USING BIOMASS FOR BOILERS – The Way Forward

(Research for this section of boilers guide was done by Uchit Nair of SRM University,Chennai)

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 How do i move from coal to biomass boilers?
what are the uncertainties that arise while using biomass?

The following page is a guide and selection tool that enables you to pick the optimal biomass, processing method and boiler to help ease the process. We have split the task into three stages: first to understand the characteristics to look for in a given biomass, second to pick the right processing method for the ease of transportation and storage and third to pick the appropriate combustion system and pollution control equipment.

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Biomass Characteristics

To set up a biomass combustion plant it is vital to understand the fuel that you are working with, hence the first step is picking the right biomass fuel. The parameters to be considered before are:

Availability and Distance of provider: The primary selection criterion for the biomass has to be the availability and distance of the supplier from the plant. The biomass fuel has to be available and in close proximity to the biomass combustion plant to be set up to reduce transportation costs.

An analysis of the properties of biomass results in a datasheet like the one below. In order to understand the data and make conclusions about the quality of the biomass we have provided a few pointers:

Parameter Comments Desired Value
Proximate Analysis
Calorific Value It is defined as the energy contained in a fuel,
determined by measuring the heat produced by the
complete combustion of a specified quantity
of it.
High
Moisture Moisture content in the biomass
reduces the calorific value and efficiency, increases
weight during transportation and causes storage problems.
The moisture in biomass fuels is usually high hence
it has to be dried and made dense till the moisture
content is around 7-8% which is ideal for
transportation, storage and combustion.
Low
Volatile Matter Volatile matter is the substances given off as gas
or vapour apart from moisture during combustion.
Low
Fixed Carbon Fixed carbon is the solid combustible residue that
remains after a fuel is heated and the volatile matter is
expelled.
High
Ash Ash is the incombustible solid mineral matter in a fuel.
The percentage of ash produced by the combustion of the fuel
is an indication of waste and loss in efficiency and also causes
slagging and fouling on boiler surfaces.
Low
Ultimate Analysis
Carbon An increasing Carbon percentage results
in an increase in calorific value of the fuel.
High
Nitrogen & Sulphur Nitrogen and Sulphur in the fuel leads
to higher production of noxious gases like
NOX and SOX.
Low
Chlorine Chlorine is a major factor in ash formation
and corrosion. It facilitates the movement of inorganic
compounds in particular potassium. Potassium chloride
is one of most stable high temperature, gas-phase, alkali
containing species, hence is the prime cause for corrosion
in boilers.
Low
Elemental Composition of Ash
Silica High percentages of silica in the ash react
with alkaline substances to lower the ash fusion temperature
which causes slagging and fouling on boiler surfaces.
Silica is also known to form crystalline silica at furnace
temperatures which could cause breathing hazards as seen
while using rice husk.
Low
Na2O & K2O Alkaline oxides in ash cause slagging and fouling on boiler
surfaces resulting in increased maintenance costs.
Low
CaO & MgO These components increase the ash fusion
temperature and hence reduce slagging and fouling.
High

 Slagging and Fouling indices

Indices Formula Tendencies
Low Medium High
Slagging : Base-Acid Ratio (Fe2O3+CaO+MgO+Na2O+K2O)/ (SiO2+TiO2+Al2O3) < 0.4 or > 0.7 0.4 to 0.7
Fouling : Alkaline Metal Content (Na2O+K2O+0.659)x ash/100 < 0.3 0.3 to 0.45 > 0.45

 

For example, given the option between the following fuels we arrive at our best option using the parameters above

