Energy and Environment II

Lecture 24/Lecture 25/Lecture26

Biomass Energy Conversion

 

Classification of Conversion Methods

The methods for converting biomass into heat (and electricity) and into other fuels (ie, into biofuels) fall into the following main classifications:

 

Thermochemical Conversion

There are three major thermochemical conversion methods:

 

1.      DIRECT COMBUSTION.  That is, the biomass, such as waste wood, bark, and agricultural residues, is directly burned in a stove or furnace for the production of heat, steam, or electricity.  System size ranges from small (the residential wood stove) to large (a steam generating boiler at a lumber or paper mill).  Generally, the biomass requires little or no pre-processing prior to combustion, though drying down to about the fiber-saturation point (about 30% moisture content in wood) is required to burn biomass in small stoves and furnaces.  Generally, the smaller the burner, the greater the pollution per unit of biomass burned.  Large biomass-fired systems mainly suffer from particulate emissions – that is, tiny fuel and char particles blown out of the furnace.  In some cases, NOx emissions can be a problem.  Small systems suffer from hydrocarbon emissions, including gases and fuel aerosol (ie, the white smoke of your neighbor’s fireplace or wood stove), carbon monoxide, and soot (tiny, mainly carbon particles).

 

Our chemical equation for complete combustion of wood is:

 

C6H9O4 + mH2O+ (1+EA)(6+9/4-4/2)O2 + 3.773(1+EA)(6+9/4-4/2)N2 Þ

6CO2 + (9/2+m)H2O + EA(6+9/4-4/2)O2 + 3.773(1+EA)(6+9/4-4/2)N2

 

Note C6H9O4 is a nominal chemical formula for wood.  The relative amounts of C, H, and O vary from wood species to species, and from fuel analysis to analysis.  Usually the C, H, and O values are not integers.  mH2O represents the amount of moisture in the wood.  The table below gives the kmols and kgs of the reactants and products for a wood combustion system burning wood (of 30% moisture content) with 50% excess air (ie, EA = 0.5):

 

Species

kmols

kg

Reactants

Wood (dry)

1

6x12+1x9+4x16 = 145

Moisture in Wood

43.5/18 = 2.417

30% of 145 = 43.5

Oxygen

1.5x(6+9/4-4/2) = 9.375

9.375x32 = 300

Nitrogen

3.773x9.375 = 35.372

35.372x28.15 = 995.7

Total Reactants

48.164

1484.2

Products

CO2

6

6x44 = 264

H2O

9/2+2.417 = 6.917

6.917x18 = 124.5

O2

0.5x(6+9/4-4/2) = 3.125

3.125x32 = 100

N2

35.372

995.7

Total Products

51.414

1484.2

 

Note:

·        The mineral matter in the wood is not included in our table.  This would add 2 or 3 kg.  Some of the main elements are calcium, magnesium, potassium, and sodium.  (This is much differ from the mineral matter in coal, which is about 10% of the coal, and is mainly composed on Fe, Si, and Al.)

·        The nitrogen in the wood is neglected.  “White” wood contains about 0.1% nitrogen by mass.  Bark and green wood contain somewhat greater amounts of nitrogen.  This nitrogen is chemically bound to the C and H in the wood.  Upon pyrolysis, it is converted to ammonia and hydrogen cyanide, which upon combustion are oxidized to NO, N2O, and N2.

·        The total number of kmols is not conserved upon combustion.

·        Total mass is conserved upon combustion.

·        The air-fuel ratio of the combustor is: A/F = (300+995.7)/(145+43.5) = 6.87

·        The air-fuel for the combustion of bone-dry wood with stoichiometric air is: (A/F)s = (300+995.7)/1.5/145 = 5.957 or about 6.  (Your auto engine runs with the A/F ratio set at the stoichiometric value, which is about 14.6.  Why does wood combustion have a significantly lower stoichiometric A/F than gasoline combustion?)

·        The molecular weight of the N2 in our table is 28.15 kg/kmol.  This is greater than the normal molecular weight of N2 (28) because our N2 includes the Argon in the air.  Argon comprises about 1% of air by mole.

