COMBUSTION TECHNOLOGIES of (On an Example of BIOFUELS )

COMBUSTION TECHNOLOGIES OF (on an Example of BIOFUELS )

A boiler plant operating on biofuel or peat consists mainly of the following units (see Figure);

·  fuel terminal, which may consist of several spaces, e.g., the fuel reception area, main storage area, automated storage or an automated section of the main storage area, etc.;

·  fuel handling equipment for the fuel conveyance from the main storage area to the automated storage and further on to the furnace

·  boiler and furnace;

·  flue gas cleaning equipment (multicyclone, bag filter, etc.) and stack;

·  ash handling equipment;

·  combustion air fans, flue-gas exhauster, safety and control devices.

Layout of the equipment in a biofuel fired boiler plant

The central technological part of a boiler plant is the boiler with a furnace. Combustion processes and furnace constructions depend highly on the properties of fuel (calorific value, volatile and moisture content, etc). In order to select the most appropriate equipment for burning biofuels and peat, combustion peculiarities of these complicated fuels must be understood,

Combustion

The fuel contains three chemical elements during the combustion of which heat evolves: carbon (C), hydrogen (H) and sulphur (S), which burn completely based on the following integrated chemical reactions:

C +O2=CO2 +heat
2H2+ O2=2H2O+heat
S+-Q2=S02 +heat

For burning atmospheric oxygen is used, and the combustion products are carbon dioxide (CO2), water vapour (H2O) and sulphur dioxide (S02). Although useful heat is released from the sulphur burning also, sulphur is considered an extremely objectionable fuel constituent, because of its environmental and corrosive impact.

Fuel combustion zones and stages

Combustion peculiarities of moist solid fuel can be studied based on the grate combustion example where some processes take place in the fuel bed and others freeboard in the combustion chamber.

The processes on the grate are the following (see Figure):

Combustion zones of moist biofuel on the sloping grate

·  after fuel has been fed to the grate the temperature of the fuel layer starts raising and drying process begins;

·  when the fuel temperature reaches 100-105 °C, the volatiles (first of all hydrocarbons) will be released. The structure of fuel particles will become porous as a result of this process;

·  depending on the type of fuel, it ignites in the temperature range of 220 ~ 300 °C:

·  carbon combustion ceases at the temperature of 800-900 °C and the ash will fall down from the grate

The processes on the grate could be split in two: endothermal or heat absorbing (drying and pyrolysis) and exothermal or heat releasing processes (burning).

The major part of heat from bioluel and peat is released in the combustion chamber and not in the fuel bed, because the volatile content of these fuels is high. Combustion of the volatiles (i.e. products of pyrolysis) begins in the furnace space at the temperature of 500-600 °C. For the ignition of volatiles, certain temperature must be achieved and in addition fresh oxygen-rich air must be fed into the combustion chamber. The air supplied under the grate is called primary air and the additional air necessary for the volatile combustion is called secondary. While burning fuels with high volatile content (biomass and peat) the demand for secondary air exceeds that of primary air.

Heat losses and combustion efficiency

Boiler efficiency is defined as the ratio of heat output and input Heat input can be calculated by using data on fuel consumption and calorific value The objective of combustion is to utilize heat input by fuels as fully as possible The calculation of combustion heat losses is based on flue gas (exhaust gas) analysis The analysis of moist or dry flue gas can be taken for the basis The methodology based on the dry flue gas analysis is described here for calculation of heat losses The methodology is well compatible with the modern measuring techniques and allows highlighting the role of water vapour in the losses from the moist fuel and hydrogen burning

The heat losses from burning include

·  heat loss with the physical heat of dry flue gas,

·  heat loss due to the content of carbon monoxide (CO), hydrocarbons (CmHn) and other combustible gaseous components in the dry flue gas The loss is the heat wasted due to the chemically incomplete combustion,

·  heat loss with bottom and fly ash that consists of two parts - physical heat of the ash and heat wasted if there is unburned carbon in the ash,

·  heat loss due to the latent heat of vapour in exhaust gas Vapour content in exhaust gas depends on moisture and hydrogen content in the fuel

Usually the water vapour in flue gases is in the form of superheated steam Hence, this loss component is the energy content in the water vapour (in particular, the evaporation and superheating heat) and it is taken into account when the higher (gross) calorific value is taken for the basis of calculating the combustion efficiency

