Lunar Oxygen Production Plant: Specification Sheet

Olivier Dubois-Matra
polard@mail.utexas.edu
August 1995

Foreword

The purpose of the Oxygen Production Demonstration Plant (O2 plant) is to test in-situ one or two process(es) of oxygen production from lunar minerals in order to prove the possibility of large-scale production for a manned base (fuel and life support). Several processes are under study, and some of them are currently adapted for lunar environment [Gibson & all]. However, there exists no plan yet for a small, automated demonstration plant which could be the payload of a small lander. Therefore, considerable work remains to be done to design this device. The following figures are only a first rough estimation based on laboratory experiments. Accurate figures would required a complete design.

The processes considered here are based on the reduction of ilmenite at high temperature. Ilmenite (FeTiO3) is a common mineral in the lunar soil, and is the most likely source of lunar oxygen [Allen & all]. Other potential feedstocks are volcanic glass and basalt. The ilmenite can be reduced either by hydrogen [Gibson & all] or by carbon [Ramohalli & all]. The respective reactions are :

FeTiO3 + H2 ---> Fe + TiO2 + H2O
H2O ---> H2 + 1/2 O2

FeTiO3 + C ---> Fe + TiO2 + CO
CO ---> C + 1/2 O2

Since we got relatively few information on the carbon process, figures are given only for a H2-type plant. The reasons for the selection of ilmenite reduction can be found in the WORLD-M proposal. Other processes may be contemplate.

Objectives

The objectives of the O2 operation are :

The O2 plant is the main purpose for the mission. Most of the equipment of the lander will be involved in the production process. The rover is a key element : it allows the choice of several different samples, making results more representative, and make possible to collect them far from the lander, thus avoiding the inconvenient of a possible contamination/modification of the soil due to the engines.

Design

Collecting the samples.

Small samples (range 10-1000 g) will be collected by the rover. Its characteristics are not yet known. For comparison, here are the figures for the Mars Pathfinder's rover [Pathfinder] : A similar lunar rover needs an improved thermal protection because of the important temperature gradient. In the other hand, it doesn't need to be fully autonomous like its martian counterpart because of the short communication delay.

In case the rover would suffer a failure, a backup system should be on board the lander, like a simple drill to collect at least one sample.

Sizing and beneficiation

Reducing the sample into small particles (<1 mm) increases the total surface of the sample and thus enhances the reaction. Since 75 to 95 % of the particles in the soil are below that size, a simple screening would be sufficient to produce a suitable raw feedstock. Moreover, a crushing device is highly power consuming, and the lunar material is abrasive, causing maintenance problems especially for an automated process . In the other hand, particles larger than 1 mm are susceptible to be richer in ilmenite, making beneficiation unnecessary. But in our case, collecting more sample and screening it would be more economic [Mason].

Beneficiation itself consists in increasing the proportion of ilmenite in the feedstock. The ilmenite reduction with H2 required 30 to 60 % ilmenite in he feedstock [Mason]. Processes involve both magnetic and electrostatic separation (ilmenite has medium magnetic and conductor characteristics). An electrostatic device has been tested on true lunar soil with field strength up to 5 kV/cm2 [Agosto], but with ambiguous results [Taylor].

Process

The process described below is the reduction of ilmenite by hydrogen. A version using a fluidized bed for a continuous large-scale operation has been studied by a team at JSC [Gibson & all]. But a batch process seems more suitable for a small test plant [Williams], using 90 g-samples. This plant is described in the World-M report [Annarella & all]. Although the final design maybe somehow complex, we can identified some major element in term of mass and power :
  1. the sample is put in a vessel heated up to 1000 C, under a flow of H2, then a reaction occurs :

    FeTiO3 + H2 ---> Fe + TiO2 + H2O

    In the Williams' experiment, the vessel was a 2 cm*11 cm stainless steel cylinder electrically heated, which lead to 1475 W of electrical power. Of course, it will be far more interesting to use directly the solar energy. With a solar flux of 1358 W/m2, however, a direct heating is not possible : the temperature of a material with solar absorptivity a and infrared emissivity e is given by :

    T^4 = G*a/e*s [Larson]

    Where G is the solar flux and s is the Stefan-Boltzmann constant, 5.67*10-8 W.m-2.K-4

    For T = 1300 K, G required is 10 to 40 times the one available on the Moon. The upper value gives us, for a cylindrical mirror, a focal length of 1 m for a width of 20 cm [Wieder]. The mirror and the furnace must be installed on the top of the lander, and a tracking device should be used.

  2. he output of the vessel contains both H2 and H2O : water must be collected and electrolyzed, providing there is enough power available. Williams used a cooler to condense the water, a drier to absorb it produced and measure the quantity, then electrolyzed it.

    The cooler used air circulation, and the electrolysis cell had a consumption of 677 W. This is definitely not suitable for the test plant. It would be interesting anyway to test an electrolysis device on the Moon, since it will probably be used on a large scale plant. I propose two alternatives:

    1. stock the water vapour produced by the reaction in a tank, then measure the quantity.
    2. design a test electrolysis cell, using a vapour phase electrolysis concept. This operation is described in [Donitz]. It works with vapour a 1000 C, which is the exhaust temperature from the vessel, and requires half less power than a liquid phase electrolysis.