·        Our table does not include the moisture of the air.

·        The heat available from burning 1 kmol of our wood in our combustor would be about 145 kg x 15,000 kJ/kg = 2.2 million kJ IF the exhaust were cooled to 25 degrees C and the H2O condensed.  Our course in a real combustion system, some heat is left in the exhaust gases, so that condensation and corrosion are prevented, and so that the exhaust plume has buoyancy and rises into the air.  Thus, only about 80-85% of the 2.2 million kJ is available to heat something (like steam).

 

2.      PYROLYSIS: In the case, the original biomass is processed to new fuels (biofuels) by heating.  The heating may be done in the absence of air, in order to limit oxidation reactions.  If wood is slowly heated and the temperature of the wood remains fairly low, the carbon in the wood forms char, and the other elements in the wood are released (mainly as CO2 and water vapor).  The original wood contains cellulose [(C6H10O5)n both long and short-random fibers] and lignin.  About 2/3rd of the wood is cellulose, about 1/3rd is lignin.  The lignin may be thought of as the “glue” that holds the cellulose fibers in place.  The lignin is mainly composed of C and H.  The cellulose contains C, H, and O.  Upon slow heating at relatively low temperature (a few hundred C), the C from the cellulose and lignin forms chains and layers of carbon – that is, it forms char.  The H and O mainly leave as H2O and CO2.  The new biofuel is charcoal.

 

If the biomass is more rapidly heated and its temperature increased, the char forming tendency is reduced, and the biomass decomposes into various hydrocarbons.  These hydrocarbons are part of the volatile matter released from the biomass.  Several of these hydrocarbons, if cooled, will condense into tars and oils.  That is, our product is pyrolysis oil, a biofuel.  Air in our pyrolysis reaction chamber must be limited so that the volatiles do not significantly oxidize, and do not ignite.  Thus, the fuel to burn in an energy system is an oil – pyrolysis oil.

 

Note the term pyrolysis means decomposition.  In the absence (or significant deficiency) of air, we say thermal pyrolysis or simply pyrolysis.  If significant air is present (as in combustion), we say oxidative pyrolysis – that is, pyrolysis in the presence of air.  The first chemical step in the combustion of all hydrocarbon fuels (include gases like methane), is oxidative pyrolysis – the decomposition of the original fuel.  In the case of methane, the heating of the fuel in the presence of air causes the methane to form CO and H2 – two very simple fuels.  In the absence of air, heating of the methane produces hydrocarbon fragments, some of which chemically and physically grow into soot particles.  In the case of biomass, the heating causes devolatilization (another word for decomposition) of the fuel, yielding char, tars/oils, light hydrocarbon gases (such as methane, ethane, and ethylene), oxygenated hydrocarbons (such as formaldehyde and methanol), CO, H2, CO, CO2, and H2O.  The respective yields of all these things depends on the type of fuel, its heating rate, its temperature, and the amount of air present.  There are primary reactions within the fuel particles, and secondary reactions in the volatiles evolving and released from the fuel particles.  With air present, the external secondary reactions include oxidation reactions.  If the ignition temperature has been reached, the oxidation reactions will be very rapid – we have flaming combustion.

 

3.      GASIFICATION: If we increase the rate of heating of the biomass, and increase the temperature of the particles, the volatile matter released from the biomass becomes significantly composed of gases.  Additionally, if we add about ½ the amount of air or O2 required for stoichiometric combustion of the original biomass, the char particles remaining from the pyrolysis stage (ie, the decomposition stage) become gasified.  Thus, we have:

 

Pyrolysis of Fuel Particles Þ Gases + Char Particles

 

Oxidation of Char Particles Þ Gases

 

Some of the important gasification reactions occurring at the surfaces of the char particles are:

 

C(solid) + 0.5O2 ® CO

C(solid) + CO2 ® 2CO

C(solid) + H2O(gas) ® CO + H2

 

Tars/oils released during the pyrolysis also undergo gasification, ie, they are partially oxidized to CO and H2.  Note: we can think of gasification as fuel-rich combustion – that is, as combustion lacking sufficient O2 to fully oxidize the fuel-carbon to CO2 and the fuel-hydrogen to H2O.  We only get as far as CO (and some CO2) and H2 (and some H2O).

