sustainable energy science and engineering center biomass for...

Sustainable Energy Science and Engineering Center

Biomass for Power Generation

References:

Donald L. Klass, Biomass for Renewable Energy, Fuels and Chemicals, Academic Press, 1998.

Peter Quaak, Harrie Knoef and Hubert Stassen, Energy from Biomass: A review of combustion and gasification technologies, World Bank Technical Paper #422, The World Bank, 1998.


Biomass Energy

Biomass resources are potentially the world's largest and most sustainable energy source

The annual bio-energy potential is about 2900 EJ, though only 270 EJ could be considered available on a sustainable basis and at competitive prices.

The expected increase of biomass energy, particularly in its modern forms, could have a significant impact not only in the energy sector, but also in the drive to modernize agriculture, and on rural development.

The share of biomass in the total final energy demand is between 7% and 27%.

Source: http://www.worldenergy.org/wec-geis/


Biomass DefinitionsOrganic material mainly composed of carbohydrate and lignin compounds, the building blocks of which are the elements carbon, hydrogen and oxygen.

Carbohydrates consist of sugars, starches, and cellulose , which contain CHO and function primarily as energy storage, energy transport and plant structure.

Cellulose (C6H10O5)n is a long-chain polymer polysaccharide carbohydrate, of beta-glucose. It forms the primary structural component of plants and is not digestible by humans.

Cellulose is a common material in plant cell walls and was first noted as such in 1838. In combination with lignin and any hemicellulose (C5H8O4)n, it is found in all plant material.

Lignin is a polymer in the secondary cell wall of woody plant cells that helps to strengthen and stiffen the wall.

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Biomass (other than wood)Agricultural and wood/forestry residues and herbaceous crops grown specifically for

energy but excludes forest plantations grown specifically for energy.

Dedicated energy plantations: 3 million ha of eucalyptus plantations used for charcoal making (Brazil); Plantation program for 13.5 million ha of fuel wood by 2010 in China; 16000 ha of willow plantations used for the generation of heat and power in Sweden; and 50000 ha of agricultural land has been converted to woody plantations, possibly rising to as much as 4 million ha (10 million acres) by 2020 in USA.

Municipal Solid Waste (MSW) is potentially a major source of energy. This source of biomass will not be considered here due to the following reasons:

the nature of MSW, which comprises many different organic and non-organic materials

difficulties and high costs associated with sorting such material, which make it an unlikely candidate for renewable energy except for disposal purposes

re-used MSW is mostly for recycling, e.g. paper

MSW disposal would be done in landfills or incineration plants.

Bio energy challenge: to device systems to overcome low combustion efficiency and health hazards.


Biomass SourcesAgricultural residues: Amount of crop residues amounted to about 3.5 to 4 billion tones annually, with an energy content representing 65 EJ, or 1.5 billion tones oil equivalent. Hall et al (1993) estimated that just using the world's major crops (e.g. wheat, rice, maize, barley, and sugar cane), a 25% residue recovery rate could generate 38 EJ and offset between 350 and 460 million tones of carbon per year. For example, that over 2 billion tones of agricultural residues were burned annually world-wide, producing 1.1 to 1.7 billion tones/yr of CO2. The most promising residues from the sugar cane, pulp and paper, and sawmill industrial sectors. Estimates are that about 1200 TWh/yr of electricity can be produced from this source.

Forestry residues: Forestry residues obtained from sound forest management can enhance and increase the future productivity of forests. Recoverable residues from forests have been estimated to have an energy potential of about 35 EJ/yr. A considerable advantage of these residues is that a large part is generated by the pulp and paper and sawmill industries and thus could be readily available.

Livestock residues: The use of manure may be more acceptable when there are other environmental benefits, e.g. the production of biogas and fertilizer, given large surpluses of manure which can, if applied in large quantities to the soil, represent a danger for agriculture and the environment, as is the case in Denmark; environmental and health hazards, which are much higher than for other biofuels.

Energy forestry/crops. Dedicated energy crops in land specifically devoted and intercropping with non-energy crops. This is a new concept for the farmer, which will have to be fully accepted if large-scale energy crops are to form an integral part of farming practices. Factors to be considered are: land availability, possible fuel versus food conflict, potential climatic factors, higher investment cost of degraded land, land rights, etc. The most likely scenario would be the use of about 100-300 million ha, mostly in developed nations, where excess food production exists.


