Contents Direct combustion Biomass energy Basic Definitions

Contents Direct combustion Biomass energy Basic Definitions
Conversion Methods - Thermochemical Methods What is combustion? Combustion of biomass Environmental impact of biomass combustion Combustion Technologies Operational problems in biomass combustion Biomass co-firing - An efficient way to reduce greenhouse gas emissions Co-firing Environmental effects of biomass co-firing Co-firing: merits and demerits Main co-firing methods Biomass co-firing by pre-mixing and co-milling Direct injection co-firing systems for biomass Indirect and parallel co-firing Technical issues with biomass co-firing

Direct combustion

Grown Biomass = Energy Crops
Biomass is the oldest source of energy As a fire burns down, it gets hotter

Biomass energy Biomass is the oldest source of energy Biomass energy is energy made from something that is or once was plant matter As a fire burns down, it gets hotter Enough Biomass resources in the world to cover the world’s energy demand

We can use fire to make steam, and make steam do work for us

Farmers often burn their crop waste to get rid of it

Biomass as energy resource
Biomass energy Biomass is biological material derived from living, or recently living organism. Biomass as energy resource Biomass is plant matter such as trees, grasses, gricultural crops or other biological material. It can be used as a solid fuel, or converted into liquid or gaseous forms for the production of electric power, heat, chemicals, or fuels Abundant energy resource It can be stored It can be transformed into energy by different technologies (mechanical, thermochemical, biological) It provides solid, liquid and gaseous biofuels to use for heating, power and transport purposes Traditional Biomass over-exploitation of natural resources, low economic valorization of biomass, low efficienct technologies Modern biomass Commercial efficient technologies environmentally sustainable production and use

Methods of utilizing biomass as a source of energy
Heat Biomass Resources Direct Combustion Gasification Conversion Processes Electricity Biomass Fuels Generator Heat Engines Mechanical Power Intermediate Fuels RESOURCE TECHNOLOGY APPLICATION

Thermochemical Conversion: Basic Definitions
Combustion Thermal conversion in the presence of excess oxygen for production of heat The oxidant is in stoichiometric excess, i.e., complete oxidation Gasification Thermal conversion of organic materials at elevated temperature and reducing conditions to produce primarily permanent gases, with char, water, and condensibles as minor products Primary categories are partial oxidation and indirect heating Pyrolysis Thermal conversion (breakdown) of organics in the absence of oxygen In the biomass community, this commonly refers to lower temperature thermal processes producing liquids as the primary product Possibility of chemical and food byproducts

Black Liquor: The industry burns black liquor in Tomlinson boilers that: feed back-pressure steam turbines supplying process steam and electricity to mills, recover pulping chemicals (sodium and sulfur compounds) for reuse. the lignin-rich by-product of fiber extraction from wood in Kraft (or sulfate) pulping.

Significance of biomass combustion
Thermochemical Conversion: Basic Definitions Significance of biomass combustion Use of biomass for energy causes no net increase in carbon dioxide emissions to the atmosphere and does not contribute to the risk of global climate change Growing plants remove carbon from the atmosphere through photosynthesis If the amount of new biomass growth balances the biomass used for energy, bioenergy is carbon dioxide “neutral” Globally, biomass meets about 14 percent of the world’s energy needs Origination process of biomass: 6 CO2 + 6H2O C6H12O6 + 6 O2 sunlight

Two Major Paths to Convert Biomass

Two Major Paths to Convert Biomass Thermochemical and biochemical routes for lignocellulose conversion

Conversion Methods There are a number of technological options available to make use of a wide variety of biomass types as a renewable energy source. Conversion technologies may release the energy directly, in the form of heat or electricity, or may convert it to another form, such as liquid biofuel or combustible biogas. Depending on its source, these processes include: combustion, pyrolysis, gasification, liquefaction, anaerobic digestion or fermentation. Thermo-chemical processes convert biomass into higher-value or more convenient products. The process releases a gas (~6 MJ/kg), a liquid (~ MJ/kg) and/or a char (~18MJ/kg), and depending on the technology one of these is the final product.

Thermochemical Methods
Thermo-chemical conversion methods represent one of the two main categories of biomass energy conversion technologies. Among the two categories, these represents the most widely used energy conversion technologies in all form of fuels. Could be categorized as: Direct combustion, Gasification, Liquefaction and Pyrolysis (carbonization, destructive distillation & fast pyrolysis) Basically used to derive energy or intermediate fuel with improved properties from a primary fuel Involve complex chemical reactions during which devolatilization and cracking take place.

