Boilers / Burners

Introduction

In a boiler the chemical energy is converted to thermal energy. The main part of a boiler is the fire box with a burner located inside. All around the fire box pipes or box sections are installed, in which a liquid (mostly water) is heated up by the thermal energy of the combustion.
There are two ways for categorising boilers:

• by the kind of fuel which is used for the combustion

(main groups: natural gas, oil, wood, biomass, other combustible materials: for example waste gases in chemical industry)

or

• by the product as heated water or steam.

The production of heat or steam in boilers is the eldest and best known utilisation of CMM, whereas special burners use CMM as fuel. These CMM burners are also used in some other applications like coal dryer, ventilation air heater or as industrial heaters. For using CMM as a fuel in boilers gases with a wide range of concentration can be utilised. Safe operating systems for methane contents from 25% up to 100% are available. On some locations utilisation of gases with methane concentration down to 20% are allowed.

CMM as additional fuel

Especially in coal fired power plants with CMM production nearby, CMM can be used as additional fuel in boilers. As the energy amount of the used CMM is small compared to the coal consumption, no special regulation is needed for the burning of the CMM in the big fire box. The safety installations depend on the local situation. In most cases a special regulation of combustion air flow is not required.

However, even if the co-firing of CMM is a simple possibility for CMM utilisation it is only reasonable, if the distance between the CMM extracting point and the power plant is acceptable.

CMM as mono fuel

In the past 20 years CMM has been used as mono fuel in boilers. Special safe burner systems are used instead of natural gas burners in gas boilers. Furthermore there are burners available for use of CMM as normal fuel and oil as secondary fuel. Sometimes a modification for fuel switch is not necessary. This kind of burners and boilers are often used on coal mine sites in Germany.

In many mines especially in Eastern countries coal fired boilers are retrofitted with a gas burner system. The grate is covered to protect it against the flame heat. In most cases a modification of the air inlet is not necessary.

Gas - Air - Mixture in CMM burners

Precise control and regulation of gas – air – mixture in CMM burners is necessary to ensure complete gas combustion, as well as a low CO content in the exhausted gas and a good efficiency of the overall system.

The burner load is regulated by the energy demand. However, the capacity of gas consumption is limited to the maximum gas volume, which can be charged to the burner. The different CMM qualities (CH₄-content) make specific demands on the control and regulation system. For example: If the content CH₄ of the CMM can range between 25% and 60%, the cross sectional area of the gas flow channel has to be dimensioned for gas with 25% CH₄ to reach full load. When the same volume of CMM with 60% CH₄ flows through this channel, the load would reach values of 240% as the gas contains more than the double energy compared to the CMM with lower CH₄ contents. Therefore maximum volume flow of CMM with 60% CH₄ has to be limited to 42% of the maximum possible volume flow for security reasons.

The proper function of the adjustment of gas and air is controlled through the measurement of oxygen concentration in the exhaust gas, which should be between 1.5% to 3% at every time.

Specifics for CMM burner installations

The most burner systems are designed for natural gas. Special attention has to be paid to the differences between natural gas and CMM to obtain a good and save working system. Three aspects are important:

• the change of calorific value (changing volume, density and CH₄-content),
• existence of water and steam in the gas and
• pollution through dust in the gas.

In opposite to natural gas burners ,air valve and gas valve should not be fixed interlinked due to the changing calorific value of the CMM. In modern constructions an open combination or electronic combination is used for the regulation of the gas valve and the air one. Through oxygen measurement in the exhaust gas a proper control is possible, provided a solid gas analyses for CH₄ and O₂.

The existence of water and steam in CMM gas do not necessarily require a drying of the CMM before burning. However, it has to be ensured that the water in the gas is not a reason for failures in the burning system. Possible condensate has to be separated and led away of the pipe. Moreover all used installations have to be dimensioned for wet gas. Especially the material from the gasket of the valves has to be qualified for these conditions. If ordinary gaskets for dry gas are used, they will swell in contact with water. Following the valve cannot work any more and the burning system fails.

Analogue the installation should be able to deal with some dust in the gas. To prevent failure of the burner the installation of a dust filter in the beginning of the burner installation is needed. The degree of pollution and necessity for filter cleaning can be determined by measurement of the regarding pressure difference. The installation of a shutoff valve is advisable to allow filter cleaning by the staff.

Further information

Canadian clean power coalition, “How a Coal-fired Power Plant works”
Indtex Boiler Pvt. Ltd
L&T MHI Boilers Pvt. Ltd.
TT Boilers, „Steam boiler – heavy duty classic 3-pass steam boilers“