Generating Energy With Zero Fuel Cost

Waste heat recovery boilers (WHRB) or steam generators are installed on the exhaust gases of gensets running on natural gas, or diesel or furnace oil. Waste heat recovery boilers utilise the heat from the exhaust gases, which are typically at temperatures of 350-600 deg C, to generate steam. This means steam energy to run the process is generated from waste, with zero fuel cost. The combined efficiency of the system (genset + WHRB) also increases by 20-30%. Waste heat recovery boilers offer a very attractive return on investment if power for the plant is generated mainly with the help of gensets.

Typical Flue Gas Waste Heat Recovery Boiler Arrangement

Typical Energy Balance with Genset Flue Gas Heat Recovery

As illustrated above, the heat recovery of 20% from stack loss of the genset helps to improve the combined efficiency of the system by up to 55%. This not only helps to save fuel, but also reduces the amount of high temperature exhaust gases released to atmosphere. If a plant relies mainly on continually running gensets for power supply, a waste heat recovery system on the exhaust gases of the genset is a must.

Factors determining WHRB Capacity

Flue gas quantity :

This will decide the amount of heat carried to the boiler. Higher the quantity of flue gases, more the heat, and higher the steam generation capacity.

Flue gas temperature :

Temperature of the flue gas at the entry of the inlet also governs the heat carried by the flue gases. Typically a temperature differential of about 50°C is required to be maintained between the WHRB outlet temperature and corresponding saturation steam temperature at maximum working pressure. Higher the temperature of flue gases at inlet, higher can be the boiler capacity.

Specific heat of flue gases (Cp) :

Cp value depends upon the gas temperature and the composition of flue gases. Higher the flue gas temperature higher will be the Cp value, thus higher can be the WHRB capacity.

Flue gas composition :

Composition of flue gas will determine the Cp value and other thermal and physical properties that have a major influence on extractable heat. Typically a higher proportion of CO2 and N2 gases would yield a higher Cp value for flue gas.

DG set loading :

Higher the DG set loading, higher will be the flue gas quantity and hence higher will be the steam generation from WHRB. At higher loading, flue gas temperatures will also be higher which will further help in steam generation.

Fouling :

For any heat exchange, fouling factors create hindrance in heat flow / transfer. Hence it is very important for the user to keep both the surfaces involved in the process of heat transfer (flue gas side and water side) clean.

Diverter Dampers for WHRB

Exhaust gases from the DG set outlet are required to pass through the WHRB for heat recovery and steam generation. However if for any reason, steam from the WHRB is not required or if the steam pressure reaches high cut-off level the flue gases should be bypassed. In order to do so, a diverter damper is used. A diverter damper is nothing but a 3 way valve on the flue gas. It is fitted at the inlet point of the WHRB, as close to boiler inlet as possible.

Since the flue gases are at high temperature, the diverter damper must be capable of withstanding these conditions. For all practical purposes, SS is the preferred material for the casing and the damper blades.

The most important specification for a diverter damper is its ‘sealing efficiency’. The sealing efficiency is usually in the range of 99.95%, and specified by the manufacturers on the basis of cross sectional area. However this figure is slightly misleading when the percentage of leaked flow to the total flow at inlet is considered. In other words, if this efficiency is converted to flow basis it is normally about 97%. With higher flue gas pressure this efficiency will further reduce.

Diverter dampers are of two types :

Double flap type :

In this design, two flaps are connected to each other by a mechanical link. The sealing is achieved by provision of SS strips welded to the flaps which press against the seating ring provided. The inlet and outlet can be either circular or rectangular. A single actuator is is provided for either ON/OFF or regulating duty. Necessary interlocks such as limit / travel switches and torque switches are also provided. The double flap damper can be closed or opened manually. The advantage of this type of damper is its simplicity and cost effectiveness.

Single flap type :

In this design a single flap is provided, with a single actuator for either on/off or regulating duty. Necessary interlocks such as limit / travel switches and torque switches are also provided.

The main advantage of this type of damper is the absence of a mechanical link between flaps.

Arrangements for DG Silencer and WHRB

There are three ways in which the DG silencer and WHRB can be arranged with respect to each other.

Silencer before WHRB :

In this arrangement, the silencer is located immediately after the DG set flue gas outlet and the ducting is then taken upto the boiler. Since the silencer is normally in the scope of DG set supplier, it is easier for them with this arrangement. It is easier for the boiler supplier as well since they do not need to worry about silencer location. This arrangement also eases the pulsating flow from the DG set outlet. Disadvantage of this arrangement is that the available gas pressure reduces.

Silencer after WHRB :

In this arrangement, care needs to be taken to ensure that silencer is located after the bypass connection, and connects to the WHRB outlet. If not properly connected, a loud noise will be heard whenever the WHRB is bypassed. In this arrangement too, the available gas pressure reduces. The silencer after WHRB arrangement is generally not preferred.

Silencer parallel to WHRB :

This is probably the best arrangement from a technical point of view. Here, the silencer is placed in the bypass path of flue gases, and whenever gases flow through the WHRB, they get silenced due to the pressure drop. The WHRB designer can take advantage of the higher gas pressure available to him.

Advantages of Waste Heat Recovery Boilers

Direct Savings :

In case of waste heat recovery, heat is being extracted from waste flue gases. No fuel is required for heat generation and hence this is a direct saving.

Attractive Return on Investment :

Fuel costs are increasing by the day. Hence the return on investment on waste heat recovery boilers is very attractive, with a shorter payback period.

Low operational costs :

Since no fuel is required, the operating costs only comprise of electrical, water and water treatment chemicals, resulting in a very low overall operating cost.

Environment Friendly :

Heat recovery helps to reduce the temperature of exhaust flue gas evacuated to atmosphere. This reduces the environmental impact due to high temperature flue gas exhaust.

Safe to Operate and Easy to Install :

Standard genset flue gas heat recovery systems are easy to install and user friendly. As no direct firing of fuel is involved, there are also considered as safe operating systems.

Low maintenance cost :

The waste heat recovery system is designed and configured with very simple components which help to keep the maintenance cost low.

Related Products : Exhaust Gas / Waste Heat Recovery Boilers (WHRB)