Measurement

The sample must be weighed before sieving and beneficiation, and just before the reaction in the vessel. The measurement can be made by an hygrometer, if the output is water, or by a mass spectrometer or a chromatograph if the output is O2. Detectable quantities must be 0.1 mg and above [Williams].

Integration

Other devices include the conveyors for the mineral during beneficiation, the tubing, the pumps, the sensors to monitor the element of the plant. Based on [Williams], the requirement for the plant and associated devices are :

ItemMass (kg)Power (W)
Vessel + mirror5 (?)Solar
Electrolysis Cell5<400 (?)
Pump3.577
Sensors.?86
Tubing1.5-
Beneficiation??
Rover + support14autonomous (?)
Conveyors??
Misceallenous (H2 tank, etc)10 (?)-

We do not include the possibility of having two processes running on the lander, which may share some element like the mirror (the carbon reduction process requires the same temperature) [Ramohalli]. Since there will be several experiments on different samples, all the elements must be reusable. Especially, the vessel should be emptied after each experiment and the tubing purged.

Interfaces

Below is a summary of the interfaces with the other subsystem. Some details :
  1. landing site: Although regolith and then ilmenite are common on the surface of the Moon, it will be more reliable to select a landing site suitable for a human occupation (not in the Highlands, for example).

  2. operation during lunar day An experiment on a sample may take a couple of hours, and we want to repeat it as many time as possible. The only limit seems to be the lunar night : the plant may not survive. Therefore, landing should occur as soon as possible in the lunar day.

  3. minimising site pollution There has been some concern on the possibility of a pollution of the site during the landing. Most of the propulsion systems exhaust contain some oxygen-based product. But according to [Fernini], the most susceptible species to be adsorbed is H2O, which is immediately released in the lunar atmosphere. However, we still don't know the thermal impact of the exhaust gas on the soil composition. It would be safe to design a landing sequence with the main engine cut off 10 m above the ground.

Input GroupsVariablesOutput Groups

Landing siteMission Planning

Operation during lunar dayMission Planning
PropulsionMinimising site pollutionPropulsion, Structure, NGC

Keeping plant almost horizontal after landingStructure, NGC
StructuresMax. mass and volume
PowerMax. Power available

Isolation of "hot" elements like furnace, cell, mirror, vapour produced.Thermal

Vibrations (screening, etc.)Structure

Tracking device for the mirrorStructure, Power, Computer

Sensors dataComputer, Com.

RoverStructure, Power, Computer, Comm., Sensors

References

Agosto W.N. : "Electrostatic Concentration of Lunar Soil Minerals" in Lunar Bases and Space Activities of The 21st Century, p. 453, W.W. Mendell, Editor, 1985.

Allen C.C., Bond G.G., McKay D.S. : "Lunar Oxygen Production : a maturing technology" in Engineering, Consruction and Operations in Space IV, American Society of Civil Engineering, 1994

Annarella & all,"Water and Oxygen Resources : a Lunar Discovery Mission", Universty of Texas, Fall 1994

Donitz W., Erdle E., Striecher R. : "High Temperature Electrochimical Technology for Hydrogen Production and Power Generation": in Electrochemical Hydrogen Technologies, Hartmut Wendt, Elsevier, New-York, 1990

Fernini I., Burns J. O. Taylor, G.J., and all : "Dispersal of Gases Generated Near a Lunar Outpost" in Journal of Spacecrafts & Rockets, Vol. 27, No. 5, Sept-Oct. 1990.

Gibson M. A., Knudsen C.W., Brueneman D.J., Kanamori H. : "Kinetic Interpretation of First Reactivity Experiments on Lunar Basalt Samples" in Engineering, Construction and Operations in Space IV (American Society of Civil Engineering, 1994)

Larson W.J., Wertz J.R. : Space Mission Analysis and Design, Microcosm, inc.,Torrance, CA, 1992

Mason W.L., "On the Beneficiation and Comminution of Lunar Regolith and Beneficiation and Comminution Circuit for the Production of Lunar Liquid Oxygen in Enginnering", Construction and Operation in Space III, American Society of Civil Engineering, 1992

MFEX: Microrover Flight Experiment Control Subsystem home page in http://robotics.jpl.nasa.gov/tasks/mfex /homepage.html, 1995

Ramohalli K. and all : "A Robotic Common Lunar Lander Concept in Support of the Space Exploration Initiative" in Space Exploration, Science and Technologies Research, The American Society of Mechanical Engineers, 1992

Taylor L.A., McKay : "Beneficiationof Lunar Rocks and Regolith : Concepts and Difficulties" in Enginnering, Construction and Operation in Space III, American Society of Civil Engineering, 1992

Wieder S. : An Introduction to Solar Energy for Scientists and Engineers, John Wiley & Sons, Inc., 1982.

Williams R.J. : "Oxygen Extraction from Lunar Materials : an Experimental Test of an Ilmenite Reduction Process" in Lunar Bases and Space Activities of the 21st Century, Lunar and Planetary Institute, 1985


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