Thermochemical Methods
These are processes in which heat is the dominant mechanism to convert the biomass into another chemical form. The basic alternatives are separated principally by the extent to which the chemical reactions involved are allowed to proceed:

Thermo-chemical conversion routes
Thermal conversion Excess air No air Partial air Combustion Gasification Pyrolysis & Hydrothermal Fuel Gases (CO + H2) Heat Liquids

Why Thermochemical Conversion?
Thermal conversion technologies are robust, and they efficiently convert a wide range of biomass feedstocks Addresses seasonal and regional variability issues Utilizes the entire biomass feedstock Thermal conversion provides for a range of fuel opportunities Ethanol, mixed alcohols, oxygenates Hydrocarbons including gasoline, diesel, Jet‐A

What is combustion? Combustion is the scientific word for burning and is a type of chemical reaction. Combustion is the reaction when a substance burns and reacts with oxygen to produce heat and light energy.

What is combustion? When a substance burns, it is said to combust.
Combustion is a rapid reaction between a substance and oxygen that releases heat and light energy. A fuel is a substance that reacts with oxygen (combusts) to release useful energy. Biomass is used as fuels because they burn easily and release a large amount of useful energy.

Fire The Fire Triangle What is needed for combustion to take place?
Three things must be present at the same time to produce fire: The Fire Triangle

Fire The Fire Triangle How to extinguish the fire?
Take away any of these things and the fire will be extinguished.

What is combustion? Combustion is the process with which everyone is familiar by which flammable materials are allowed to burn in the presence of air or oxygen with the release of heat. The basic process is oxidation. Combustion is the simplest method by which biomass can be used for energy, and has been used for millennia to provide heat. This heat can itself be used in a number of ways: Space heating Water (or other fluid) heating for central or district heating or process heat Steam raising for electricity generation or motive force.

Direct combustion Combustion of biomass has been widely used in the past to generate heat At present, it is making a comeback in many industrial applications including generation of electricity, Straightforward conversion of thermal energy into mechanical or electric power results in considerable losses It is not possible to raise the ratio of thermal to mechanical power above 60%. However, if the low temperature waste heat can be used productively, for instance for drying or heating purposes, much higher overall efficiencies can be obtained.

Direct combustion Biomass materials mainly comprised of cellulose, hemicelluloses and lignin, each consists of three basic elements: C, O and H. Further, elements such as S and N could be present. In such case, the chemical equation can be expressed in a very convenient form, as a stoechiometry formula written for one atom of carbon as CHxOyNzSu. For pure and dry biomass fuels of the lignocellulosic type, nitrogen and sulfur are usually negligible and the chemical formula may be rewritten as follows: CHxOy with x ≅1.44 and y ≅0.66 describing the average composition of typical biomass used for combustion, i.e., wood, straw, or similar material.

Direct combustion When the flammable fuel material is a form of biomass the oxidation is of predominantly the carbon (C) and hydrogen (H) in the cellulose, hemicellulose, lignin, and other molecules present to form carbon dioxide (CO2) and water (H2O). The above, and other, molecules within the biomass also contain other atoms in different quantities and some of these too can be oxidized, with the oxide released as gas in the flue gasses, or as solid as ash or slag. All carbohydrates, such as cellulose, also contain oxygen in the molecular structure. Other atoms potentially found in biomass include: Nitrogen (N) Phosphorus (P) Potassium (K) Silicon (Si) Sulphur (S).

Direct combustion Cooking with firewood in developing countries, typical cookstove efficiency is 10-20%, 30% with improved stoves (vs % with gaseous fuels) Pellet heating, central Europe in particular District heating in Sweden, Atlantic Canada Issues include ash content (which is related to the non- combustible silica in the biomass, which can be high) and K and Ca in the fuel, which can cause agglomeration in boilers

Direct combustion Humans have been combusting biomass for heating and cooking for millions of years Robust process, easy to maintain Boilers convert heat to hot water or other medium or generate steam for distribution Thermodynamic limits on efficiency Inefficient systems create more air pollution and require more fuel to operate

Radiation to Surrounding Convective Heat to Surrounding
Direct combustion Heat Energy Fuels and Combustion Biomass combustion Air and Fuel Combustion Unit Combustion Products Light Volatile Matter Hot Flue Gas Flame Front Entrained Air Burning Char Ash Wood Conduction to Wood Radiation to Wood Radiation to Surrounding Convective Heat to Surrounding

Direct combustion Overview Air Heat Conduction to Fuel
Fixed Carbon (Char) Volatile Matter Fuel Heat Glowing Combustion Flaming Combustion Flue Gas Heat Radiation to Fuel Light Infrared Radiation Conduction Convection Radiation

Direct combustion Properties of Fuels Solid. Density Moisture Content
Volatile Matter and Fixed Carbon Sulfur Content Ash Calorific Value Fuel Volatile Matter Fixed Carbon Ash Paddy Husk 63.3 14.0 22.7 Bagasse 74.0 19.3 6.7 Wood Lignite 43.0 46.6 10.4 Anthracite Coal 5.0 80 15

Combustion of biomass Volatiles and Fixed Carbon
Volatiles: Flaming combustion Fixed carbon: Glowing combustion

Combustion of biomass Combustion Controlling Factors
Physical and chemical properties of the fuel; Fuel/air ratio; Temperature of the flame/envelope; Mode of fuel supply; Primary and secondary air supplies.

