Water cooled grates

Water cooling of grates is used to protect the grate. Water is used as a cooling medium to capture heat from the burning waste bed and use it elsewhere in the process. It is common that the heat removed is fed back into the process for preheating the combustion air (primary and/or secondary air) or heating the condensate. Another option is to directly integrate the water-cooling into the boiler circuit, operating it as an evaporator.

These grates are applied where the net calorific value of the waste is higher, typically above 10MJ/kg. At lower calorific values their application is more limited. Increases in the calorific value of municipal waste seen in Europe have increased the application of this technique.

There are other reasons for the use of water-cooled grates – these are discussed in section 2.3.1.2.5.

2.4.4.5 Flue-gas condensation [5, RVF, 2002]

Water in the flue-gas from combustion comprises evaporated free water from the fuel and reaction water from the oxidation of hydrogen, as well as water vapour in the combustion air.

When burning wastes, the water content in the flue-gas after the boiler and economiser normally varies between 10 and 20 % by volume, corresponding to water dew points of about 50 – 60 °C.

During cleaning of the boiler with steam the water content in the flue-gas increases to about 25 %.

The minimum possible dry gas temperature at this point is 130 - 140 °C using normal boiler construction material. This temperature is mostly determined in order to be above the acid dew point, linked to the SO3 content and the H2O content in the flue-gas.

Lower temperatures result in corrosion. The boiler thermal efficiency (steam or hot water from waste) will, under these conditions, be about 85 %, as calculated based on the calorific value of the waste input. However, if there is more available energy in the flue-gas, a water vapour will result which has a latent specific energy of about 2500 kJ/kg and dry gas with a specific heat of about 1 kJ/(kg °C).

Return water from district heating at a temperature of 40 - 70 °C (system configuration dependent), can be used directly to cool and condense the water vapour in the flue-gas. This system is common at plants burning bio-fuel, which normally is very wet and gives water dew points of 60 - 70 °C in the flue-gas.

Example: Stockholm/Hogdalen (Sweden):

At the Stockholm/Hogdalen (Sweden) plant this system is used with three conventional grate fired steam boilers and one with a circulating fluidised bed. Flue-gases from the conventional grate fired boilers are cooled in shot cleaned waste heat boilers to about 140 °C. Return water from district heating is used as the cooling media.

FGT starts with a dry cleaning system for each boiler in which dry hydrated lime is injected and mixed with the flue-gas in a reactor. The acid impurities react with the lime and solid salts are formed which are removed in a fabric filter together with fly ash and the excess of lime. The final reaction takes place in the dust cake on the bags. The fluidised bed boiler has a slightly different reactor as re-circulated dust from the fabric filter is slightly humidified before it is mixed with fresh lime and injected into the flue-gases.

The second cleaning stage includes wet scrubbers, which saturate the flue-gas and remove the rest of the acid gases, particularly hydrogen chloride (HCl) and sulphur dioxide (SO2). The saturated gas leaving the wet scrubbers has a temperature of about 60 °C. It is sucked to a tube condenser, which is cooled by return water from the district heating at a temperature of 40 - 50 °C. One wet system is used for all three grate boilers, although the CFB-boiler has its own.

If the return water temperature is 40 °C (the normal case for this plant but very low in comparison with the majority of European climates) 14 % additional energy is recovered in the condenser. On the other hand, if the return water temperature is 50 °C only about 7 % additional energy is recovered. For extreme cases, when the return water temperature is as high as 60 °C, no extra heat is recovered.

In the Stockholm/Hogdalen case the flue-gas is reheated before the ID fan and stack and for this reheating some MW of low-pressure steam is consumed. It is also possible to operate without this reheat but with a wet fan and stack.

Figure 2.39: Pollution control and additional heat recovery by condensation of flue-gas water vapour at the Stockholm/Hogdalen waste-fired CHP plant

Source [RVF, 2002 #5]

This simplified example shows that condensation can be effective only if there is a comparatively big temperature difference between the water dew point in the flue-gas and the cooling water (normally district heating return water). If this condition is not fulfilled heat pumps can be installed (see below).

It should be noted that, in this case, it is the cold district heating water return that provides the energetic driver for the condensation of the flue-gases. This situation is only likely to exist in regions with the lower ambient temperatures found mostly in Northern Europe.

2.4.4.6 Heat pumps [RVF, 2002 #5]

The main purpose of heat pumps is to transform energy from one temperature level to a higher level. There are three different types of heat pumps in operation at incineration installations.

Theses are described below with examples.

2.4.4.6.1 Compressor driven heat pumps

This is the most well known heat pump. It is, for instance, installed in refrigerators, air conditioners, chillers, dehumidifiers, and heat pumps used for heating with energy from rock, soil, water and air. An electrical motor normally drives the pump, but for big installations steam turbine driven compressors can be used.

In a closed-circuit, a refrigerant substance (e.g. R134a), is circulated through a condenser, expander, evaporator and compressor. The compressor compresses the substance, which condenses at a higher temperature and delivers the heat to the district heating water. There the substance is forced to expand to a low pressure, causing it to evaporate and absorb heat from the water from the flue-gas condenser at a lower temperature. Thus the energy at low temperature in the water from the flue-gas condenser has been transformed to the district heating system at a higher temperature level. At typical incineration conditions, the ratio between output heat and compressor power (heat to power ratio) can be as high as 5. The compressor driven heat pump can utilise very much of the energy from the flue-gas.

2.4.4.6.2 Absorption heat pumps

Similar to the compressor type pump, absorption heat pumps were originally developed for cooling. Commercial heat pumps operate with water in a closed loop through a generator, condenser, evaporator and absorber. Instead of compression the circulation is maintained by water absorption in a salt solution, normally lithium bromide, in the absorber. The diluted water/salt solution is pumped to the generator. There the water is evaporated by hot water or low-pressure steam and is then condensed in the condenser at a higher temperature. The heat is transferred to the district heat water. The concentrated salt solution is circulated back to the absorber. The process is controlled by the pressure in the system, in relation to the vapour pressure of the liquids, water and lithium bromide.

Electrical power consumption is very low, limited to a small pump between the absorber and generator, and there are few moving parts. The ratio between the output heat and absorber power is normally about 1.6.

2.4.4.6.3 Open heat pumps

The third heat pump is sometimes called open heat pump. The principle is to decrease the water content of the flue-gas downstream of the condenser using a heat and humidity exchanger with air as intermediate medium.

The higher water content in the flue-gas in the condenser means a higher water dew point, and a bigger difference between the water dew point and the dew point of the return water from the district heating system.

2.4.4.6.4 Example data of different heat pumps

The following table has been collated from data from three different plants in Sweden, each using a different type of heat pump, as described above.

As it can be seen from the table, the use of heat pumps consumes electricity; therefore the net electrical output is reduced. However, the thermal heat output is increased.

Example 1 Example 2 Example 3

Heat pump type Compressor driven Absorption heat pump Open heat pumps Net heat output using

heat pump 82 80 81

Net heat output

without heat pump 60 63 70

Variation in heat

output +37 % +28 % +16 %

Net electricity output using

heat pump

15 15 0

Net electricity output

without heat pump 20 19 0

Variation of electricity production

-25 % -21 % 0

Data refer to an energy input of 100, therefore all numbers are percentages.

Example 3 does not produce electricity

Source: Data have been collated from 3 examples of plants in Sweden.

Table 2.14: Example data showing the variation in heat and electricity output when using various