Parameters Rice Husk Switch grass Sugarcane Bagasse
Calorific Value 3200 4200 4500
Proximate Analysis (% Dry Fuel)
Fixed Carbon 16.22 14.34 11.95
Volatile Matter 63.52 76.69 85.61
Ash 20.26 8.97 2.44
Ultimate Analysis (%)
Carbon 38.83 46.68 48.64
Oxygen 35.47 37.38 42.82
Nitrogen 0.52 0.77 0.16
Sulphur 0.05 0.19 0.04
Chlorine 0.12 0.19 0.03
Elemental Composition of Ash (%)
SiO2 91.42 65.58 46.61
Al2O3 0.78 4.51 17.69
TiO2 0.02 0.24 2.63
Fe2O3 0.14 2.03 14.14
CaO 3.21 5.60 4.47
MgO < 0.01 3.0 3.33
Na2O 0.21 0.58 0.79
K2O 3.71 11.60 0.15
SO3 0.72 0.44 2.08
P2O5 0.43 4.50 2.72
Slagging & Fouling Indices
Base – Acid Ratio 0.08 0.32 0.34
Alkaline Content 1.15 1.12 0.03
From the above table we identify sugarcane bagasse as our best option as it has a higher calorific value, lower ash content, low Nitrogen, Sulphur and Chlorine content, less Silica and Alkali values in its ash and has a good enough base to acid ratio and fouling factor.

Biomass Densification

Biomass needs to be made dense to provide ease of mechanical handling and feeding, uniform combustion in boilers, reduced cost of transportation and ease of storage. The types of dense biomass to choose from are pellets, briquettes or loose biomass that has been dried before being fed into the boiler. They can be picked based on the following parameters:

Parameter Comments Desired Value
Unit Density & Bulk Density A high unit density and bulk density reduces transportation costs and increases ease of storage. High
Energy Density A higher energy density decreases transportation and storage costs. High
Dimension The dimensions of the dense biomass has an effect on the ease of transport and storage as well as the combustion performance based on the system being used Based on combustion system used
Moisture Content Moisture present in the biomass can increase weight, decrease efficiency and hence affect transportation and storage costs Low
Combustion Performance The combustion of more dense, uniform biomass is more efficient. High
Ash Content The ash content produced can be different based on the densification methods due to adulteration and addition of inorganic binders used in each method. Low
Fuel Feeding System  Fuel can be fed into the boiler either through fully automated systems or semi-automated systems Based on user

Pellets: Pellets are very dense, cylindrical with a diameter of 6-8mm and a length of about 38mm. They are produced through an extrusion process using a piston press.

Briquettes: They are similar to pellets but not as densely packed and much bigger in size with a diameter greater than 25mm.

Parameters Pellets Briquettes Loose Biomass
Bulk Density (kg/m3) 550 – 700 ~ 350 60 – 80
Energy Density (GJ/m3) 9.8 – 14 ~ 6  <<6
Dimensions (mm) 6-8(dia)x38 mm 32 (dia) x 25 (thick)  N/A
Combustion Performance Very Good Moderate Poor
Transportation Cost ~ 7-8x lesser than loose biomass ~ 4x lesser than loose biomass Most Expensive
Procurement Cost ~ 10x more than that of loose biomass ~5x more than that of loose biomass Cheapest
Moisture Content 7-8% 7-8% > 8%
Ash Content Less More More
Fuel Feeding System They can be used in a completely automated or a semi-automated system They can only be used in a semi-automated system They can only be used in a semi-automated system
    From the above data we infer that,
  • Briquettes are preferred for plants with lesser capital and semi-automated fuel feeding systems as the combustion performance is good enough and transportation and storage costs are reduced compared to using loose biomass.
  • For plants with more capital, pellets are preferred as they can be used in fully automated systems and have a better combustion performance but briquettes can also be used as a less optimal substitute.
  • Loose biomass is the cheapest but transportation and storage costs are high, combustion performance is poor, hence its use is advisable only when capital costs have to be reduced

 Biomass Combustion

Once the right biomass fuels have been narrowed down and the densification process has been selected we must then move to the third stage of selection that identifies the appropriate combustion system, condenser to be used as well as the pollution control equipment to be used.

Biomass Combustion Systems:

Biomass thermal conversion uses combustion of the fuel to provide heat, power generation or both. There are primarily three types of combustion systems:

Fixed Bed Combustion

  • Grate Furnace
  • Under Feed Stoker

Suspension Combustion

Fluidised Bed Combustion (FBC)

  • Bubbling FBC
  • Circulating FBC

Fixed Bed Combustion:

Grate Furnace: The combustion of the biomass occurs on a grate with a primary air flow underneath the bed and the combustible gases produced are burnt with a secondary air flow. These systems place the fuel on a grate and the ash produced is disposed off automatically. It has a few types such as fixed grate, moving grate, travelling grate and vibrating grate.