Combustion of biomass Biomass combustion is a complex process that consists of consecutive heterogeneous and homogeneous reactions. The main process steps are drying, devolatilization, gasification, char combustion, and gas phase oxidation. The time used for each reaction depends on the fuel size, properties, temperature, and combustion conditions. Different types of pollutants can be distinguished from the combustion process: Uncombusted pollutants such as CO, condensable organic compounds (COC, also referred to as "tar”), and polycyclic aromatic hydrocarbons (PAH), soot, carbon, H2, HCN, NH3, and N2O Pollutants from complete combustion such as NOX (NO and NO2), CO2, and H2O Ash and contaminants such as ash particles (KCl, etc.), SO2, HCl, Cu, Pb, Zn, Cd etc.

Combustion of biomass

Combustion of biomass - Processes and temperatures in a burning piece of wood

Combustion of biomass Products of Incomplete Combustion PIC PIC Heat

Combustion of biomass Wood is mainly just carbon, hydrogen, and oxygen: [CH2O]x Combustion: CH2O + O CO2 + H2O + heat Why doesn’t wood emit only CO2 and H2O when it is burned? Answer: Incomplete combustion – unavoidably, some of the wood carbon is not completely combusted into CO2.

Burning biomass is almost never complete
Burning biomass is almost never complete. There is always incomplete combustion. Incomplete versus complete combustion

Incomplete combustion
If there is a shortage of air (oxygen), incomplete combustion of biomass takes place. Instead of producing just carbon dioxide and water, incomplete combustion also produces carbon monoxide and/or carbon (soot). These form tiny particle in the air (particulates). It also releases less energy than complete combustion. Carbon monoxide is a poisonous gas because it reduces the ability of blood to carry oxygen. Biommas contain sulfur compounds. When the fuel burns, these sulfur compounds produce sulfur dioxide.

Combustion of biomass -Chemical Composition of Wood Smoke
avg ppm carbon monoxide up to 100,000 methane 20,000 VOCs* (C2-C7) 15,000 total particle mass particulate organic carbon 10,000 oxygenated mono-aromatics 5,000 alkyl benzenes 3,000 aldehydes 2,000 benzene acetic acid oxygenated PAHs 300 substituted naphthalenes 250 Chemical avg ppm nitrogen oxides 200 sulfur dioxide naphthalenes guaiacol 150 syringol substituted furans formic acid 70 methyl chloride 20

Environmental impact of biomass combustion
Biomass combustion exhibits relatively high emissions of NOX and particulates in comparison to the combustion of natural gas or light fuel oil. Biomass combustion contributes significantly to particulate matter (PM), ozone, and NO2 in the ambient air. For wood combustion, a life cycle assessment (LCA) indicates that 38.6% of the environmental impact of a modern automatic wood furnace is attributed to NOX, 36.5% to PM 10, only 2% to CO2 and 22.9% to all other pollutants (SOX, NH3, CH4, NMVOC, primary energy, residues, and others) In case of poor combustion conditions in manually operated wood stoves or boilers, PM emissions can be higher than assumed in the cited LCA by a factor of 10

Environmental impact of biomass combustion
Biomass does not contribute to the greenhouse effect, since the CO2 generated in the combustion is reabsorbed by means of photosynthesis in plants necessary for its production

Indoor pollution concentrations from typical woodfired cookstove during cooking
Indoor Levels International Agency for Research on Cancer (IARC) Group I Carcinogens Typical standards to protect health

Environmental performances
Impacts of emissions from biomass combustion Component Biomass Sources Climate, environmental and health impact Carbon dioxide (CO2) Major combustion product from all biomass fuels Climate: Direct GHG. However, biomass is a CO2-neutral fuel Carbon monoxide (CO) Incomplete combustion of all biomass fuels Climate: Indirect GHG through ozone formation. Health: Reduced oxygen uptake especially influences people with asthma, and embryos. Suffocation in extreme cases. Methane (CH4) Climate: Direct GHG. Indirect GHG through ozone formation. Non Methane Volatile Organic Components (NMVOC) Health: Negative effect on human respiratory system Polycyclic Aromatic Hydrocarbons (PAH) Environment: Smog formation Health: Carcinogenic effects Particles Soot, char and condensed heavy hydrocarbons (tar) from incomplete combustion of all biomass fuels. Fly ash and salts Climate and environment: Reversed greenhouse effect through aerosol formation. Indirect effects of heavy-metal concentrations in deposited particles. Health: Negative effect on the human respiratory system. Carcinogenic effects

