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Modelling Gasifier Heat Flow

Page history last edited by jim mason 3 years, 5 months ago

 

Summary of gasifier energy balance.

 

Some rough comparisons and calculations for 1 kg of biomass input, with proportional air and moisture amounts.  The details of derivation follows under the summary below.

 

Water in biomass heating loads

(atm liquid water to vapor at 100C = 2574 kj/kg / atm liquid water through vapor at 1000C = 4446 kj/kg)

Heating loads at various moisture contents (Dry basis)

     - 129/222 kj for water portion of 1kg biomass at 5% moisture (Dry Basis)

     - 257/445 kj for water portion of 1kg biomass at 10% moisture

     - 386/667 kj for water portion of 1kg biomass at 15% moisture

     - 514/890 kj for water portion of 1kg biomass at 20% moisture

     - 772/1,333 kj for water portion of 1kg biomass at 30% moisture 

     - 1,030/1,778 kj for water portion of 1kg biomass at  40% moisture 

     - 1,287/2,223 kj for water portion of 1kg biomass at  50% moisture (Dry Basis) / 33% moisture (Wet Basis)

 

Biomass/char/tar through pyrolysis and combustion

     - 1462kj/kg

 

Incoming air to oxidation reaction temp

     - 888 kj/1.6kg 

 

Losses to Vessel Walls:

      - @250-500kj at 15hp/hr

 

Heat available for mining in product gas output

     - 1560 kj/2.6kg of potential heat to mine

 

Heat available in the IC exhaust 

     - 5000 kj/2.6kg of potential heat to mine as waste from engine.

 

 

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The Details

 

I'm long remiss in doing the math for the energy balance of a gasifier.   To start this i'm going to try to answer the subset issue of concern for me currently: the relative importance of the various thermal drags on combustion in a downdraft gasifier.  As we know, the main work of combustion heat is to run reduction.   But in a downdraft, this combustion heat also has to run pyrolysis and drying, as well as heat incoming air for oxidization reactions, as well as heat char to combustion temps on the way to reduction.   Putting these together more formally, I see 5 thermal drags on combustion in a downdraft gasifier.

 

   1. drying (potentially with onward propagation of resulting moisture)

   2. pyrolysis

   3. heating incoming air to auto-ignition temp of tar (@580C)

   4. heating char to combustion temps as it passes through combustion zone to reduction.

   5. losses to vessel walls

   (6. and finally, the real work of the combustion heat- reduction)

 

What are the relative importance of these 5 factors?  in what order should we be worrying about them?  Here's my very rough numbers (emphasis on the "very rough".  Please help fix the specifics.)

 

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Some constants needed to do the figuring:

 

Specific Heat Capacities:

   Water (steam)    2.080  kj/kg     

   Water (liquid)     4.1813 kj/kg

   Wood, pine: 2.5 kj/kg

   Charcoal: 1 kj/kg

   Air: 1 kj/kg

   Output Gas: ~1 kj/kg

 

Heat of vaporization of water

   100C liquid to 100C gas = 2260 kj/kg

(this is the killer.  note it takes more than 7x the energy to vaporize water at 100C than it does to heat the water from 25C to 100C)

 

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To start i'm going to do this figuring by mass type, not by gasifier zones.  This keeps me figuring one substance at a time, which seems easier.  We'll divide back into the zones later.

 

Water:

Drying and heating vapor to Combustion Temp

   - heat water 25C to 100C = 75C * 4.1813 kj/kg =      314 kj/kg

   - heat to vaporize water 100C to 100C =                     2260 kj/kg

   - heat vapor 100C to 1000C = 900 * 2.08 kj/kg=       1872 kj/kg 

Subtotal: 4446 kj/kg       (adjust to 10-20% of biomass weight, or whatever the moisture content of the biomass)

 

 

Biomass / Char:

Heating and Pyrolysis of Biomass/Char/Tar to Combustion Temp

   - heat wood 25C to 350C = 325*2.5 =                 812.5 kj/kg

   - heat charcoal 350c to 1000C = 650*1 =            650 kj/kg     

(note aug 09. need to fix this.  i dropped the tar mass portion here.  changing to char specific heat only is a mistake.  this math is not what i described in the summary below)

Subtotal: 1462 kj/kg

 

(note april 09: i still can't get the proper figures for energy needed in pyrolysis from 220-350C.  above 350C it is self-sustaining, slightly exothermic.  in general, we know the energy for pyrolysis is very small, so i'm going to ignore the mass reduction during pyrolysis from 220 - 350C, and treat pyrolysis energy as the specific heat of unreacting wood, at a constant mass, even though it is reducing by multiples.  i'll bet these two cancel out to an approximation.  above 350C let's assume that char and tar are about the same specific heat, so we can ignore further solid mass reduction.  both the char and the tar need to be heated to combustion temps, so both are a drag on combustion.)

