Pyrolysis study on cattle manure: From conventional analytical method to online study of pyrolysis photoionization time-of-flight mass spectrometry

Highlights

• The pyrolysis behavior of cattle manure was analyzed by PIMS and GCMS.

• Products mainly contained ketones, phenols and N-heterocyclic compounds.

• Maillard reaction had great effect on the pyrolysis process of cattle manure.

• A predicted generation pathway of major products was proposed.

Abstract

Cattle manure is a kind of abundant agriculture waste but potential renewable resource to be converted into fuel by pyrolysis. The pyrolysis behavior of cattle manure was analyzed by three methods in this work: thermogravimetric analyzer (TG), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and pyrolysis photoionization time-of-flight mass spectrometry (Py–PI–TOFMS). The pyrolysis process of cattle manure included four stages, dehydration stage, lignocellulose decomposition stage, lignin and protein decomposition stage, char and mineral matter decomposition stage. The products were mainly classified into six groups: ketones, aldehydes, phenolic compounds, acids, hydrocarbons and N-containing compounds. Based on the time-evolved profiles and temperature-programmed profiles, the characteristics of these major products were analyzed and discussed. Maillard reaction has great effects on the pyrolysis process of cattle manure because cellulose can easily react with protein to produce abundant Amadori products and then generate cyclopentanes and furanmethanol. In addition, some Amadori products and amino acids can also be cyclized to generate N-heterocycle compounds like pyrroles and pyrazines. The decomposition of lignin basic units with more methoxy groups require higher reaction temperature, and the removal of hydroxy and methoxy group on phenols mainly occurred at around 350 °C. Furthermore, primary decomposition pathways of cattle manure were also proposed and discussed.

Introduction

The depletion of non-renewable fuels and the escalation of environmental problems have made biomass energy attract increasing attention in the past decades [1]. Currently, the biomass energy accounts for around 80 % of the total renewable production [2]. One of the richest biomass resources in China is livestock manure, of which the fresh production has been over 2.1 billion tons since 2011 and would reach 3.7 billion tons by 2030, and cattle manure would be the main source [3]. Unlike most agriculture waste that can return to the fields, livestock manure cannot be used as fertilizers directly for containing potentially dangerous pathogens and nitrates [4]. Therefore, the excessive manure may cause serious environmental and food safety problems, provided that its utilization rates at present is only 0.9 % [5]. One promising way to utilize livestock manure is to produce biofuels and value-added products by thermochemical conversion methods [6,7]. Lopez et al. [8] classified biomass based on a Van Krevelen diagram and found that the protein and lipid in livestock manure showed lower O/C and higher H/C ratios than lignin and cellulose. Wang et al. [9] compared the properties of cattle manure and corn stover and found that cattle manure had higher H/C ratios and higher heating value on dry and ash-free basis, which could produce biocrude with higher HHVs than that produced from lignocellulose biomass [10]. Since pyrolysis is the fundamental of thermochemical conversion processes, a thorough understanding of the pyrolysis behaviors and mechanisms of livestock manures is necessary to improve its utilization efficiency.

In terms of the manure thermal decomposition, thermogravimetric (TG) analysis is a most commonly used technique to analyze the weight loss of feedstocks. Wu et al. [11] analyzed the thermal decomposition behavior of dairy manure and showed that the temperature range of 160–360 °C was the critical step where the most thermal decomposition occurred. Janković [12] found that the pyrolysis process of swine manure could be depicted by two-portion process model, including the process of hemicelluloses and fats/proteins decomposition as well as the process of cellulose and lignin decomposition. Our team previously has calculated kinetic parameters of cattle manure pyrolysis by different mathematical methods based on TG analyses at five various heating rates (from 10 to 50 °C/min) [13].

TG analysis can only provide the mass loss characteristics, thus the pyrolysis mechanism was further studied by product testing. A couple of studies have investigated the product distribution and pyrolysis pathways of livestock manure. Erdogdu et al. [14] researched the chemical composition of the bio crude from goat manure at temperatures from 300 °C to 600 °C and reported that the seven main component in bio crude. Jeong et al. [15] analyzed the oil products of swine manure pyrolysis by FTIR spectra and indicated the presences of phenols, alkyne, ketones, aldehydes, esters and aromatics in the bio-oil. Das et al. [16] analyzed the bio-oil derived by chicken manure pyrolysis and revealed the major components were fatty acids, N-hererocyclics, phenols, sterols, diols and alkylbenzenes. Kazi et al. [17] separated N-heterocyclics from the bio-oil generated in chicken manure pyrolysis and identified the composition of these N-heterocyclics detailedly, such as pyrazine, benzoquinoline and carbazole by Pyrolysis Field Ionization Mass Spectrometry (Py-FIMS) and Electrospray Ionization Mass Spectrometry (ESI-MS). Schniter et al. [18] decomposed chicken manure by Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and found that N- heterocyclics and aliphatics accounted for 42 % and 38 % of all compounds identified respectively and 1-methylpiperidine accounted for the largest proportion of N- heterocyclics was.

For a deeper understanding of pyrolysis mechanism, not only the pyrolysis products are needed but also the pyrolysis intermediates. Online photoionization mass spectrometric (Py–PI–TOFMS) is an effective method to analyze pyrolysis intermediates and products as a function of temperature and time. Compared with the conventional electron ionization method applied in mass spectrometers, photoionization mass spectrometric enables online analysis of the product chemical compositions and obtains the fragment-free mass spectra in real time [19].

Cattle manure is one of the most abundant and typical livestock manures, which contains little hemicellulose and lipid. This work was aimed to investigate the pyrolysis process and mechanism of cattle manure, using TG, Py-GC/MS and Py–PI–TOFMS technologies. The weight loss of feedstock, mass spectra, temperature-programmed profiles, and time-evolved profiles of typical intermediates and products were observed, and the reaction mechanism of cattle manure pyrolysis were elucidated.