Bioenergy Flashcards
What are the three main types of Biomass
- Wood: short rotation forestry
- Energy Crops: miscanthus grass and switch grass
- Waste: Industrial, animal etc.
What ar the two dominant functions of solar radiation on green plants?
- Temperature control for chemical reactions to proceed
- Photo-excitation of electrons for the production of oxygen and
carbon structural material
What is the theoretical max efficiency of photosynthesis?
33%
What is the realistic efficiency of photosynthesis to create biomass
Closer to 0.5%, therefore large amount of land required to harvest significant amounts of energy.
Example question:
Estimate the amount of electricity in kWh that could be
produced each month (30 days) from a biomass-fired
power station that burns biomass grown over an area of
15km by 15km. Assume biomass yields 0.5 W/m2
(MW/km2) and the power station has an efficiency of 35%
Biomass power = 15 x 15 x 0.5 = 112.5 MW
Electricity power= 112.5 x 0.35 = 39.375 MW
Electricity = 39375 x 24 x 30
= 2.835x107 kWh
Side note: same area of PV cells might harvest up to 10x as much.
What is the big selling point of Biomass fuels?
It can be carbon-neutral. Amount of CO2 released in conversion is the same as removed from the atmosphere during photosynthesis
What are the six types of Biomass conversion processes?
- Combustion
- Gasification
- Anaerobic digestion
- Pyrolysis
- Fermentation
- Transesterification
Describe the biomass conversion process of combustion.
A biomass (eg. wood) can be burned to provide a process or heat.
The combustion of biomass can also be used to raise
steam to drive engines / turbines which are coupled to
generators producing electricity
Describe the biomass conversion process of Gasification
Thermochemical process by which a gaseous fuel is
produced from solid fuel
House waste, or compost are heated to a high temperature (over 700 Celsius) with a controlled atmosphere of oxygen/steam
These components undergo reactions with steam and
oxygen to produce ‘syngas’
Describe what ‘syngas’ is and how it can be used to produce energy
Syngas mainly consists of Hydrogen and Carbon Monoxide and is a product of gasification.
Low calorific value, but burns relatively cleanly with low emissions
Conversion efficiency of 60-70% - lower because energy is used to dry out damp biomass before burning
Syngas can be combusted for heat/electricity or processed into transportation biofuels.
Describe the biomass conversion process of Pyrolysis and the products that are formed.
Thermal degradation of biomass by heating/burning in the absence of oxygen
Products are waste gas, liquid (pyrolysis oil) and charcoal (bio-char)
Condensed gas produces liquid called bio-oil
What are functions of the bio-oil?
Bio-oil can be purified and synthesized into chemicals
and fuels using catalysts, e.g. methanol
Describe the biomass conversion process of Anaerobic digestion
Anaerobic Digestion involves the breakdown of organic waste (biomass) by bacteria in an oxygen-free environment
Products of this process are biogas and agricultural
fertiliser (rich in nitrogen, phosphorous, potassium)
What are functions of Biogas?
Biogas can be used
to fuel a gas engine or gas turbine, or burned in a boiler to provide heat or to raise steam
What is the composition of Biogas?
Biogas is a combustible gas composed primarily of roughly 64% methane (CH4) and 34% CO2
What are some benefits of anaerobic digestion?
Compared to aerobic digestion, much less solid residue is
produced and much more gas
Can be artificially instigated in designated digestors. By
increasing the temperature to 40-60 celsius, the process can
provide biogas after a few weeks.
Describe the digesters used in anaerobic digestion
Consists of feedstock holder,
digestion tank with mixing system, biogas and residue recovery, and heat exchangers to maintain
temperature
Small digestors for heat
productions, larger units can be used to generate electricity
What are two reasons that AD will become more important soon
- Recovery of the methane from land-fill for energy use
- Stop the methane being released into the atmosphere as Methane is 23 times more potent as a greenhouse has than carbon dioxide, so reducing it’s emission is important
What is the efficiency of energy conversion from glucose to biogas?
greater then 90% efficiency
What are the four solid biomass conversion technologies and what are the products.
- Combustion - combust biomass to produce heat and work.
- Anaerobic Digestion - produces biomethane which can be used directly in gas turbine or burned for heat.
- Gasification - produces syngas which burns cleanly and can be synthesised for transportation fuel or combusted to produce heat.
- Pyrolysis - produces liquid oil (bio-oil) which can be synthesised to be a fuel
What are the two important biofuels to power vehicles?
- Bioethanol
- Biodiesel
How is bioethanol produced?
Ethanol is produced through
fermentation from sources of sugar or starch, e.g. corn (US), wheat and corn (China) and sugar cane (Brazil and
India)
In production of ethanol from starch (corn) the starch biopolymer must be broken down. Enzymes are used to catalyse the decomposition of starch to glucose which is then fermented to form ethanol.
What is the FER and gallons of ethanol/acre of corn switchgrass and sugarcane
Corn: 1.2 - 1.4, roughly 300
Stitch grass: 3 - 5, roughly 400
Sugarcane: 8, 650
Define the meaning of FER
Fossil energy ratio
FER = energy supplied to consumer / fossil energy used
How is biodiesel produced?
Biodiesel is derived chemical ‘transesterification’ of
vegetable oils such as rapeseed and sunflowers
This is when triglycerides (vegetable oils) are mixed with methanol and a catalyst producing fatty acid methyl esters (FAME), a term
used interchangeably with biodiesel which is better for a diesel engine then original vegetable oils
what is the efficiency and FER of transferring vegetable oil to biodiesel?
Greater then 97% so very high. FER = 3.2
What is a summary of the chain of processes in bioethanol manufacturing?
Cellulose plants -> pre treatment -> Hydrolysis -> Fermentation -> Ethanol
Starchy plants -> Hydrolysis -> Fermentation -> Ethanol
Glucose plants -> Fermentation -> Ethanol
What are some benefits of bioenergy?
- Can be carbon neutral
- Can be harvested in liquid
form for transport
applications - Large choice of biomass
materials that can be
matched with local climate - Sustainable energy source
(replenishment from Sun) - Security of supply
- Waste materials can be used (e.g fertiliser)
What are some impacts of bioenergy?
- Low energy per unit area
- Social impact of replacing
good food producing land
with energy crops - Only viable as a
component of an overall
energy delivery system - There are some often
some emissions, but
reductions in CO2
emissions relative to fossil
fuels - Deforestation
