Promoting effects of polyacrylamide on ignition and combustion of Al/H₂O based fuels: Experimental studies of polyacrylamide aqueous solution flash pyrolysis

Abstract

In this work, the flash pyrolysis of polyacrylamide (PAM) solution in furnace-type pyrolyzer with an initial normal pressure in argon was investigated at 300, 450 and 600 °C, respectively, combined with off-line analysis. Gas chromatography tandem mass spectrometry (GC/MS) was employed to detect the volatile products. Some important common peaks in the mass spectra of the PAM pyrolysis products were discovered, and about 20 pyrolysis products were identified. Among all these chemicals, some active compounds were confirmed to exist for the first time, which should be of importance in improving the ignition of Al/H₂O/PAM based fuels. Moreover, the flash pyrolysis residues were analyzed by FTIR. The results indicate that O radical can accelerate the pyrolysis of PAM and result in the formation of some special chemicals. The internal interaction mechanism of PAM solution is discussed from the aspect of reaction pathways.

Introduction

Polyacrylamide, simplified as PAM, is one of the most widely used polymers, and has the chemical structure as shown in Scheme 1. Due to its excellent water-soluble characteristic and relative high viscosity of the solution, polyacrylamide has the important application as a kind of wastewater treatment polymer. It is also used as a flocculant, thickener, cross-linker, reducer of hydraulic resistance in liquids, and oil liquefier [1], [2], [3], [4], [5].

In recent years, portable electronic devices (mobile phones, notebook computers, palmtop computers, etc.) are becoming more power-demanding, which leads to the rapid development of fuel cell systems with higher specific energy [6], [7]. Among variety of fuel cells, hydrogen fuel cells are considered to have many future applications. They have outstanding performances compared to direct methanol fuel cells (DMFCs), such as anti-poisoning for electrode, low toxicity, higher power density and double conversion efficiency [8], [9]. The traditional hydrogen fuel cells are mixtures, mainly made up of compounds with high content of hydrogen. The hydrolysis of these compounds will generate a great amount of hydrogen. It has been found that borohydrides of light metals (Li, Be, Na, Mg, Al) are excellent sources of hydrogen, which can easily react with water to generate hydrogen [10].

However, the practical solution strength is limited to 30 wt% borohydride, results in decreasing maximum H₂ yield from theoretical value 10.8–6.3 wt% [11]. Furthermore, the catalysts should be introduced into the mixture to help initiating reaction, which is difficult, particularly for portable electronics applications. Recently, this problem is overcome in combustion-based approaches to hydrogen generation, which require only ignition and no catalysts [12]. In these approaches, some light metals (aluminum or magnesium) and special substances (macromolecules or polymers), whose solutions are of relative high viscosity, are used as additives. The latter component can usually dissolve to give viscous solution, aiming to provide as a media for the light metals to uniformly disperse. To date, several low cost water-soluble polymers are used for this purpose, such as PAM and poly(ethylene glycol) (PEG). In addition, these combustion-based fuel cells have also been introduced to rocket propulsion system, as power supplies for both space and underwater vehicles.

Nowadays, the application of underwater propulsion has sparked worldwide interest [13], [14], [15], [16], [17], [18], [19]. Since the combustion of aluminum powders with seawater is extremely exothermic, and it is not necessary for water (oxidizer) to be carried onboard, the vehicles’ performances, i.e., speed, range and submersible depth, will be improved a lot compared to the traditional solid rocket ramjet engine. The previous researches have showed that the viscosity of the mixture in engines played a key role in the ignition step, as well as the combustion sustaining process [15], [16]. The reactions initiated by free radicals in solutions are very rapid, and therefore, one important problem must be addressed, which is whether these water-soluble polymers just act as a physical dispersant agent and pyrolysis through their own pathways, or also take part in the combustion and react with water to form some flammable substances. Few researches have focused on this subject. Therefore, it is of great importance to investigate thermal behaviors of polymer solutions.

The previous studies allow us to define different thermal stages of the thermal degradation of PAM. Thermal degradation of PAM is a two-stage process [20], [21]. The first stage mainly involves a deamination process and forms imides, similar to those observed on intermolecular or intramolecular deaminations in the pyrolysis of other amides [10], [11]. This polymer material is the precursor for the formation of a polyaromatic carbonaceous structure as a residue which undergoes pyrolysis to generate polymer backbone. Meanwhile, thermogravimetric kinetic computation was carried out by Yang, and the thermal decomposition reaction was found to have the activation energies of 137.1 and 190.6 kJ mol⁻¹ for the first and the second degradation stages, respectively, in a nitrogen atmosphere [20].

In a violent combustion process, e.g., explosion and propellants combustion, the reaction behavior of mixtures can approximate to a rapid-heating process that usually includes a one-step pyrolysis reaction occurring in the condensed phase behind the flame front where the temperature may soar over 600 °C within several milliseconds [22]. Thus, the study of flash pyrolysis of PAM solution could simulate the combustion situation to certain extent, and will definitely reveal the pyrolysis mechanism in some basic-formula propellants. Also, understanding flash pyrolysis of PAM solution is important for the post-treated burning in solid slurry disposal, where PAM acts as a flocculant for sedimentation of wastewater.

In this work, with initial atmospheric pressure, the flash pyrolysis of PAM solution was carried out at three different temperatures in argon. The gas and liquid pyrolysis products were detected and identified by pyrolysis-gas chromatography tandem mass spectrometry (Py-GC/MS), and the residue was analyzed by FTIR. The flash pyrolysis mechanism of PAM solution was also discussed in detail.