 

The goal with most gasifiers is to create a synthetic gas mainly composed of CO, H2, CO2, H2O, (and sometimes) CH4.  If the gasifier is air-blown, a lot of N2 will be present in the product gas, yielding a gas with a heating value of only about 100-200 BTU/scf (std cubic foot).  This is called a low-BTU gas. For reference, the heating value (ie, the higher heating value) of natural gas is usually about 1000 BTU/scf.  If the gasifier is oxygen-blown, N2 will be nearly absent in the product gas, and the heating value of the gas will be much higher, though not as high as natural gas.  Some gasifiers use steam as the gasifying agent.  These gasifiers (called steam reforming gasifiers) produce a gas high in H2.  The oxygen-blown and steam reforming gasifiers produce a medium-BTU gas.

 

There are many types of gasifiers:

·        Air-blown, oxygen-blown, steam-reforming.

·        Fixed-bed, fluidized-bed, entrained-flow.

·        Down-draft, up-draft.

·        Small-scale, medium-scale, large-scale (high pressure)

 

Gasifiers for biomass, coal, and oil have been around for a long time, though they have not been without problems and the need for enlightened engineering.  A major problem is the cleanup of the gas, removing the tars, particulate matter, sulfur (as H2S if coal is the fuel), and ammonia/hydrogen cyanide (from the small amount of fuel-bound-nitrogen in the biomass or other fuel).  Thus, there is still significant technology development and systems integration to perform.  An example of a system would be IGCC – Integrated Gasification Combined Cycle, the components of which are.

·        Air Separation Unit (if oxygen-blown gasifier)

·        Gasifier

·        Gas Cleanup System

·        Gas Turbine Engine (driving an electrical generator)

·        Heat Recovery Steam Generator

·        Steam Turbine (driving the second electrical generator)

Biochemical Conversion

Two major biochemical conversion methods are discussed below.  These result in useful biofuels.

 

1.      ALCOHOLIC FERMENTATION.  This leads to the production of ethanol.  The sugar fermentation reaction is:

 

C12H22O11 + H2O Þ 4C2H5OH + 4CO2

 

Yeast is used to catalyze this reaction.  The biofuel product is ethanol (C2H5OH).  In a typical process, there is considerable water present, leading to a product of about 10% ethanol in water.  Distillation increases the ethanol content to about 95% purity.  This is followed by purification steps leading to anhydrous ethanol (ie, ethanol without water).  Now we have our biofuel.  Ethanol is an attractive fuel.  It is a good engine fuel if blended with 5-15% gasoline (to overcome the cold starting problem of pure ethanol).  Since it is a liquid, it can be easily transported.  Its higher heating value is 29700 kJ/kg.  Its lower heating value is 26900 kJ/kg.  (The respective values for gasoline are 47300 kJ/kg and 44000 kJ/kg.)  Feedstocks for our sugar fermentation reaction above are cane sugar, beet sugar, and fruit sugar. 

 

There are some downsides, however.  Current fermentation-distillation-purification methods are quite energy intensive.  Especially, the distillation step requires energy.  Thus, the heating value of a gallon of ethanol may not be much greater than the energy that went into producing the ethanol.  Besides the energy required for the fermentation-distillation-purification process, energy is required to make the fertilizers used in the growing of the sugar crop, and energy is used in harvesting the sugar crop and transporting it to the refinery.  (Energy required to acquire, transport, and refine oil to gasoline is about 15% of the heating value of the gasoline.)

 

Additionally, other types of biomass must be hydrolyzed to sugar before the fermentation step can be performed.  That is, the H2O to C ratio of the biomass must be brought to about the ratio for sugar.  While it is relatively easy to hydrolyze starchy biomass, such as grains and roots (ie, cassava), it is difficult to hydrolyze woody (cellulose) biomasses.

 

Research and development is underway to decrease the energy required for the sugar fermentation-distillation-purification process.  R&D is also underway to improve the feasibility of converting woody biomass into ethanol.