Under feed Stoker is limited to capacities up to 6MW and can only be used for biomass with very
low ash content and small particle sizes, hence grate furnaces  are preferred.

Suspension Combustion is not efficient as it requires a lot of air to prevent slag formation in the
combustion chamber. Hence this system is not preferred.

Fluidised Bed Combustion: In this system a bed of silica, sand and dolomite is suspended and behaves like a fluid by the air that is blown through a perforated bottom plate. This fluidised bed is then heated to the ignition temperature of the fuel and combustion of the fuel occurs. This system is highly efficient as heat transfer is much faster and better due to fluidisation. There are two types:

Bubbling FBC: This system fluidises the bed through a perforated distributor plate and the heattransfer tubes are placed in the bed as well as in the convection zone before the flue duct.

Circulating FBC: This system fluidises the bed by utilising a higher air velocity and collects the solids expelled from combustion using solid separators and sends the solids back to the fluidised bed

Hence out of the above combustion systems grate furnaces and fluidised bed combustion are most widely used. We shall therefore compare the characteristics of grate and FBC systems.

Parameters Grate Furnace Bubbling FBC Circulating FBC
Capital Cost Capital cost is low for plants > 20MWth Capital cost is high. Useful if plant capacity > 20MWth Capital cost is high. Useful if plant capacity > 30MWth
Emissions High NOX and SOX Low NOX and SOX Low NOX and SOX
Fuel Size Any fuel size can be used. (Pellets/Briquettes/Loose) Pellets are preferred but crushed briquettes can also be used. Pellets are preferred but crushed briquettes can also be used.
Fuel Type High Alkali fuel can be used High Alkali fuel cannot be used due to bed agglomeration High Alkali fuel cannot be used due to bed agglomeration
Fuel Flexibility Special design changes have to made to incorporate fuel mixtures It can handle fuel variations It can handle fuel variations
Efficiency Excess Oxygen (7-8%) reduces efficiency Low excess Oxygen      (3-4%) gives high efficiency Low excess Oxygen     (1-2%) gives higher efficiency
Maintenance High maintenance costs due to higher slagging, fouling tendencies Low maintenance costs Lower maintenance costs as corrosion tendencies are reduced from BFBC

From the above data we can infer that if one is to set up a biomass plant, the combustion system to use would be

  • For a high alkali fuel and a plant capacity below 20MWth a grate furnace is preferred.
  • For a low alkali fuel and a plant capacity below 30MWth a bubbling FBC is preferred.
  • For high capacity plants > 30MWth circulating FBC is always preferred as CFBC has higher
    efficiency than BFBC, lesser corrosion and more fuel flexibility in handling fuel mixture
    compared to BFBC.

Pollution Control Equipment:
The combustion of biomass produces a large amount of particulate matter that can penetrate deep into our lungs. This particulate matter needs to be collected and disposed off safely to reduce air pollution. The pollution control equipment to be used are of the following types:

Cyclone separator: Particulate-laden combustion gas is forced to change direction; and gets separated from the gas stream due to inertia of the particles. They have a low capital cost, low maintenance but are not very efficient in collecting fine sized particulate.

Baghouse Filter: This device utilizes fabric filter tubes or a cartridge to capture fly ash and other particulates from the combustion gases. They have high collection efficiency and can withstand most flue gas flow rates but they get damaged at high temperatures, cannot handle corrosive gases and cannot filter moist gas streams.

Wet scrubber: High-energy liquid spray is used to remove particulate and gaseous pollutants from the combustion flue gases. They can withstand high temperatures and high particle loading as well as explosive gases but are prone to corrosion and the scrubbing liquid can pollute water.

Electrostatic precipitator: This is used to remove very fine particulates by inducing a negative charge to the particles and collecting them on a positively charged or grounded plate and are very efficient (usually more than 99%). They have a very high efficiency and can handle any flow rate of flue gas but they are expensive to install and have high operational costs due to the amount of electricity required.

  • Baghouse filter can used when theflug gastemperature is not very highand gas does not have ,moisture hence it can be used with FBC systems
  • ESP can used where high capital requirements can be mat as it has a very high efficiency.
  • Cyclones can be used to save capital cost in small scale plants.