Biomass CHP

CHP technologies based on biomass combustion represent a great potential to reduce CO2 emissions since they are based on utilisation of renewable energy sources (e.g. wood fuels or sawdust).

 

Typical fields of applications for biomass CHP plants are wood processing industries and sawmills, district heating systems (newly erected or retrofitted systems) as well as industries with a high process heat and cooling demand. These applications represent a great market potential. In order to achieve an ecological and cost effective plant operation it is a basic requirement that not only the electricity but also the heat produced as process or district heat are utilised (heat controlled system of the overall system). From an ecological and economic point of view, the total annual utilisation rate (heat and electrical produced/fuel energy input; overall efficiency rate) should not be less than 60% and ideally exceed 80%. Furthermore, since the energy density of solid biofuels is relatively low, biomass CHP technologies should be primarily applied for decentralised applications.

The annual full load operating hours of the CHP plant have the largest influence on the electricity generation costs. A minimum value of 5,000 hours for economic operation can be recommended which shows the importance of an optimal design of the CHP unit according to the annual heat output line of the district or process heating network. An additional important side constraint for biomass CHP are long term supply contracts for the biofuels used.

Over the past few years CHP technologies based on biomass combustion have been newly or further developed and plants have been successfully implemented in many European countries. Several different technologies for medium and large scale application are available on the market and have proven their technological maturity. CHP technologies in the power range below 200 kWel. (small scale systems) are in the demonstration stage at the moment and should be commercially available within the next few years.

CHP technologies based on biomass gasification processes also represent a future potential but have not yet achieved a level of development which allows commercial application. Several demonstrations plants based on biomass gasification however have already been in operation for several thousands of hours. It is a considerably more complex process than biomass combustion but may offer higher electrical efficiencies, which makes it attractive as a future option.

 
CHP technologies based on biomass combustion
  

Various technologies are available for the electricity production based on biomass combustion: the steam turbine process, the steam piston engine process, the steam screw type engine process, the ORC process and the Stirling engine process.
These Technologies are applicable for the following power ranges:

  • up to 100 kWel.: the only applicable technology for small scale CHP plants is the Stirling engine process which is at a demonstration phase at present.
  • from 200 - 2000 kWel.: suitable technologies for medium scale CHP plants are steam engines, steam turbines, and especially the ORC process. These technologies are already available on the market.
  • larger than 2000 kWel.: the steam turbine process is the most relevant technology for large scale CHP plants.

For power production through biomass combustion, steam turbines and steam piston engines are available as proven technology. While steam engines are available in the power range from approximately 50 kWe to 1 MWe, steam turbines cover the range from 0.5 MWe up to more than 500 MWe with the largest biomass-fired, steam turbine plant around 50 MWe.

Small-scale steam turbines are usually built with a single expansion stage or few expansion stages, and operated at quite low steam parameters as a result of the application of firetube boilers. Plants smaller than 1 MWe are usually operated as backpressure CHP plants and aim for electricity net efficiencies of typically 10% - 12%. The backpressure heat can be used as process heat. Steam piston engines can also be used for small-scale applications, enabling efficiencies of 6% - 10% in single stage and 12% - 20% in multi-stage mode. Steam engines are relatively robust - even saturated steam can be used.

For large steam turbine plants, water tube boilers and superheaters are employed, thus enabling high steam parameters and the use of multi-stage turbines. Furthermore, process measures such as feed water preheating and intermediate tapping are implemented for efficiency improvement. This results in electricity efficiencies of around 25% in plants of 5-10 MWe. In plants around 50 MWe and larger, up to more than 30% is possible in cogeneration mode and up to more than 40% if operated as condensing plant.

Another interesting development for small-scale biomass power production is the externally fired Stirling engine. A 30 kWel prototype plant has reached approximately 20% electricity efficiency in CHP operation. Up to 28% efficiency is aimed at by improving the process and scaling up to 150 kWel. It is expected that Stirling engines may enable economic small-scale power production by biomass combustion in the future. In spite of the high complexity, closed gas turbine cycles or hot air turbines may become attractive for medium-scale applications. Before market introduction, however, development of process and component design (especially heat exchanger and/or hot gas particle separation) is needed.

 

 

 
Steam Turbine
 

Steam turbines are one of the most versatile and oldest prime mover technologies still in commercial production. Power generation using steam turbines has been in use for about 100 years, when they replaced reciprocating steam engines due to their higher efficiencies and lower costs. The capacity of steam turbines can range from 50 kW to several hundred MW for large utility power plants. Steam turbines are widely used for CHP applications.

A steam turbine is a thermodynamic device that converts the energy in high-pressure, high-temperature steam into shaft power that can in turn be used to turn a generator and produce electric power. Unlike a gas turbine where heat is a by product of power generation, steam turbine CHP systems normally generate electricity as a by product of heat (steam) generation. A steam turbine requires a separate heat source and does not directly convert fuel to electric energy. The energy is transferred from the boiler to the turbine through high-pressure steam, which in turn powers the turbine and generator. This separation of functions enables steam turbines to operate with an enormous variety of fuels, from natural gas to solid waste, including all types of coal, wood, wood waste, and agricultural by products (sugar cane bagasse, fruit pits, and rice hulls). In CHP applications, steam at lower pressure is extracted from the steam turbine and used directly or is converted to other forms of thermal energy.

The basic process behind steam power generation is the "Rankine Cycle".

In the thermodynamic cycle liquid water is converted to high-pressure steam in the boiler and fed into the steam turbine. The steam causes the turbine blades to rotate, creating power that is turned into electricity with a generator. A condenser and pump are used to collect the steam exiting the turbine, feeding it into the boiler and completing the cycle. There are several different types of steam turbines:

  1. A condensing steam turbine is for power-only applications and expands the pressurized steam to low pressure at which point a steam/liquid water mixture is exhausted to a condenser at vacuum conditions;
  2. Extraction turbines have openings in their casings for extraction of a portion of the steam at some intermediate pressure for process or building heating;
  3. Back-pressure turbines exhaust the entire flow of steam to the process or facility at the required pressure. Non-condensing steam turbines are also referred to as "back pressure" steam turbines. Here, steam is expanded over a turbine and the exhaust steam is used for to meet a facilities steam needs. The steam is expanded until it reaches a pressure that the facility can use.

The attainable electric annual use efficiency (= annual electricity production / annual fuel input based on its net caloric value) depends on the live steam parameters (temperature, pressure) and on the other hand on the necessary temperature level for the process and/or district heat consumers. Electric annual use efficiencies are usually between 18 and 30 % for biomass CHP plants in the capacity range between 2 and 25 MWel.

Regarding steam turbine technology backpressure turbines and extraction condensing turbines have to be distinguished. If there is a constant heat demand in form of hot water or low pressure steam all over the year backpressure turbines are used. At projects with the need of uncoupling the electricity and heat production extraction condensing turbines are applied, using the steam which is not or only to a low part required for heat supply in the low pressure part of the turbine to increase electricity production.

 
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