 

Brazil has been a leading producer and user of ethanol.  The ethanol is used as a transportation fuel.  However, when the price per barrel of oil is low, it is difficult for the ethanol to economically compete.  Ethanol used in the USA for blending with gasoline is produced from corn.  Many urban areas are required by the EPA to use gasoline blended with an oxygenated component, the two major ones being ethanol and methyl tertiary butyl ether (MTBE).  Typically, the oxygenated component makes up about 10% of the gasoline.  Recently, the EPA ruled against continued use of MTBE, meaning more pressure may be put on ethanol supplies, or the EPA may need to reconsider its rule about the oxygenation of gasoline.  Oxygenated gasoline reduces the CO emission of automobiles, especially in the winter months.  The oxygenated components also improve the octane number of the gasoline.  MTBE has found to be a water pollutant.

 

2.      ANAEROBIC DIGESTION.  In this, certain types of bacteria in the absence of air promote the reaction of biomass with water to form methane and carbon dioxide.  The reaction is:

 

CxHyOz + aH2O Þ bCH4 + dCO2

 

The elemental balance equations are:

C:        x = b + d

H:        y + 2a = 4b

O:        z + a = 2d

 

Solving we obtain:

a = x-y/4 –z/2

b = x/2+y/8-z/4

d = x/2-y/8+z/4

 

If the biomass has the chemical formula Cx(H2O)z, then y = 2z and our chemical equation becomes:

 

CxHyOz + (x-z)H2O Þ x/2CH4 + x/2CO2

 

For example, for cellulose, we would have:

 

C6H10O5 + H2O Þ 3CH4 + 3CO2

 

On the other hand, for our wood, we would have:

 

C6H9O4 + 1.75H2O Þ 3.125CH4 + 2.875CO2

 

That is, the product gas is 52% methane and 48% carbon dioxide.

 

Anaerobic digestion occurs in many situations and locations, eg:

·        Swamps

·        Rice patties

·        Landfills

·        Sewage treatment plants

·        Animal wastes in vessels.

The nominal composition of landfill gas is 55% methane, 45% carbon dioxide.  This indicates the organic component of the landfill waste digested has an H/O ratio somewhat greater than that of our wood.  The nominal composition of the digester gas at sewage treatment plants is 65% methane and 35% CO2.  This indicates even more H compared to O in the waste material.

 

The digester gas at sewage treatment plants is burned in engines to generate electricity and heat for the plant.  In other cases, the digester gas is cleaned up and put into the natural gas distribution system.  King County does both.  Stationary engine manufacturers have for years made large-bore reciprocating engines to burn digester gas.

 

Many landfills around the world collect the gas and either burn it in engines (large-bore reciprocating engines of about 1 MW power or gas turbine engines of about 3 MW or greater power), generating electricity for the grid, or clean up and sell the gas.  A local firm makes equipment that cryogenically converts the landfill gas to LNG (liquified natural gas).  This requires clean up of the gas (a significant step) and separation of the methane from the CO2.

 

Some diary farms are installing systems to convert the cow waste to digester gas.  The digester gas can be burned in engines to generate electricity for the farm or grid.

 

Here is an interesting units conversion: kwh of electricity per cow!

 

In the developing world, methane from the anaerobic digestion of animal wastes and crop residues is relatively simple and economically attractive to set up.  Individual farms can do this.  Simply using the methane for cooking has several benefits over cooking with wood or other solid biomass:

·        The many hours of gathering wood (and other solid biomass) are eliminated, freeing up time for other pursuits.

·        The methane is much more cleanly and efficiently burned than solid biomass in cooking stoves, leading to a much more healthful interior living environment.  The food preparation process is improved and takes less time.

·        On a somewhat larger scale the methane can burned in small engine generator sets.

 

Biodiesel

See the old 342 web site:

http://www.me.washington.edu/~malte/engr342

Click “Notes with Links”.

Go to the 2000 lectures, Lecture #25, Monday, 06 Mar 2000, “Lecture on Biodiesel”.