Lecture 9

Question 1

Thermal processing - Objective

Answer 1

• thermal release of volatiles (mainly H2O) from regolith
• done in Low T (cold traps), Medium T , High T

Question 2

Low Temperature thermal processing

Answer 2

• lunar ice exists in cold traps at high latitudes
• concentrations presum. <10wt%
• Thermal extraction at 100-200K

Question 3

Medium Temperature thermal processing

Answer 3

• H2 released up to 80% at T <600°C
• CO & CO2 released at >700°C -> mineral decomposition, reaction of carbon-bearing phases
• N2 hard to distinguishe from CO (mass = 28) but may result from trapped gases
• H2S & SO2 (700-1300°C) are chemical reaction products, mainly from troilite (FeS)
• O2 liberated from Fe at > 1300°C

Question 4

High Temperature Thermal Processing

Answer 4

• Pyrolysis
• Vacuum thermal decomposition partially decomposes oxides to metals& oxygen
• No reactants & no limitations on feedstock
• Process was demonstrated on regolith simulants with a solar furnace (due to high energy requirement)
  
  • Losses by absorption, reflection, scattering need to be accounted for
  • simplest /most mass efficient: Fresnel lense

Question 5

Thermal properties of regolith - Thermal conductivity

Answer 5

• heat transfer by radiation
  
  • T
  • optical properties of fluid & particles
  • size of voids
• Heat transfer through inter particle contacts:
  
  • external forces
  • mechanical properties & morphology
• Smoluchowski effect: pressure dependence-> thermal conductivity decreases with decreasing pressure
  
  • particle diameter
  • mean free path

Question 6

Thermal Properties of regolith - Specific heat capacity

Answer 6

• ~200-1500J/kgK in relevant T range

Question 7

Heating concepts - overview

Answer 7

• Rotary dryer
  
  • direct type
  • indirect type
• Rotary kiln
• Fluidisation

Question 8

Heating concepts - Rotary dryer

Answer 8

• Main application is drying the feedstock
• T usually up to 800°C
• Direct heat type
  
  • Drying gases come in direct contact with the material
  • Flight blades lift material to improve drying process
  • Commonly used in mining & agriculture
• Indirect heat type
  
  • Drying gases don’t come into contact with material (avoids reaction)
  • Instead of gases, steam, water or oil may be used
  • heating is via radiation & conduction from the internal walls
  • Usually applied in processing food-grade materials, combustible materials

Question 9

Heating concepts - Rotary kiln

Answer 9

• Main application is heating & reacting the feedstock
• T usually up to 1300°C
• Materials & alignment of lining bricks is crucial for the operation
• Rotation speed & inclination determine residence time & quality of heat transfer
• Widely used for thermal processing of cement & lime, calcination & agglomeration of rock & minerals or pyrolysis of waste

Question 10

Heating concepts - Fluidisation

Answer 10

• Fixed bed
  
  • Increase of fluid velocity leads to an increase in pressure drop (Bernoulli)
• Point A -> fluidisation point, friction forces are overcome & fluidisation starts
• Fluidised bed:
  
  • increase of fluid velocity leads to increase in porosity but pressure drop remains consist
• Entrainment:
  
  • particles leave the bed (conveyance)
• Main influence parameters:
  
  • Particle diameter
  • Density difference solid-fluid

Question 11

Fluidisation - Groups

Answer 11

• Group A
  
  • Aeratable
  • small particle size &/ low particle density
  • easy to fluidise
  • controlled bubbling
  • preferred
• Group B
  
  • sandlike
  • fluidise well with vigorous bubbling action & bubbles that grow large
• Group C
  
  • cohesive
  • very fine powders
  • hard to fluidise
• Group D
  
  • spoutable
  • large &/ dense particles
  • difficult to fluidise with erratic behaviour
  • bubbling
  • spouting
  • channelling

Question 12

Rotary vs. Fluidised systems

Energy consumption, Thermal eff., feedstock flex., operational comp.

Answer 12

• Energy consumption: Rotating lower
• Thermal efficiency: Fluidise higher
• Feedstock flexibility: Rotating higher
• Operational complexity: Rotating lower
• Rotating is for heavy-duty, Fluidised is more efficient for uniform feedstock

Question 13

Types of Mining

Answer 13

• Thermal mining
  
  • sunlight chanelled into craters
• “Gradient Gas Dynamic” mining
• Subsurface Deposit mining
• rocket mining
  
  • fires half-second pulses -> fluidise regolith -> Magnetic separator -> electrostatic sorter -> gasses captured by cold-plates

Question 14

In-situ Heaters - contactless

Answer 14

• Optical
• Infrared
• Microwave
• Laser
• Ultrasonic ?

Question 15

In-situ Heaters - via contact

Answer 15

• Electrical/resistive
• capacitive
• chemical?
• friction?
• shock?
• Magnetic?