Temperature Influence and Distribution in Three Phases of PAHs in Wet Sewage Sludge Pyrolysis Using Conventional and Microwave Heating

Abstract

Pyrolysis of wet sewage sludge with a moisture of 86.47 wt % was conducted using electric heating (conventional pyrolysis, CP) and microwave heating (microwave pyrolysis, MWP) separately on two sets of batch reactors. Sixteen polycyclic aromatic hydrocarbons (PAHs) characterized by United States Environmental Protection Agency (U. S. EPA) as priority pollutants were determined by gas chromatography/mass spectroscopy (GC/MS) in gas, liquid, and solid phases of pyrolysis products to get detailed transmission characteristics of PAHs in sewage sludge pyrolysis. Temperatures of 400−900 °C and microwave power of 400−900 W were investigated. Production yields in MWP and CP behaved similarly for different power in MWP and for different temperatures in CP, whereas liquid yielded more than 75% of the total weight for the high content of water in wet sewage sludge. Liquid yield went through a high peak at 500 W for MWP and 700 °C for CP. Total yield of PAHs had trend of increase in MWP, whereas it peaked at 700 °C in CP. Naphthalene, fluorene, phenanthrene, and anthracene contributed most of the yield change, which can be explained by integration of the secondary pyrolysis of solid, decomposition of PAHs and synthesis of gas in different temperature. CP yielded more PAHs than MWP, which is more obvious in gas fraction than oil fraction. Distillation effect had great influence on PAHs distribution between gas and oil fraction. PAHs yield in solid for MWP decreased with increase of power, which might be attributed to the special effect of microwave on the microscope characteristic of solid, whereas PAHs in solid for CP were even lower and would cause fewer environmental problems. The results also indicated "nonthermal" impacts of microwave on the process.

生活垃圾焚烧残灰中有毒成分的排放特性

“…All the concentration values were based on the dry base of residual ash samples. The detailed pretreatment steps, including solution concentration, multistep purification, and volume measured, were according to the previous studies (Yan et al, 2010;Dai et al, 2014). Then samples in organic solution were injected into gas chromatography with electron capture etector (GC-ECD, 6890N, Agilent, USA) for PCBzs detection and gas chromatography with mass spectrometry (GC-MS, Scientific ISQ, Thermofisher, USA) for PAHs detection.…”

Strategies for producing biochars with minimum PAH contamination

“…with temperature (at least in the temperature range suitable for biochar production) [6,9,[11][12][13]35,36]. PAH yield at a particular temperature consists of PAH formation and destruction (conversion of PAHs into lighter hydrocarbons / gases [12,14] and condensation to form high molecular weight PAHs / pyrolytic carbon [8,9,13]). This concept is illustrated in Figure 3.…”

“…Consequently, the peak of PAH yield (Figure 3a) indicates the temperature where the highest accumulated yield increase is reached (slope change Figure 3b). At even higher temperatures, the PAH yield decreases until PAH destruction equals PAH formation (PAH yield is zero) and the maximum accumulated PAH yield is reached (Figure 3a, b) which is between 750-900°C [13,14,35,36].…”

“…Studies that dealt with PAHs and pyrolysis mostly investigated PAH formation and concentrations in pyrolysis liquids / gases [6,9,[11][12][13]. Recently, attention has shifted to PAH concentrations in pyrolysis solids because of the potential application of biochar to soil for soil improvement, -remediation and carbon sequestration [8,[14][15][16][17][18][19]. In order to avoid possible negative effects on soil ecosystems and to comply with environmental legislation, it is essential to produce biochars with low PAH The few systematic studies on dependence of PAH concentrations on pyrolysis conditions that exist provide different perspectives and no overall trend is observed [8,[14][15][16][17][18][19]22].…”

“…Recently, attention has shifted to PAH concentrations in pyrolysis solids because of the potential application of biochar to soil for soil improvement, -remediation and carbon sequestration [8,[14][15][16][17][18][19]. In order to avoid possible negative effects on soil ecosystems and to comply with environmental legislation, it is essential to produce biochars with low PAH The few systematic studies on dependence of PAH concentrations on pyrolysis conditions that exist provide different perspectives and no overall trend is observed [8,[14][15][16][17][18][19]22]. In Hale et al (2012) [16], the effects of highest treatment temperature (HTT -maximum temperature material is exposed to), residence time and feedstock was investigated by analysing 59 biochars, however, due to the highly variable technologies used for biochar production only limited conclusions could be drawn.…”