 

 

Air:

Heating Air to Combustion Reaction start temp

    - heat air from 25C to 580C (auto ignition temp of tar) = 555 * 1 kj/kg =         555kj/kg     (adjust to ratio of air in vs biomass in vs gas out) 

Subtotal: 555 kj/kg 

with 1.6kg air for each 1kg biomass, total air thermal load is 888kj/1.6kg

 

 

Losses to Vessel Walls:

   - @250-500kj at 15hp/hr

(bear can you add your figuring in here?)

 

 

Product Gas Output

   - 1kg of gas cooled from 650C after reduction to 50C = 600 * 1kj/kg =  600 kj/kg  

Subtotal: = 600 kj/kg

(Adjust for gas out for 2.6x the mass of biomass in, or 1560kj/2.6kg.  See mass flow balance for specifics.)  

 

 

 

Heat in IC Exhaust

   -  Wood gas has an energy density of around 5-6 MJ/m3.  Each 1kg of biomass in produces 2.6kg of woodgas out, or 2.738m3, or 5.5(2.738) = 15MJ/kg in.  This cooresponds to 20MJ/kg wood energy density input.   At least one third of this  energy is going out the exhaust pipe, so 15/3=5MJ / kg biomass in.  This is over 3x the heat energy available in the gasifier output (1,560kj).

 

 

 

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Some rough comparisons over rough calculations over rough appoximations for 1 kg of biomass input, with proportional air and moisture amounts.

 

Water heating needs (atm through vapor at100C (2574 kj/kg) / atm through vapor at 1000C (4446 kj/kg) )

     - 129/222 kj/kg of biomass + 5% moisture

     - 257/445 kj/kg biomass + 10% moisture

     - 386/667 kj/kg biomass + 15% moisture

     - 514/890 kj/kg biomass + 20% moisture

     - 772/1,333 kj/kg biomass + 30% moisture 

     - 1,030/1,778 kj/kg biomass + 40% moisture 

     - 1,287/2,223 kj/kg biomass + 50% moisture

 

Biomass/char/tar through pyrolysis and combustion

     - 1462kj/kg

 

Incoming air to oxidation reaction temp

     - 888 kj/1.6kg

 

 

Losses to Vessel Walls:

      - @250-500kj at 15hp/hr

 

Heat available for mining in product gas output

     - 1560 kj/2.6kg of potential heat to mine

 

 

Alternative Description

(Jim feel free to move this somewhere else)

Woody biomass is a composed of both (holo)cellulose and lignin. Chemically, cellulose has primarily singly bound carbon rings with an oxygen thrown in and many hydrogen and hydroxyl connections. Lignin has a much more tightly bound solid carbon benzene ring and as a result much less hydrogen content. The holocelloluse is further made up of cellulose and hemicellulose (xylan, galactan, and mannan). In general, hardwoods are 40-50% cellulose, 22-40% hemicellulose, and about 20% lignin. Softwoods are made up of 40-45% cellulose, 24-37% hemicellulose, and 25-30% lignin.

 

The burning of wood can be divided into four stages :

  1. drying/heating
  2. pyrolysis
  3. combustion of volatiles
  4. oxidation/reduction of char

 

The specific heat of dry wood varies with temperature; a reasonably good "curve fit" is given by Wilkes [1]:

 

H_sp(d)=0.1031 + 0.003867 t kJ/kg/K

 

where t is the temperature in degrees Kelvin. The specific heat of dry wood at 0C (32F, 273K) is 1.113 kJ/kg/K and 1.598 kJ/kg/K at 100C (212F, 373K). This line fit was developed frmo observations form 280K to 420K. The specific heat of green (wet) wood can be calculated by extending the previous relationship with a rule for the specific heat for mixture with

