sludge to coal (Xs), heating rate and incineration atmosphere on co-combustion kinetics of sludge and coal under the O2/CO2 atmosphere were analyzed. The obtained results indicated that a higher heating rate results in a higher activation energy (E) of the mixed sample (Xs=0.50) at a low temperature zone (<360℃), and the activation energy of the mixed sample is similar to that of sludge at the low temperature zone (<360℃). A higher heating rate resulted in a lower activation energy of the mixed sample (Xs=0.50) at a high temperature zone (>360℃). The activation energy of the mixed sample (Xs=0.50) increases obviously with increasing O2 concentration at the high temperature zone. The distribution of activation energy for the mixed sample (Xs=0.50) under 21%02/79%CO2 is the same as that under 21%02/79%N2.3) The migration characteristics of heavy metals during co-combustion of sludge and coal were carried out in a fixed-bed reactor. The effects of incineration temperature, mass ratio of sludge to coal (Xs) and incineration atmosphere on migration characteristics of heavy metals during co-combustion of sludge and coal were analyzed. The obtained results indicated that the release fraction of Hg is near 100% at the incineration temperature above 700℃. The incineration temperature can promote the release of As, Sn and Pb significantly. The release fractions of Zn, Pb, As and Sn increase with increasing mass ratio of sludge to coal. The release fractions of Zn, Pb, Sn, Cr, Ni and Mn in 21%02/79%N2 atmosphere is higher than that combusted in 21%02/79%CO2 atmosphere. A higher O2 concentration resulted in a higher release fraction of Pb, Cr and Ni in O2/CO2 atmosphere, however, the release fraction of As decreases with increasing O2 concentration.4) Based on HSC Chemistry software, speciation distribution of heavy metals during co-combustion of sludge and coal was simulated. The obtained results indicated that Hg, As, Sn and Cu are mainly existed as the forms of HgO(g), AlAsO4, SnO2, CuO·Al2O3 at incineration temperatures ranging from 700 to 1000℃, respectively. The incineration temperature has a significantly effect on the speciation of Zn, Pb, Cr, Co, Ni and Mn. The increasing of mass ratio of sludge to coal can promote Pb transfer from solid phase to gas phase, and affect the speciation of Co, Cr and Mn, however all of them are still mainly existed as the forms of solid phase. The speciation of heavy metals under 21%O2/79%CO2 is the same as that of under 21%O2/79%N2 atmosphere. The increasing of O2 concentration in O2/CO2 atmosphere can promote Pb transfer from solid phase to gas phase, and affect the speciation of Cr and Mn, but all of them are still existed as the forms of solid phase. |
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