Thermal Properties of Biomass

Important properties relating to the thermal conversion of biomass are:

Moisture content

Ash content

Volatile matter content

Elemental composition

Heating value

Bulk density


Moisture and Ash Contents

Moisture content is the quantity of water in the material (% of material’s weight)

Wet basis: percentage of the sum of the water weight, ash and dry and ash-free matter

Dry basis: percentage of the ash weight, ash and dry and ash-free matter

Dry and ash free basis: percentage of the dry and ash-free matter content

The ash content (inorganic component) in general is expressed on dry basis

Ash Value:

0.5 % in wood

5~10% in agricultural crop material

30 ~40% in rice husks and milfoil

Chemical composition is also important


Volatile Matter Content & Elemental Composition

During the combustion or gasification process , the biomass decomposes into volatile gases and solid char. Biomass typicallyhas up to 80% volatile matter content.

0.0-0.2SSulfur

0.12-0.60NNitrogen

41-50OOxygen

5.5-6.7HHydrogen

44-51CCarbon

Weight % (dry & ash free basis)

SymbolElement


Heating ValueHeating value: Energy chemically bound in the fuel with reference to a standardized environment (temperature, state of water and the combustion products.

The heating value is given in J/kg or BTU and is always given with respect to a reference state.

Lower Heating Value (LHV): reference state of water is the gaseous state

Higher Heating Value (HHV): reference state of water is liquid state

Biomass always contains water, which is released as vapor upon heating. Some of the heat liberated during chemical reactions is absorbed by the evaporation process. Hence, the net heating value decreases as the moisture content of the biomass increases.

For typical biomass type, the value of HHV on dry and ash free basis is about 20,400 kJ/kg (± 15%).

In practice, the maximum allowable moisture content must be 55% on wet basis to ignite the fuel and extract energy from it. Typical LHV is then will be about 7500 kJ/kg.


Bulk Density

Bulk density: Weight of the material per unit volume, expressed on an oven dry weight basis (moisture content is zero percent), or as is basis with an indication of moisture content (MCw).

Typical values are 150 -200 kg/m3 for straws to 600-900 kg/m3 for solid wood.

Together, heating value and bulk density determine energy density– energy /unit volume.

Typically, biomass energy densities are about one-tenth that of fossil fuels (petroleum or high quality coal)


Fuel Characteristics

0.5-61-1025,000-32,000Charcoal19914,000Rice husks

0.25-1.710-608,400-17,000Wood210-2013,000-15,000Maize

4818,000Coconut Shells1.7-3.840-607,700-8,000Bagasse

Acd(%)MCw (%)LHVW(kJ/kg)Type

Acd : ash content


Thermal Properties of Biomass


Environment Contaminants

Nitrogen content: thermal NOx formation takes place at temperatures above 950oC from nitrogen contained in the combustion of air. Fuel NOx formation occurs at lower temperatures from the nitrogen contained in the fuel.

Volatile hydrocarbons: In combustion and gasification processes, volatile hydrocarbons( CxHy) are formed. These can be burned when they are in a hot combustion zone.

Sulfur (0.01 – 0.18% on dry weight basis) and chlorine (0.01-1.48%) may be present in the biomass.


Environment Contaminants


Thermal Conversion Processes

Combustion:

Biomass+ Stoichoimetric oxygen Hot combustion products

Gasification:

Biomass + Limited oxygen Fuel gas

Pyrolysis: breaking down of material by heat, it is the first step in the combustion or gasification of biomass

Biomass + Heat Charcoal, oil and gas

When biomass is heated in absence of air to about 350oC. It forms charcoal, gases and tar vapors


Gasification Processes

Source: Hand book of biomass down draft gasifier engine systems, SERI SP-271-3022, 1988


Flaming Match



Biomass Pyrolysis



Combustion Systems

FurnaceFuel

Air

Ash

Flue gas

Thermal Energy

Boiler

Thermal Energy

Flue gas

Combustion process stages:

Drying: evaporation of the water content

Pyrolysis and reduction: thermal decomposition of the fuel into volatile gases and solid char

Combustion of the volatile gases above the fuel bed

Combustion of the char in the fuel bed


The FurnaceChemical-bound energy in the fuel is converted into thermal energy in the form of hot flue gas.

Energy loss: thermal energy not transferred to flue gas.

Release of hot ash, unburned pyrolysis gases and CO in the flue gas

Efficiency of the furnace:

ηcomb = thermal energy available in the flue gas/ chemical energy in the supplied fuel

ηcomb = 65% - 99%

Completeness of the combustion process and the heat loss from the furnace.

Excess air factor λ: 1 – 5

At stoichiometric combustion λ= 1

At higher values of λ, the flue gas leaving the furnace has lower temperature. Highest flue gas temperature is obtained at λ= 1.


Stoichoimetric Combustion

Complete combustion of biomass consists of the rapid chemical reaction (oxidation) of biomass and oxygen, the release of energy and thesimultaneous formation of the ultimate products of organic matter - CO2and water. For combustion of wood, the combustion is represented by the empirical formula of cellulose, (C6H5O5)n is

C6H10O5( )n + 6nO2 → 6nCO2 + 5nH2O+"energy"


Adiabatic Flame Temperature

Theoretical furnace temperature at complete combustion without any heat losses and heat transfer by radiation.

