Hydrothermal ethanol flames in Co-flow jets

Highlights

• Autoignition of hydrothermal flames in a co-flow burner are reported.

• Progression from downstream incipient flames to stable hydrothermal flames is observed.

• Flame luminosity and sooting variation with fuel to air ratio is reported.

• Spectral emissions for non-sooting flame zones show weak OH* signal at 310 nm.

• Observed spectral emission bands between 350 nm–540 nm are attributed to CO2*.

Abstract

Results on the autoignition and stabilization of ethanol hydrothermal flames in a Supercritical Water Oxidation (SCWO) reactor operating at constant pressure are reported. The flames are observed as luminous reaction zones occurring in supercritical water; i.e., water at conditions above its critical point (approximately 22 MPa and 374 °C). A co-flow injector is used to inject fuel (inner flow), comprising an aqueous solution ranging from 20%-v to 50%-v ethanol, and air (annular flow) into a reactor filled with supercritical water at approximately 24.3 MPa and 425 °C. Results show hydrothermal flames are autoignited and form diffusion flames which exhibit laminar and/or turbulent features depending upon flow conditions. Two orthogonal camera views are used; one providing a backlit shadowgraphic image of the co-flow jet and the other providing color images of the flame. In addition, spectroscopic measurements of flame emissions in the UV and visible spectrum are discussed.

Introduction

Proper waste management for long duration space missions has remained a long standing technical challenge for NASA’s mission planners due to the increased emphasis on resource reclamation and the cumulative volumes associated with any extended space exploration mission. Even short duration human space missions, such as the past missions of Skylab and Space Shuttle, and the current science missions carried out on the International Space Station (ISS), generate a considerable amount of waste. This waste is usually wet, voluminous, and biologically unstable with the major constituents comprising plastics (about 30% on average) and water (also approximately 30%) [1].

The water in the waste comes from a combination of food residues stuck to food pouches, hygiene wipes, and free liquids remaining in the drink pouches after consumption. It has been estimated that for a Lunar outpost each inhabitant generates between 6.8 kg to 9.6 kg of waste per day. Waste accumulation remains a significant problem and will require serious attention in the planning and design for the long duration space exploration missions currently envisioned; particularly with the Mars transit mission. In order to reduce the waste management system’s “total equivalent mass” to minimize re-supply missions, it is essential to move toward closure of the environmental control and life support system [2]. This drive toward effective closure will be enabled by technologies that allow resource reclamation from the air, water, and waste streams. In addition, regenerative systems such as those designed to grow plants for food will require extensive resource reclamation (e.g., carbon dioxide, water and plant nutrients) from bio-waste streams in order to be practicable.

Supercritical Water Oxidation (SCWO) is a process where organic compounds can be efficiently oxidized in water above its critical point at approximately 374 °C and 22 MPa. Under these conditions organic compounds and gases become completely soluble leading to extremely high reaction rates between dissolved oxygen and organic materials [3,4]. SCWO is often considered a “green” technology because of its ability to recover energy and reclaim water from wet waste streams without producing pollutants such as NO_x or SO_x, which require further scrubbing. The primary products of this oxidation mechanism are carbon dioxide and water, with the inorganic material precipitated out of solution as salt or converted into acids which can be neutralized in the effluent stream. SCWO is a promising technology [5] for processing solid entrained liquid waste streams since (i) pre-drying of waste is not required, (ii) product streams are benign, microbially inert, and easily reclaimed, (iii) organic waste conversion is complete and relatively fast, and (iv) with proper design and operation, reactions can be self-sustaining. In addition, because of the absence of inter-phase reactant transport due to the single phase nature of SCWO reactions, reaction timescales are greatly reduced and many of the complications associated with two-phase transport and processing in reduced gravity environments are eliminated, which is an advantage for space missions.

Hydrothermal flames were first described by E.U. Franck who noted that flames could be generated in supercritical water because of the high miscibility of hydrocarbons (in this case methane) and oxygen in the medium [6]. As such, a “hydrothermal flame” is a classification of flames that occur in conditions when an environment is largely comprised of water at supercritical conditions. Hydrothermal flames have been studied in batch and semi-batch reactors [7].

Historically, supercritical water oxidation (SCWO) technologies have depended on maintaining conditions in the SCWO reactor where spontaneous ignition of localized hydrothermal flames was suppressed and the complete oxidation of hydrocarbon wastes occurs at relatively low temperatures. It was recognized that these flames, if not properly controlled in reactors for which these conditions were not designed, would lead to accelerated thermal wear on reactor components, would enhance corrosion, and depending on the reactant stream, would result in increases in NO_x or other unwanted products [8].

Recently, however, a number of SCWO technologies and/or advanced reactor concepts have been proposed where controlled hydrothermal flames are used beneficially. This includes hydrothermal flames for thermal augmentation to initiate or sustain reactions or as a means of increasing conversion efficiencies for traditionally difficult waste streams, or for new applications, such as for hydrothermal spallation drilling [[9], [10], [11], [12], [13]].

Results reported in this study demonstrate the feasibility of spontaneously igniting and stabilizing hydrothermal flames in a SCWO reactor operating at constant pressure. Hydrothermal flames are observed as luminous reaction zones that occur when appropriate concentrations of fuel and oxidizer are present in supercritical water. A co-flow injector is used to inject fuel (inner flow), comprising an aqueous solution ranging from 20%-v to 50%-v ethanol, and air (annular flow) into a reactor filled with supercritical water at approximately 24.3 MPa and 425 °C. Two orthogonal camera views are used; one providing a backlit shadowgraphic image of the co-flow jet and the other providing color images of the flame geometry. In addition, spectroscopic measurements of flame emissions in the UV and visible spectrum are reported. The present work is an extension of the study presented in [14] on supercritical water jets to reacting jets and is complementary to experimental [15] and modeling [16,17] work on flame autoignition.