Article pubs.acs.org/est
Nitrogen Conversion in Relation to NH3 and HCN during Microwave Pyrolysis of Sewage Sludge Yu Tian,†,‡,* Jun Zhang,‡ Wei Zuo,‡ Lin Chen,‡ Yanni Cui,‡ and Tao Tan‡ †
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (SKLUWRE, HIT), Harbin 150090, China ‡ School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin, 150090, China S Supporting Information *
ABSTRACT: The nitrogen conversions in relation to NH3 and HCN were investigated during microwave pyrolysis of sewage sludge. The nitrogen distributions and evolution of nitrogen functionalities in the char, tar, and gas fractions were conducted. The results suggested that the thermal cracking of protein in sludge produced three important intermediate compounds, including the amine-N, heterocyclic-N, and nitrile-N compounds. The deamination of amine-N compounds resulted from labile proteins cracking led to the formation of NH3 (about 7.5% of SS-N) between 300 and 500 °C. The cracking of nitrile-N and heterocyclic-N compounds in the tars from the dehydrogenation and polymerization of amine-N generated HCN (6.6%) from 500 to 800 °C, respectively. Moreover, the ringopening of heterocyclic-N in the char and tar contributed to the release of NH3 accounting for about 18.3% of SS-N with the temperature increasing from 500 to 800 °C. Specifically, the thermal cracking of amine-N, heterocyclic-N and nitrile-N compounds contributed to above 80% of the total (HCN+NH3) productions. Consequently, it might be able to reduce the HCN and NH3 emissions through controlling the three intermediates production at the temperature of 500−800 °C.
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INTRODUCTION The rapid increase of sludge production generates a real challenge in the field of biological wastewater treatment. For the energy recovery from sewage sludge, pyrolysis, as an alternative promising option, has raised increasing attentions in recent years.1−4 The pyrolysis of sewage sludge is a process in which the organic materials in the sludge are transformed into biogas (noncondensable), biotar (condensable volatiles) and biochar residue in the absence of air. Biogas can be considered as gas fuel, whereas the biotar can either serve as liquid fuel or as raw material for chemicals.2 Biochar, which has an inherent energy value, can be burnt as fuel or be upgraded to porous adsorbents.2,3 It is believed that pyrolysis can reduce the volume of sewage sludge by 93% and recover energy of sludge up to 80%.5,6 Although pyrolysis indeed enjoys many obvious advantages, it should be noted that there is a release of nitrogen-containing gases during the pyrolysis process. The nitrogen content in sewage sludge was reported to be higher than 3 wt.% (daf) in comparison with that in coal (