 

H_sp(g)=(H_sp(d)+4.178*MC)/(1+MC)

 

where MC is the moisture content of the wood on a dry basis (not percent) and it is assumed that the specific heat of liquid water is 4.178 kJ/kg/CK  Further accuracy can be added by including a correction factor to account for the additional energy in the wood-water bond. This relationship is used until the liquid water must vaporize at about 100C.  After the water has vaporized, the dry wood specific heat can be used when applied to the dry wood weight. The net effect of a higher moisture content is to increase the specific heat of wood. Higher moisture content also increases the conductive heat transfer rate of wood. It also limits the temperature of the core of the biomass (chips, chunks, or pellets) until all of the water can be vaporized.

 

The pyrolisis of the dry wood has the following temperature ranges

  • hemicellulose : 500-600K
  • cellulose : 600-650K
  • lignin : 500-773K

Pryolysis consists of cleaving all of the "extras" off of the carbon rings to produce volatile gasses and char. Char is predominantly pure carbon. As a chemical reaction, the elements that breakdown at lower temperatures are slightly endothermic and those that breakdown at higher temperature are slightly exothermic. The pyrolysis of cellulose (and hemicelloluse) produce a smaller percentage (15%) of char on a weight basis than lignin (60%).

 

The pyrolysis process does not require any oxygen, just heat to raise the temperature above the needed thresholds for the chemical breakdown to occur. In most cases this heat is provided by the oxidation of the volatile compounds released during pyrolysis (combustion).

 

Combustion of the volatiles releases about 65% of the chemical energy stored in the wood. In the reduction (remove oxygen) that occurs over the reacting char bed, is an endothermic process that pulls heat from the products of combustion. The carbon in the char, at these high temperatures, pull an oxygen off of CO2 or pull the oxygen off of H20 to create a steady supply of CO and H2. This is one of the primary reasons a high moisture content fuel can be the downfall of a gasifier. The combustion temperature (adiabatic flame temp) decreases rapidly as water is added to the combustion process. The net result is there is less high temperature energy to drive reduction reaction. One effect of this reduction process over the reacting char bed is to recover some of the thermal energy released during the combustion of the volatiles as chemical energy that can then be released durign combustion in an internal combustion engine. ( I need to find details on the amount of thermal (kinetic) energy that can be converted to chemical (potential) energy) The remaining 35% of the chemical energy in the char is directly converted into the chemical energy stored in the generated CO and H2.

 

 

[1]  Wilkes, K. E., "Thermo-Physical Properties Data Base Activities at Owens-Corning Fiberglass," pp. 662-677, in Thermal Performance of the Exterior Envelopes of Buildings, ASHRAE SP 28, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., Atlanta, GA, 1981. 

 

(will continue to work on this later - Jay Martin)

 

 

 

 

Comments (2)

Daniel Chisholm said

at 12:52 pm on Jan 30, 2009

Re: losses to vessel walls, Jim, how hot are the exterior walls? Above boiling, below? How much surface area?

On my beer brewing kettle (uninsulated thin stainless, heated with electric immersion elements) the heat loss to free air is on the order of 1kW, over an area of perhaps 2000 square inches. You might be able to use the figure of "half a watt per square inch for 100C surface temp" to get a bit of a SWAG going on your vessel's heat losses.

jim mason said

at 5:06 pm on Apr 7, 2009


on 4/6/09 i redid my previous figures with better understanding of the mass and volume relationships between air and biomass in and gas out. these are of course important when calculating heat capacity of the various flows.

one important mistake in my previous calcs was not correcting the heat capacity of each mass for the amount of each present. the summary was all related to 1kg, which is not of course the ratio between the different masses. correcting this now gives us more heat in the output gas to do with work, and still leaves the thermal intake capabilities of the air in about half of the gas out. the other half, as well as losses through the vessel walls should go to drying and preheating the fuel, which is the point of the heated auger add-ons. i've added more detail on the thermal sink of moisture at various concentrations, as well as to various interim steps in the total temp rise to combustion.

the summary is biomass itself at 15% moisture content is about 2.5x the load on combustion as the air is. at 30% moisture, the fuel/water load is about 3.5x the air load. thus air preheating is good, but it is not sufficient. bigger wins are yet ahead through insitu drying and fuel heating.








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