Impacts of emissions from biomass combustion Component Biomass Sources Climate, environmental and health impact Nitric oxides (NOX = NO and NO2) Minor combustion product from all biomass fuels containing nitrogen. Additional NOx may be formed from nitrogen in the air under certain conditions Climate and environment: Indirect greenhouse effect through ozone formation. Reversed greenhouse effect through aerosol formation. Acid precipitation. Vegetation damage. Smog formation. Corrosion and material damage. Health: Negative effect on the human respiratory system. NO2 is toxic Nitrous oxide (N2O) Minor combustion product from all biomass fuels containing nitrogen Climate: Direct GHG. Health: Indirect effect through ozone depletion in the stratosphere Ammonia (NH3) Small amounts may be emitted as a result of incomplete conversion of NH3 from pyrolysis/ gasification Environment: Acid precipitation. Vegetation damage. Corrosion and material damage. Health: Negative effect on the human respiratory system. Sulphur oxides (SOX = SO2 and SO3) Minor combustion product from all biomass fuels containing sulphur. Climate and environment: Reversed greenhouse effect through aerosol formation. Acid precipitation. Vegetation damage. Smog formation. Corrosion and material damage. Health: Negative effect on the human respiratory system, asthmatic effect

Impacts of emissions from biomass combustion Component Biomass sources Climate, environmental and health impact Heavy metals All biomass fuels contain heavy metals to some degree, which will remain in the ash or evaporate Health: Accumulate in the food chain. Some are toxic and some have carcinogenic effects Ground level ozone (O3) Secondary combustion product from atmospheric reactions, including CO, CH4, NMVOC and NOX Climate and environment: Direct GHG. Vegetation damage. Smog formation. Material damage. Health: Indirect effect through ozone depletion in the stratosphere. Negative effect on the human respiratory system, asthmatic effect Hydrogen Chloride (HCl) Minor combustion product from all biomass fuels containing chlorine Environment: Acid precipitation. Vegetation damage. Corrosion and material damage. Health: Negative effect on the human respiratory system. Toxic Dioxins and Furans PCDD/PCDF Small amounts may be emitted as a result of reactions including carbon, chlorine, and oxygen in the presence of catalysts (Cu) Health: Highly toxic. Liver damage. Central nervous system damage. Reduced immunity defense. Accumulate in the food chain

Combustion technologies
How to produce electricity by direct combustion

Combustion technologies convert biomass fuels into several forms of useful energy for commercial or industrial uses: hot air, hot water, steam and electricity A furnace is the simplest combustion technology: biomass fuels burns in a combustion chamber converting biomass into heat energy (hot gases contains 85 % of the fuel’s potential energy) either direct or indirect use of heat exchanger to use the hot gases in the form of hot air or hot water combustion of wood can be divided into four phases: Drying: water inside the wood boils off Degasification: gas content is freed from the wood Gasification: the gases emitted mix with atmospheric air and burn at a high temperature Combustion: the rest of the wood (mostly carbon) burns

A biomass-fired boiler is a more adaptable direct combustion technology because a boiler transfers the heat of combustion into steam steam can be used for electricity, mechanical energy and heat boiler’s steam output contains 60 to 85 % of the potential energy in biomass fuel major types of biomass combustion boilers: pile burners, stationary or travelling grate combustors, fluidized-bed combustors Pile burners: consist of cells, each having an upper and a lower combustion chamber biomass fuel burns on a grate in the lower chamber, releasing volatile gases the gases burn in the upper combustion chamber operator must shut down pile burners periodically

Fluidized-bed combustors: burn biomass fuel in a hot bed of granular material, such as sand injection of air into the bed creates turbulences resembling a boiling liquid the turbulences distributes and suspends the fuel the design of a fluidized-bed reactor increases heat transfer and allows for operating temperatures below 950°C, reducing nitrogen oxide emissions fluidized-bed combustors can handle high-ash fuels, agricultural residues and sewage sludge

Cogeneration: using a boiler to produce heat and electricity conversion efficiency 85% for comparison: electricity production from steam-driven turbine- generators 17 to 25 % conversion efficiency Direct-Fired Gas Turbine Technology: fuel pre-treatment reduces biomass to a particle size less than 2 mm and a moisture content of less than 25 % fuel is burned with compressed air turbine electricity Co-Firing: biomass is used as secondary fuel e.g. in coal-burning power plants could help to reduce sulphur dioxide and nitrogen oxide emissions decreases net carbon dioxide emissions from the power plant (if the biomass fuel comes from a sustainable source)

Comparison of Direct Combustion Technologies Application Type Typical Size (MW) Fuels Ash (%) Water content (%) Manual Wood stoves Dry wood logs < 2 5 - 20 Log wood boilers Log wood, sticky wood residues 5 - 30 Pellets Pellet stoves and boilers Wood pellets 8 - 10 Automatic Understoker furnaces Wood chips, wood residues 5 - 50 Moving grate furnaces All wood fuels and most biomass < 50 5 - 60 Pre-oven with grate Dry wood (residues) < 5 5 - 35 Understoker with rotating grate 2 - 5 Wood chips, high water content Cigar burner 3 - 5 Straw bales 20 Whole bale furnaces Whole bales Straw furnaces Straw bales with bale cutter Stationary fluidized bed 5 - 15 Various biomass, d < 10mm Circulating fluidized bed Dust combustor, entrained flow 5 - 10 Various biomass, d < 5mm < 20 Co-firing* total 5 -60 total Cigar burner straw straw 5 – 20 Dust combustor in coal boilers total 100 – 1000 Various biomass, d < 2-5mm

