Biomass & Electricty Generation

Question 1

How do thermal plant efficiencies range?

Answer 1

5% for domestic HP, 63% for maximum.

Question 2

Define GCV/HHV and NCV/LHV

Answer 2

GCV: Gross Calorific Value. HHV: Higher Heating Value
The total heat released from burning fuel when the combustion products are returned to 25C.

NCV: Net Calorific Value. LHV: Lower Heating Value.
Total heat released from burning the fuel when the combustion products are returned to 150C.

Question 3

When can you recover full GCV, why doesn’t this occur?

Answer 3

Only recover the full GCV if you condense the water in a flue gas, Flue gases are corrosive, condensing them will form acids which will corrode the chimney.

Question 4

Discuss moisture in solid fuels.

Answer 4

Biomass and coal are not dry. Biomass is 40-60% moisture. Some of the energy in the fuel will be used to vapourise the water.

Question 5

Define biomass.

Answer 5

A non-fossilised and biodegradable organice material originating from plants, animals and micro-organisms.

Question 6

Discuss advantages of biomass

Answer 6

1) Renewable
2) Absorbs CO2 during growth
3) Lag between releasing and absorbing CO2 is short compared to fossil fuels.

Question 7

What are the main drivers for using biomass?

Answer 7

1) Already a major source in developing countries
2) Environmental benefits
3) More wood in construction
4) Socio-political benefits
4) Technical benefits

Question 8

What are the negatives of biomass?

Answer 8

1) Limited Resource
2) Sustainability is costly
3) More particulate emissions
4) Logistical problems due to low energy density.

Question 9

What is gasification?

Answer 9

Partial oxidation process in air, oxygen or steam, where biogas is converted to syngas.

Question 10

What is pyrolysis? What can the solid/liquid products be used for?

Answer 10

Thermochemical decomposition in absence of an oxidising agent. Typically at 300-700C.
Solid product: Soil improver
Liquid product: Fuel for boilers, engines and turbines.

Question 11

Discuss the two types of anaerobic digestion.

Answer 11

Landfill gas (uncontrolled): Feedstock is the organic compounds in black bag waste. Gas collection using pipes distributed around landfill sites.

Biogas generation (controlled): See diagram in notes.

Question 12

Discuss the two different types of digester.

Answer 12

Mesophillic: 36-42C. Slow and stable, most common.
Thermophillic: 48-55C. Faster, prone to ammonia poisoning. Greater gas yields.

Question 13

Draw diagram for extraction and transesterification.

Answer 13

See notes.

Question 14

Define “biorefinery”. Draw diagram

Answer 14

The sustainable processing of biomass into a spectrum of marketable products and energy.

Question 15

Discuss the generations of biofuels

Answer 15

1st: Conventional biofuels. Crops can also be used for food. Land availability issues, high fertilizer requirement. Ethanol, butanol, methane, hydrogen.
2nd: Advance biofuels: Wider range of feedstock, higher productivity, compatible with current fuels. Methanol, DME, gasoline, jet fuel.
3rd: Algae derived biofuels. Very high yields, compatible with current fuels.

Question 16

What are the three most common combustion system technologies?

Answer 16

Moving bed, fluidised bed, pulverised fuel.

Question 17

Discuss the two main types of boiler.

Answer 17

Fire tube boilers: Small, low pressure, saturated steam, for heat not power.
Water tube boilers: Large, high pressure, superheated steam, can be used for power.

Question 18

Discuss emission control in a furnace.

Answer 18

CO + Particulates: Fuel / Air ratio control
Thermal NOx: Secondary air injection. Flue gas re-circulation.
SOx: Lime addition.

Question 19

Discuss particulate control, post combustion.

Answer 19

Cyclones: Heavy particles forced to outside by inertia, hit wall then fall to bottom. Clean gases exit at the top.
Bag Filters: Flue gases pass through bag, particulates stick to bag, cleaned using air pulses.
Electrostatic Precipitors: Flue gases pass through highly charged electrodes. Particulates become charges and are attracted to plates.

Question 20

Describe NOx control.

Answer 20

Downstream solution is selective catalytic reduction using ammonia.

Question 21

Discuss Geo-Thermal Organic Rankine Cycle.

Answer 21

Essentially the same as steam cycle.
Uses working fluid with lower boiling point.
Generally lower efficiencies.

Question 22

Discuss concentrating solar.

Answer 22

Tracking mirrors reflect sun onto collector.

Heat can be stored in molten salt to provide overnight power.

Question 23

Discuss the two types of turbines.

Answer 23

Impulse turbines: High head/low flow. Use velocity of water hitting a bucket.
Reaction Turbines: Generally low head/high flow. Combination of pressure and velocity.

Question 24

Define solar irradiance.

Answer 24

The power per unit area received from the Sun in the form of electromagnetic radiation as reported in the wavelength range of the measuring instrument.

Question 25

What affects PV output?

Answer 25

Tilt, azimuth angle, shading, temperature, dust.

Question 26

What is the differing control used by different sized turbines?

Answer 26

Small turbines employ stall control. Blades are designed to decrease aerodynamic torque above a set wind speed.
Larger turbines use pitch control: Blades are rotated to reduce aerodynamic torque.

Question 27

What is the general equation for turbine power?

Answer 27

P=Cp(pAV3/2)

Question 28

Discuss affects of wind speeds on turbines.

Answer 28

Turbines start generating at 2-3.5m/s, they typically cut out at 25m/s to prevent damage. Betz proved that a turbine could not be more than 59.3% efficient.

Question 29

Discuss the two methods of hub height wind speed correction.

Answer 29

Power Law and Log Law.

Question 30

How do you calculate wave power?

Answer 30

P=0.5H2Te

Question 31

Discuss the two types of shoreline wave power.

Answer 31

Over-Topping: Waves go over a wall and water is returned to the sea via a turbine.
Oscillating Water Column: Wave compresses air in tubes and drives an air turbine.