It is a function of λ and moisture content of the fuel.


Flue Gas Thermal Energy UtilizationBoiler: Switches thermal energy from the flue gas to the process medium (e.g. water, steam or air).

Energy Loss: the flue gas that contains thermal energy being released to the environment.

ηboiler = thermal energy available in the water or steam/ thermal energy in the entering flue gas

ηboiler = 60 ~ 95% based on LHV

ηboiler = 50 ~ 90% based HHV

Flue gas outlet temperature = 200oC

The flue gas exit temperature can not be too low, since it contains water vapor, Nox, HCI and SO2. If the flue gas is cooled too much, water vapor will condense, absorbing HCI, SO2 and NO forming acid components. As a result, the flue gas exit temperatures always kept higher than about 120oC.


Fluidized-Bed Furnace SystemIn a fluidized-bed combustor, the fuel is burned in a hot (700-1000oC) bed of sand or any other non-combustible material that is kept in turbulent suspension by blowing air. At temperatures lower than 850oC, incomplete combustion occurs, resulting in unwanted emissions. The residence time of all flue gases in the combustion zone should be longer than 1.5 seconds.

Because of the intensive mixing, the heat exchange rates are high and complete combustion is realized using low excess air factors (1.2 ~ 1.4). The furnace and heat exchanger are integrated to minimize heat losses.

Emissions: The flue gas is mixed with dust and particles. Cyclones are typically used for removing the particles and bag filters are used to collect the dust and smaller particles.


Steam Cycle


Steam Cycle

1. Boiler feed water is pressurized by the feed-water pump

2. In the boiler, the water is evaporated and superheated (boiler efficiency)

3. The superheated steam is fed into a steam turbine, where it expands to a low pressure and temperature, the magnitudes of which are determined by the condenser (turbine efficiency)

4. The expanded steam (may contain water) is fed from the condenser into the deaerator

5. In the deaerator, the noncondensible gases are removed

At low capacities (< 500 kW) a steam engine may be considered due to low efficiency of small turbines ( a research topic).

ηoverall = net electric output/fuel input


Steam Cycle Efficiencyηoverall = net electric output/fuel input

Net electric efficiency = ηcomb x ηboiler x ηturb x ηgen x φfuel x LHVfuel - Pparasitic

Fuel input = φfuel x LHVfuel

φfuel : mass flow of the entering fuel

Turbine efficiency


Fluidized Bed Boiler for District Heating

Source: KTH


Wood powder

Combined Heat and Power

Source: KTH


Source: Ohlström M. et al. “New concepts for biofuels in transportation”, VTT research notes 2074, Technical Research Center of Finland, Espoo 2001.

Gasification Systems


Gasification Process

Converts biomass into combustible gases that ideally contain all the enrgy originally present in the biomass, In practice, gasification can convert 60 -90% of the energy in the biomass into energy in the gas.

Direct gasification process: uses air or oxygen to generate heat through exothermic reactions

Indirect gasification process: transferring heat to the reactor from the outside

The gas can be burned to produce industrial or residential heat, to run engines for mechanical or electrical power or to make synthetic fuels.


Updraft Fixed-Bed GasifierThe first commercial gasifier of solid fuels is developed during 1830’s to produce synthetic gas (syn gas).

Simple in principle and very appealing and proven technology.

Drying zone: biomass is dried

Distillation or Pyrolization zone: Biomass is decomposed into volatile gases and solid char

The heat for pyrolization and drying is mainly delivered by the upward flowing gas and partly by radiation from hearth zone.

Reduction zone: carbon is converted into gas that mostly contains carbon monoxide and hydrogen

Hearth zone: the remaining char is combusted providing heat, carbon dioxide and the water vapor for reactions occurring in the reduction zone

High charcoal burnout; low gas-exit temperature; high gasification efficiencies and fuels with high moisture content can be used

High amounts of tar; extensive gas cleaning is required.


Oxidation & Reaction Zones

C +O2 ⇔CO2 + 401.9kJ /mol

H +12

O2 ⇔ H2O+ 241.1kJ /mol

Burning 1 mole of carbon to carbon dioxide releases a heat quantity of 401.9 kJ

Oxidation zone:

Reduction zone:

C + CO2 +164.9kJ /mol ⇔ 2COC + H2O+122.6kJ /mol ⇔CO+ H2

C + H2 + 42.3kJ /mol ⇔CO+ H2OC + 2H2 ⇔CH4

C + 3H2 ⇔CH4 + H2O+ 205.9kJ /mol

Require heat and as a result the temperature will decrease during the reduction

Water gas equilibrium


Gasifiers

Updraft DowndraftSource: Hand book of biomass down draft gasifier engine systems, SERI SP-271-3022, 1988

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