Comparison of different types of direct combustion technologies Parameter Pile Combustion Stoker Combustion Suspension Combustion Fluidized Bed Combustion Grate Fixed / Stationary Grate Fixed or moving grate No grate or moving grate No grate Fuel Size Uniform size of the fuel in the range of mm is desired & % fines should not be more than 20% Uneven fuel size can be used Preferable for high % of fins in the fuel Uniform size fuel in the range of mm Difficult to maintain good combustion due to : Air fuel mixing is not proper, Bed height is in stationary condition resulting in clinker formation, Difficult to avoid air channeling Due to intermittent ash removal system it is difficult to maintain good combustion. The combustion is better & an improved version of pile combustion. Since most of the fuel is burnt in suspension the heavier size mass falls on the grate. If the system has a moving grate the ash is removed on a continuous basis & therefore the chances of clinker formation are less. It is similar to stoker combustion, but since the fuel sizes is small & even the combustion efficiency is improved as maximum amount of fuel is combusted during suspension. Best combustion takes place in comparison with the other types since the fuel particles are in fluidized state & there is adequate mixing of fuel & air.

Comparison of different types of direct combustion technologies Parameter Pile Combustion Stoker Combustion Suspension Combustion Fluidized Bed Combustion Bed temperature ºC ºC ºC Moisture High moisture leads to bed choking & difficult combustion conditions Combustion condition not very much disturbed with 4-5 % increase in moisture Same as Stoker Combustion It can handle fuels with high moisture condition up to % but high moisture in the fuels is not desirable, & adequate precautions are to be taken up in the design stage itself. Draft Conditions Natural Draft / Forced Draft/ Balance Draft Forced Draft / Balance draft Balance draft Maintenance Not much maintenance problems Frequent problems due to moving grate Variation in fines in fuel leads to delayed combustion thereby affecting the boiler tubes Erosion of boiler tubes embedded in the bed is quite often

Advantages and disadvantages
Advantages and disadvantages of modern firing (bulk goods vs. bulk solids) Advantages Disadvantages firing of wood log - low investment costs - low stock requirements for the solid fuels - high efficiency (up to 90 %) - high operating expense - buffer storage to avoid light load operation firing of wood chips - user friendly and low-maintenance - automatic provision of heat - very high efficiency (more than 90 %) - also weak wood residuals useable - higher costs of investment - higher stock requirements for the solid fuels necessary firing of wood pellets - very high efficiency (up to 95 %)

Operational problems in biomass combustion
The nature and severity of the operational problems related to biomass depend on the choice of combustion technique. In grate-fired units deposition and corrosion problems are the major concern. In fluidized bed combustion the alkali metals in the biomass may facilitate agglomeration of the bed material, causing serious problems for using this technology for herbaceous based biofuels. Fluidized bed combustors are frequently used for biomass (e.g. wood and waste material), circulating FBC are the preferred choice in larger units. Application of biomass in existing boilers with suspension- firing is considered an attractive alternative to burning biomass in grate-fired boilers. However, also for this technology the considerable chlorine and potassium content in some types of biomass (e.g. straw) may cause problems due to deposit formation, corrosion, and deactivation of catalysts for NO removal (SCR).

Currently wood based bio-fuels are the only biomasses that can be co-fired with natural gas; the problems of deposition and corrosion prevent the use of herbaceous biomass. However, significant efforts are aimed at co-firing of herbaceous biomass together with coal on existing pulverized coal burners. For some problematic fuels, esp. straw a separate auxiliary boiler may be required. In addition to the concerns about to deposit formation, corrosion, and SCR catalyst deactivation, the addition of biomass in these coal units may impede the utilization of fly ash for cement production. In order to minimize these problems, various fuel pretreatment processes have been considered, including washing the straw with hot water or using a combination of pyrolysis and char treatment. Some types of biomass contain significant amounts of chlorine, sulfur and potassium. The salts, KCl and K2SO4, are quite volatile, and the release of these components may lead to heavy deposition on heat transfer surfaces, resulting in reduced heat transfer and enhanced corrosion rates.

Complete combustion produces minimum pollution and depends on the combustion chamber temperature, the turbulence of the burning gases, residence time and the excess O2 These parameters are governed by: combustion technology (e.g. combustion chamber design, process control) settings of the combustion (e.g. primary and secondary air ratio, distribution of the air nozzles) load condition (full- or part-load) fuel characteristics (shape, size distribution, moisture content, ash content, ash melting behaviour). Biomass is difficult to handle and combust due to low energy density and presence of inorganic constituents. The release of alkali metals, chlorine and sulfur to the gas-phase may also lead to generation of significant amounts of aerosols (sub-micron particles) along with relatively high emissions of HCl and SO2.