In order to find the combustion efficiency, the so-called indirect heat balance method is mainly used

efficiency = 100 -total loss,

where the total loss and efficiency are given in percents

The heat loss from the combustion must not be considered the same as that from the boiler or boiler plant, because the latter contains still several additional losses, for example losses due to radiation and convection from the outer boiler surface, blowdown losses in case of steam boiler, losses due to air flow through the boiler in hot reserve, etc

The most significant heat loss is that via the flue gas physical heat and it depends on the temperature and on the excess air factor A The last one is defined as a ratio of actually supplied combustion air to that of theoretically required for the complete combustion The excess air factor A is used as the most important characteristic describing the combustion process

Based on the flue gas analysis, the following simplified relationship can be used for defining the excess air factor

CO2 max values for some fuels

Combustion technologies

Considering the wide scale of properties of biofuels and peat, many different techniques for combustion of these fuels can be applied

·  pulverized combustion - is used in single cases, for example for co-combustion of wood grinding powder together with liquid fuel,

·  grate combustion technologies with a wide range of grate construction Grate types might be divided into two groups - fixed or solid grates and moving grates,

·  combustion in fluidized bed – either bubbling or circulating fluidized bed,

·  fuel gasification and combustion in the oil or gas firing boiler

A certain combustion technology is either technically or economically most expedient in the certain range of boiler capacities

The boilers could be classified also according to field of their implementation In each field there is expedient to use boilers with certain capacity, most appropriate technological solution and preferred automation level

Grate furnaces

The grate furnace technologies are the most widespread in the medium and low capacity range

A great number of different grate types could be classified as follows:

·  solid or fixed grate;

·  moving inclined grate;

·  travelling grate;

·  special grates for fuels of specific properties, for example for burning municipal waste.

Underfed pre-furnace with a conical grate (retort grate) for burning very wet fuel,

1 - screw feeder; 2 - fuel funnel 3 - inclined grate; 4 - horizontal grate; 5 - combustion chamber; 6 - ash scraper; 7 - screw conveyer for ash

Furnace with the grate consisting of fixed and moving inclined parts

furnace with checkered grate bars

Travelling (chain grate in the boiler of the Boras Sweden

Fluidized bed furnaces

With gradual increasing of the velocity of combustion air forced into the fuel bed, a state can be reached where the fuel bed is carried up by the air, the fuel particles start to suspend in the air flow. It seems like the bed begins to boil and this is where the term fluidized bed comes. The described fluidized bed is called a bubbling or stationary fluidized bed. The moisture, released volatiles, ash and also fine fuel particles are carried out of the fuel bed. Fine fuel particles and volatiles are burning in the combustion chamber above the fluidized bed

When the air velocity is increased to the level higher than needed for the bubbling fluidized bed, the burning fluid particles are carried away with the airflow. In the cyclone-separator solid particles are separated from the air and gas flow and circulated back to the furnace. Since the burning fuel circulates between the furnace and separator, the term used for this combustion technology is the circulating fluidized bed.

Schematic diagram of furnaces with bubbling (A and circulating B fluidized beds)

Both the bubbling and circulating fluidized beds are good for burning biofuels, peat and waste. For oil shale and coal the circulating fluidized bed suits better.

WHY, I WILL EXPLAIN IN LECTURE?

One of the advantages of fluidized bed technology is the possibility to burn different low quality (REARK FOR LECTURE: BUT I NOT CAN CHANGE QUALITY OF FUELS ON CONCRET DESING BOILRES) fuels reducing at the same time emission of hazardous air pollutants. In fluidized bed the temperature is relatively low (about 850 °C) and therefore no ash melting and furnace slagging is likely to happen. Also, nitrogen emissions decrease at this temperature

fluidized bed

gas tube boiler of the 1-5 MW capacity

Fhoster Wheeler:

The fluidized bed technology is today an important contributor to the reduction of greenhouse gases and has a good potential to provide zero CO2 emission solutions

in the next ten years.

Principle scheme of the pellet combustion system

Technological solutions for the pellet feeding and combustion systems

The Malle 20 kW boiler

The small boiler Pelle with the peilet container, feeder and burner developed at Tallinn University of Technology

Two views of the pellet burner

Shell Boilers

Shell boilers may be defined as those boilers in which the heat transfer surfaces are all contained within a steel shell. Shell boilers may also be referred to as 'fire tube' or 'smoke tube' boilers because the products of combustion pass through the boiler tubes, which in turn transfer heat to the surrounding boiler water. Several different combinations of tube layout are used in shell boilers, involving the number of passes the heat from the boiler furnace will usefully make before being discharged.

Shell boiler - Wet and dry back configurations

17