Bio-Fuel Technology: Gasification

Part 4: Biofuels Technology - Gasification

Waste to Energy Technology: Thermochemical Processes

• Comparison of Thermochemical Processes:

  • Incineration / Combustion:

    • Oxygen: High

    • Temperature: $1000°C - 1500°C$

    • Catalyst: No

    • Heat Input: Direct Heat

    • Gas Products: -

    • Oil Products: -

    • Char Products: -

  • Gasification:

    • Oxygen: Low

    • Temperature: $800°C - 1200°C$

    • Catalyst: Yes

    • Heat Input: Low

    • Gas Products: 50-60wt% (CO, CO2, CH4, H2)

    • Oil Products: Very Low (40-50wt%)

    • Char Products: -

  • Pyrolysis:

    • Oxygen: No

    • Temperature: $400°C - 500°C$

    • Catalyst: Yes

    • Heat Input: High

    • Gas Products: 20wt%/40wt% (CO, CO2, CH4, H2)

    • Oil Products: 10wt.%

    • Char Products: 50-70wt.%

  • Catalytic Process:

    • Oxygen: Low

    • Temperature: $700°C - 800°C$

    • Catalyst: Yes

    • Heat Input: High

    • Gas Products: 40-50wt.% (H2, CO, CO2)

    • Oil Products: 10wt.%

    • Char Products: 40-50wt.%

Incineration / Combustion

• Direct, complete combustion of waste materials.

• Converts waste into flue gas, heat, and ash.

• Chemical Equation: $C<em>nH</em>mNS + O<em>2 \rightarrow CO</em>2 + H_2O + CO + NOx + SOx$

• Flue gas carries heat.

• Remaining solid is ash (generally has no use).

• Minimum combustion temperature: $850°C$

• Minimum residence time: 2 seconds.

• Emission limits for release to atmosphere:

  • Sulphur Oxides (SOx)

  • Nitrogen Oxides (NOx)

  • Hydrogen Chloride (HCl)

  • Hydrogen Fluoride (HF)

  • Dust

  • Heavy Metals

  • Dioxins and furans

Incinerator Designs

• Rotary Kiln

  • Rotating combustion chamber keeps waste moving, allowing it to vaporize for easier burning.

• Fluidized Bed

  • Combustion chamber filled with floating particles (ash, small stones).

  • Waste mix burns in the floating area.

  • Gas/air mix injected at the bottom.

  • Optimal oxygen level is crucial for good combustion.

  • Too much air cools the chamber; too little leads to incomplete combustion.

  • Flue gas + air or adding sand particles can be used.

• Moving Grate

  • Waste moves slowly down by conveyor and burns with oxygen.

• Multiple Hearth

  • Multiple circular hearths stacked on each other.

  • Material fed from the top and moved by rotating rabble arms.

  • Oxidizing gases flow upward, counter-current to the waste flow.

  • Works at low temperatures for simple, low-ash waste.

• Waste-gas Flare

  • Burns waste gases (CO, unburned hydrocarbons) with oxygen.

  • Fuel is added to start ignition.

  • Pilot is added as an ignitor (using gaseous fuel).

Incineration: Advantages vs. Disadvantages

• Advantages:

  • Decreases waste quantity by 80-90%.

  • Produces heat and power.

  • Reduces pollution compared to landfills.

  • Saves on waste transportation.

  • Controls odor and noise.

  • Prevents methane gas production.

  • Kills bacteria and viruses.

• Disadvantages:

  • High initial cost.

  • Potential long-term problems.

  • Releases dioxins, heavy metals, NOx, SOx.

  • Ash waste can potentially harm people and the environment.

Gasification

• Partial combustion of waste at high temperatures ($800°C$ to $1200°C$) with low oxygen content.

• Hydrocarbon materials converted mainly into syngas (CO, CO2, CH4, H2).

• Chemical Equation: $C<em>nH</em>m + \frac{1}{2}O<em>2 \rightarrow nCO + nCO</em>2 + mH<em>2O + mH</em>2 + yCH_4$

• Needs heat source to start combustion.

• Syngas carries heat.

• Remaining solid is char (used as solid fuel, fertilizer, or active carbon).

• Minimum combustion temperature: $800°C$ up to $1200°C$

• Minimum residence time: 2-5 seconds.

Gasification Kinetics

• Drying process / Dehydration at $100-200°C$

  • Evaporates all moisture in the waste, producing water vapor/steam.

• Pyrolysis at $200-400°C$

  • Volatiles are released, and char is produced, resulting in up to 70% weight loss.

• Combustion process at $800°C$

  • Volatile products and some char react with oxygen to produce carbon dioxide and small amounts of carbon monoxide.

  • Provides heat for subsequent gasification reactions.

  • $C<em>nH</em>m + \frac{1}{2}O<em>2 \rightarrow nCO + nCO</em>2 + mH_2O$

• Gasification process at $800-1200°C$

  • Char reacts with steam and carbon dioxide to produce carbon monoxide and hydrogen.

  • Needs heat for reactions.

  • $C + CO_2 \rightleftharpoons 2CO$

  • $C + H<em>2O \rightleftharpoons CO + H</em>2$

Gasification Reactions

Gasification Problems

• Ash melting (at high temperature above $900°C$).

• Waste with high metal and nonmetal content melts at high temperatures.

• Causes problems in the reactor/furnace.

Gasification: Advantages vs. Disadvantages

• Advantages:

  • Energy-efficient: Low energy input – High energy output.

  • Decreases waste quantity by 80-90%.

  • Produces Syngas with high energy content (HHV).

  • Reduces pollution: Less likely to pollute than incinerators & landfills.

  • Effective metal recycling / Uses of char.

  • Char can be used as fuel or active carbon.

  • Kills bacteria and viruses.

• Disadvantages:

  • High initial cost.

  • Needs regular maintenance.

  • Requires larger space.

  • Releases hazardous compounds: CO2, NOx, SOx.

  • Syngas needs filtration system before using it.

Gasifier Designs

• Updraft Gasifier

• Downdraft Gasifier

• Cross-draft gasifier

• Fluidized bed reactor

• Entrained flow gasifier

• Plasma gasifier

Gasifier Design: Fluidized Bed

• Combustion chamber is full of floating particles, like small stones/sand, where the waste mix burns.

• Gas/air mix is injected at the bottom.

• Floating waste is based on the speed of air injection from the bottom (Air injection OR Air + Sand injection).

• More air injection leads to complete combustion and makes the waste fly out.

• Less air makes the waste fall. Sand or stone is used in the process.