Biomass co-firing - An efficient way to reduce greenhouse gas emissions

The cofiring of biomass with coal is becoming popular as biomass is considered carbon-neutral.

Co-firing Definition: Co-combustion or co-firing is to burn or to convert two or more fuels together in one boiler system - the combustion or cogasification of coal and biomass, or the combustion of coal with biomass-derived fuel gas Co-firing is a very attractive option for the utilisation of biomass and for the delivery of renewable energy, in terms of the capital investment requirement, the security of supply, the power generation efficiency and the generation cost. Objective: to achieve emission reductions This is not only accomplished by replacing fossil fuel with biomass, but also as a result of the interaction of fuel reactants of different origin, e.g. biomass and coal. Waste reduction with energy utilization

Why Biomass and co-firing?
Unlike fossil fuels, biomass fuel is renewable and CO2-neutral in the sense that the CO2 it only releases recently fixed carbon when combusted thereby closing the carbon loop on a short time Partial substitution of coal for combustion Legislation on CO₂ reduction to meet Kyoto target and EU’s target to reduce CO₂ missions by 20% by 2020. Hence the combination of oxy-fuel combustion with biomass fuel become a CO2 sink for power plants.

Power Generation Coal Used extensively to generate electricity and process heat for industrial applications. Poses significant world environmental problems: global warming (CO2) acid gases (NOx and SO2) Biomass: as a fuel source Biomass fuels are CO2-neutral, hence reduce global warming effects. The sulphur and nitrogen contents are often lower. Differences between biomass and coal Higher moisture content (= low net calorific value) Higher Cl content Low heating value Low bulk density Higher content of volatile matter (80%:coal 30%)

Biomass characteristics
Lower density Higher moisture content, often up to 50% Lower calorific value Broader size distribution, unless pre-conditioned by screening, crushing or pelletising The variability of the material as a fuel will be greater Such variations in fuel quality, compared to coal, may have a number of implications for plant applications that include process design and operation, and potentially, plant availability.

Emission reduction Theoretical decrease in CO2 emissions by co-firing of wood with coal

Emission reduction

Synergy effects Fuel Composition of fuels

Effects of biomass co-firing on NOx emissions
Can reduce NOx through lower N content (depends on biomass) and higher volatiles release in the fuel rich zone of the flame, but amount of NOx produced does not follow simple additivity. Also gives lower flame temperature, reducing thermal NOx but may affect the SCR – larger quantities of alkalis such as K, Na, Ca and phosphorus may blind or poison the catalyst, leading to higher NOx emissions and potentially high ammonia slip Can need earlier catalyst change

Effects of biomass co-firing on SO2 emissions
Coal blend principally affecting SO2 emissions are: The total sulphur content (represents maximum amount of sulphur oxides that could be formed) The ash composition (since typically 5-10% of the SO2 is generally captured by alkalis in the coal ash) Biomass generally has much lower contents of sulphur, together with higher concentrations of alkalis in its ash, so SO2 emissions are generally considerably reduced when co- firing.

Effects of biomass co-firing on particulates emissions
Chemical and physical properties of fly ash particulates from biomass combustion are different from those of coal Can give higher release of trace metals Reduces fly ash loading Can increase overall collection efficiency of ESPs due to larger particulates and ease of agglomeration But may instead reduce collection efficiency, due to high resistivity of fly ash, and increase PM2.5 emissions

Co-firing biomass with coal reduced emissions of major pollutants
Plots for average emission impacts of co-firing coal with biomass

Co-firing: merits Some biomass fuels can be grown on redundant agricultural or set-aside land, improving local economics and creating jobs. Increased plant flexibility in terms of fuels utilised. Improved plant economics through the use of zero/low cost fuel feedstocks. Fuel feedstocks may be available locally, reducing transport costs. Replacement of part of the coal feed can reduce dependence on imported fuels and help maintain strategic national reserves of coal. Reduced emissions of main classes of pollutants through reduction in amount of coal burnt. This can occur through simple dilution or via synergistic reactions between biomass feedstocks and coal. Several types of combustion and gasification technology may be applicable to a particular combination of feedstocks. These may include pulverised fuel, bubbling fluidised bed combustion and circulating fluidised bed combustion.

