EVC6: Large-scale combustion for heat and power

Large scale-combustion refers to industrial applications with a high thermal heat demand or with the main focus on power production. However, frequently plants with large-scale combustion applications aim to provide both - heat and power. The simultaneous production of heat and power is also known as “cogeneration” or “combined heat and power” production (CHP).

The most relevant and widespread application using solid biomass fuels are CHP systems which produce superheated steam. Most relevant technical parts of such applications are:

• Combustion system/furnace: The combustion system is used to convert the chemical energy of the biomass into thermal energy of hot flue gases. In most cases the combustion system/furnace as well as the boiler represent one large unit.
• Boiler: The boiler is used to transfer the thermal heat of the hot flue gas to the working fluid, i.e. water, via three types of heating surfaces (evaporator, superheater, economizer). Most commonly water tube boilers are used in large industrial applications. The type of boiler could be further differentiated according to the arrangement of heating surfaces as well as the water/steam circulation in natural circulation, forced circulation and once-through boilers.
• Turbine: Within the turbine the high-pressure superheated steam is expanded and kinetic energy is transferred to mechanical energy. Via a generator the mechanical energy is further transformed into electrical energy (power). In principle, three different types of turbines can be used – condensing turbines for maximum power production and backpressure turbines as well as extraction turbines for combined power and heat production (e.g. steam for process heat and/or hot water for district heating).
• Heat exchanger/Condenser: Heat exchangers downstream the turbine are used for heat extraction for process heat and/or district heating and for condensation of low-pressure steam into liquid water.
• Feedwater pump: The feedwater pump(s) increase the pressure level of feedwater upstream the boiler.

The thermodynamic process behind such applications is also known as the “Clausius-Rankine-Cycle” (or “Rankine-Cycle”).

Typically coal fired power plants with an electrical efficiency in the range of 30% - 40% are operated according to the previously described thermodynamic process. For such plants a supercritical pressure level of the steam up to 270 bar and a superheating temperature of 600°C are possible. However, in biomass-based plants the pressure as well as temperature levels are significantly lower due to the challenges of slagging and fouling as well as the risk of high temperature corrosion within the boiler at the heating surfaces.

The combustion systems/furnaces can be categorized in three main technologies:

1. Fixed bed combustion systems, e.g. grate systems or underfed firing systems up to 100 MW thermal power output
2. Fluidized bed combustion systems, e.g. bubbling fluidized bed (5 MW – max. 80 MW thermal power output) or circulating fluidized bed systems (> 30 MW thermal power output)
3. Pulverized fuel firing systems, enable highest thermal power outputs, e.g. up to 3 GW_th

Beside the thermal heat output, the type of fuels and chemical and physical fuel parameters (e.g. size distribution of fuel and potential pre-treatment efforts, content of sulphur (S) or nitrogen (N)) are crucial in order to select an appropriate combustion system. Depending on national regulations and the size of the plant also the need of potential flue gas treatment systems has to be considered in order to comply with the respective emission limit values. Therefore, for economic and operational considerations the need of secondary emission reduction technologies plays also an important role (e.g. PM, NO_x or SO_x reduction) for selection of the combustion system.

Another possibility to produce power and heat in large-scale industrial applications is the use of gas turbines. For these, ambient air is compressed by a compressor followed by a thermal heat input at the high-pressure level via combustion and the subsequent expansion of hot and high-pressure flue gases within the gas turbine. After the expansion the thermal energy of the low-pressure flue gases can be further used for hot water and/or steam production using a heat recovery boiler or a heat recovery steam generator. The electrical efficiency of applications using gas turbines is typically about 35%.

The thermodynamic process behind such applications is also known as the “Jule/Brayton-Cycle”.

Typically, gas turbines are fired with fossil fuels (e.g. oil or natural gas). However, it is also possible to use biomass-based gas (e.g. “green” natural gas) or fine pulverized biomass fuels for internal combustion in gas turbines (direct combustion). However, especially for pulverized biomass the risk of slagging and corrosion on the turbine blades which result in operational problems is high.

Therefore, another possibility is to transfer the thermal energy from biomass combustion via a high temperature heat exchanger to the high-pressure working fluid. For example, hot air turbines with external biomass combustion (indirect combustion) can be used for such an application.

Combining the Jule/Brayton-Cycle (gas turbine) and the Rankine-Cycle (steam turbine) is possible. This application is also known as combined cycle gas turbine (CCGT) plant. Therefore, the thermal energy of the hot (low-pressure) flue gases downstream the gas turbine is used to run an additional Rankine cycle. With CCGT plants high electrical efficiencies up to 60% are possible. Moreover, using such applications as cogeneration plants, an overall efficiency (heat & power) > 85% is possible.

Some other possibilities for combined heat and power production is the use of packaged CHP or ORC systems.

For packed CHP systems, the power is produced by an internal combustion engine, which is for example fuelled by low-calorific syngas produced from biomass gasification. The released excess heat from the combustion process is used for district heating or low-pressure steam production. Thereby, the heat is exchanged from the water and oil cooling circuit as well as from the heat recovery boiler of the hot flue gases downstream the engine. Packed CHP systems are typical installations of medium-scale CHP applications (up to 10 MW electrical output).

A Rankine-Cycle which is operated with an organic working fluid instead of water/steam is called ORC process (Organic Rankine Cycle). The advantage of an ORC application compared to a water/steam-based Rankine-Cycle is the potential usability of thermal energy at a lower temperature level (> 120°C) for power production. Therefore, it is possible to use low temperature waste heat from industrial processes for power production. Biomass based ORC systems are typically in the range of 0.2 MW – 3 MW electrical power output and can be therefore categorized as a small- to medium scale CHP application.