Co-firing: merits Encourages development of feedstock infrastructure
Creates a market for residues and energy crops Co-firing represents a cost effective, short term option at a large scale Co-firing provides means for emissions reduction Reducing NOx emissions Biomass blending decreases SO2 emissions Trace organic compounds Particulates The Existing Power Plant Existing equipment is still utilized Easier to meet environmental regulations and hedge future regulations Cost savings Tax incentives Fuel supply options

Co-firing: demerits Feedstock pre-preparation may be required. For instance, wood requires chipping, straw may require chopping up, etc. resulting in increased energy requirements. Some biomass materials have low bulk density (e.g. straw), this resulting in the handling and storage of large quantities of materials. Moisture content may be high, reducing overall plant efficiency. Depending on the feedstock, the complexity of fuel feeding requirements may be increased; some materials can be co-fed using a single feed system whereas others require a separate, dedicated system. Contains potassium: it can cause corrosion Existing boilers/systems designed (exclusively) for fossil fuels Negative impact on existing boilers Cl-based corrosion Negative impact on boiler capacity

Summary: Combustion attributes of biomass vs coal
Benefits from Biomass Carbon neutral No SO2 emissions Lower NOx emissions Lower ash content No mercury Benefits from Coal Higher heating value No dioxin emissions Challenges with Coal High SO2 formation Need to control mercury Need to control heavy metals High NOx formation Potential of fouling High CO2 Emissions Challenges with Biomass Lower heating value Potential of fouling Dioxins if fuel contaminated

Co-firing of biomass methods
1. Co-milling of biomass with coal 2. Separate milling, injection in pf-lines, combustion in coal burners 3. Separate milling, combustion in dedicated biomass burners 4. Biomass gasification, syngas combusted in furnace boiler 5. Co-milling of torrefied biomass with coal Each co-firing route has its own (unique) operational requirements and constraints and specific demands on the fuel quality

Main co-firing methods
Most earlier conversions, suitable for a range of biomasses, were achieved by mixing the material with the coal on the conveyor feeding the existing mills (1) This allowed rapid installation of co-firing, at modest capital cost, for a range of biomasses, but at co-firing fractions only up to about 10% thermal Most recent projects use injection of milled biomass into the pulverised coal pipes (2), allowing much higher proportions of biomass to be co-fired, up to 100% Other methods are available, including adding dedicated biomass burners

Biomass co-firing methods
The co-firing of solid biomass by pre-mixing with the coal and processing the mixed fuel through the installed coal handling, milling and firing systems, The direct co-firing of milled solid biomass by pneumatic injection into the furnace, through dedicated biomass burners or through the existing coal burners, The indirect co-firing of solid biomass by gasification and co- firing of the product gas, The parallel co-firing of solid biomass in a dedicated biomass boiler, with utilisation of the steam in the power generation system of a large coal power plant, and The co-firing of liquid biomass materials as a replacement for fuel oil, for light-up/mill support and for load carrying.

Direct co-firing Two methods were developed:
Blending the biomass and coal in the fuel handling system and feeding blend to the boiler. Separate fuel handling and separate special burners for the biomass, and thus no impact to the conventional coal delivery system.

Biomass co-firing by pre-mixing with coal and co-milling: general aspects
Co-firing by co-milling is commonly the preferred approach for stations embarking on co-firing activities for the first time. The capital investment, at least for the initial trial work, can be kept to modest levels, and the expenditure is principally on the biomass reception, storage and handling facilities. The project can be implemented in reasonable time. This approach is particularly attractive when there are concerns about the security of supply of the biomass materials, and about the long-term security of the subsidy payments for co-firing.

Biomass co-firing by pre-mixing and co-milling
In general, this approach permits co-firing at levels up to 5-10% on a heat input basis. The key constraints are: The availability of suitable biomass supplies, The limitations of the on-site biomass reception, storage and handling facilities and The limitations associated with the ability of the coal mills to co-mill biomass materials. There are also safety issues associated with the bunkering and milling of the mixed coal-biomass material.

The co-milling of biomass with coal in coal mills
A range of biomass materials are being co-milled with coal in ball and tube mills, and in vertical spindle ball and ring, and roller mills. These mills depend on the coal particles being subject to brittle fracture, and this does not apply to most biomass materials. There is a tendency for the biomass particles to accumulate in the mill, during normal operation, and to take longer to clear from the mill during shutdown. With vertical spindle mills there is a tendency for the mill differential pressure and the mill power take to increase when co-milling biomass. The mill product topsize tends to increase, due to the lower particle density of the biomass, i.e. larger biomass particles can exit the classifier. When co-milling wet biomass materials there will be an impact on the mill heat balance, and this may be a limiting factor.

Safety issues when co-milling biomass in large vertical spindle coal mills
The key issue in mill safety is avoiding hot primary air coming into direct contact with dry fuel. This is particularly important during certain mill operations such as planned shutdowns, emergency shutdowns and restarts after emergency shutdowns, loss of coal or intermittent coal feed incidents, etc. Biomass has high volatile matter content and combustible volatiles are released in significant quantities at temperatures above about 180oC, i.e. at much lower temperatures than for bituminous coals. It is usually advisable to reassess and modify the mill operating procedures to allow the co-milling of biomass safely.

Direct injection co-firing

Direct injection co-firing systems for biomass - basic options
Direct injection co-firing involves by-passing the coal mills and can increase the co-firing ratio. The biomass can be pre-milled either off-site or on-site. All direct injection co-firing systems involve the pneumatic conveying of the pre-milled biomass from the fuel reception and handling facility to the boiler house. There are three basic direct injection co-firing options: Direct injection into the furnace with no combustion air, New, dedicated biomass burners, and Injection of the biomass through modified burners or into the pulverised coal pipework.

Direct injection through dedicated burners
If the existing coal-firing capability is to be maintained, additional burners are required for biomass firing. Appropriate locations for the biomass burners are not easy to find, particularly as a retrofit. Additional furnace penetrations and burner support structures are required. Fuel and air supply systems for the biomass burners have to be installed. Flame monitoring equipment for the biomass flames is required. The impact of exposure of the ‘out of service’ biomass burners to the coal-fired furnace gases needs to be assessed. The impacts of the new biomass burners on the coal-firing system have to be assessed. Overall, the installation of dedicated biomass burners is an expensive and relatively high risk approach to biomass co-firing.

Direct injection to modified burners
May be necessary with some fuels, e.g. chopped cereal straws, Recent projects have involved modification of both wall- fired and corner-fired furnaces, Biomass metering and pneumatic conveying systems to each burner are required, The burner modifications involve significant additional cost, There are risks of interference with the coal combustion process and NOx emission control, Overall, this is a viable, if relatively expensive, approach to direct injection co-firing.

Direct injection into the pulverised coal pipes
Direct injection into the existing coal firing system is relatively simple and cheap to install, and this is generally the preferred option. The preferred injection locations are into the pulverised coal pipework at the mill outlet or local to the burners. The mill air and fuel flow rates have to be reduced in line with the biomass conveying air flow rate, and the heat input to the mill group from the biomass. Both the mill and the burners are maintained within their normal operating envelopes for both the heat input and primary air flow rate. The maximum heat input from the mill group is maintained. There are new interfaces between the mill and biomass conveying system controls, covering permits to operate, biomass system shutdowns, start-ups and trips, etc. These systems have been in successful operation since 2005, firing a wide variety of pre-milled biomass materials.

Biomass co-firing via pre-gasification (Indirect)
Indirect co-firing Indirect co-firing for gas-fired boilers Biomass co-firing via pre-gasification (Indirect)

Parallel co-combustion (steam-side coupling)

Technical issues with biomass co-firing at elevated levels
The procurement of large quantities of biomass, Fuel quality/flexibility issues, and off-site biomass storage and pre-processing arrangements and costs. Fuel deliveries/reception, on-site handling, storage and pre-processing of very large quantities of biomass. Direct injection of pre-milled biomass at high biomass co-firing ratios, and the impact on combustion/NOx control, The increased risks of excessive ash deposition, and fireside boiler tube corrosion. The production of mixed biomass/coal ashes and the risks to the normal ash utilisation/disposal routes.

Biomass ash effects Most biomass materials have low ash contents (<5%), compared to most power station coals. The biomass ashes are very different chemically from coal ashes, i.e. they are not an alumino-silicate system, but a mixture of simple inorganic compounds, of Si, K, Ca, P and S. There are concerns about increased rates of deposition on boiler surfaces and the surfaces of SCR catalysts. There are concerns about increased rates of high temperature corrosion of boiler components, with high chlorine biomass materials. Biomass co-firing tends to increase the level of submicron aerosols and fume in the flue gases, and may impact ESP collection efficiency. There may be utilisation/disposal issues with mixed coal/biomass ashes.

The effect of biomass ash on Ash Fusion Temperatures and fouling behaviour
Coal ash slagging For coals with high ash fusion temperatures, the addition of relatively small amounts of some biomass ashes can reduce the DT by as much as 200ºC. For low ash fusion temperature coals, the effect is much less dramatic. For predictive purposes, the normal coal Slagging Indices can be applied to mixed biomass-coal ash systems. Empirical correlations permit estimation of the Deformation Temperatures of mixed ashes. Coal ash fouling Fouling indexes for mixed biomass/coal ashes are based on the alkali metal contents of the fuels.

Forms of wood-based biomass for cofiring
Wood chips – established for low firing ratios mixed with coal before milling Wood pellets – most widely used, suitable up to high cofiring ratios, milled separately from coal in vertical spindle mills; a commodity fuel with sustainability standards, product standards, consistency, large production facilities, large export/import facilities developed Steam exploded pellets – less established – favoured by some utilities Torrefied wood – demonstration stage, aimed at being straight partial or full replacement fuel, minimal change to plants

Drivers of co-firing biomass
Reduces the emissions of greenhouse gases and other pollutants Co-firing in coal plants would strongly increase biomass use Lowest capital cost option for increasing the use of biomass to produce electricity Co-firing biomass and coal takes advantage of the high efficiencies obtainable in large coal-fired power plants Improves combustion due to the biomass higher volatile content Jobs creation. Technical barriers Thermal behavior and efficiency Fouling and corrosion of the boiler (alkalis, chlorine) Environmental constraints - emissions

Contents Direct combustion Biomass energy Basic Definitions

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