Combined Heat and Power Systems in Biomass Industry

Combined Heat and Power (CHP) is the simultaneous generation of multiple forms of useful energy (usually mechanical and thermal) in a single, integrated system. In conventional electricity generation systems, about 35% of the energy potential contained in the fuel is converted on average into electricity, whilst the rest is lost as waste heat. CHP systems use both electricity and heat and therefore can achieve an efficiency of up to 90%.

CHP technologies are well suited for sustainable development projects because they are socio-economically attractive and technologically mature and reliable. In developing countries, cogeneration can easily be integrated in many industries, especially agriculture and food-processing, taking advantage of the biomass residues of the production process. This has the dual benefits of lowering fuel costs and solving waste disposal issues.

CHP systems consist of a number of individual components—prime mover (heat engine), generator, heat recovery, and electrical interconnection—configured into an integrated whole. Prime movers for CHP units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. A typical CHP system provides:

  • Distributed generation of electrical and/or mechanical power.
  • Waste-heat recovery for heating, cooling, or process applications.
  • Seamless system integration for a variety of technologies, thermal applications, and fuel types.

The success of any biomass-fuelled CHP project is heavily dependent on the availability of a suitable biomass feedstock freely available in urban and rural areas.

Rural Resources Urban Resources
Forest residues Urban wood waste
Wood wastes Municipal solid wastes
Crop residues Agro-industrial wastes
Energy crops Food processing residues
Animal manure Sewage

Technology Options

Reciprocating or internal combustion engines (ICEs) are among the most widely used prime movers to power small electricity generators. Advantages include large variations in the size range available, fast start-up, good efficiencies under partial load efficiency, reliability, and long life.

Steam turbines are the most commonly employed prime movers for large power outputs. Steam at lower pressure is extracted from the steam turbine and used directly or is converted to other forms of thermal energy. System efficiencies can vary between 15 and 35% depending on the steam parameters.

Co-firing of biomass with coal and other fossil fuels can provide a short-term, low-risk, low-cost option for producing renewable energy while simultaneously reducing the use of fossil fuels. Biomass can typically provide between 3 and 15 percent of the input energy into the power plant. Most forms of biomass are suitable for co-firing.

Steam engines are also proven technology but suited mainly for constant speed operation in industrial environments. Steam engines are available in different sizes ranging from a few kW to more than 1 MWe.

A gas turbine system requires landfill gas, biogas, or a biomass gasifier to produce the gas for the turbine. This biogas must be carefully filtered of particulate matter to avoid damaging the blades of the gas turbine.  

Stirling engines utilize any source of heat provided that it is of sufficiently high temperature. A wide variety of heat sources can be used but the Stirling engine is particularly well-suited to biomass fuels. Stirling engines are available in the 0.5 to 150 kWe range and a number of companies are working on its further development.

A micro-turbine recovers part of the exhaust heat for preheating the combustion air and hence increases overall efficiency to around 20-30%. Several competing manufacturers are developing units in the 25-250kWe range. Advantages of micro-turbines include compact and light weight design, a fairly wide size range due to modularity, and low noise levels.

Fuel cells are electrochemical devices in which hydrogen-rich fuel produces heat and power. Hydrogen can be produced from a wide range of renewable and non-renewable sources. A future high temperature fuel cell burning biomass might be able to achieve greater than 50% efficiency.

Biomass Gasification Power Systems

Biomass gasification power systems have followed two divergent pathways, which are a function of the scale of operations. At sizes much less than 1MW, the preferred technology combination today is a moving bed gasifier and ICE combination, while at scales much larger than 10 MW, the combination is of a fluidized bed gasifier and a gas turbine.

Larger scale units than 25 MW would justify the use of a combined cycle, as is the practice with natural gas fired gas turbine stations. In the future it is anticipated that extremely efficient gasification based power systems would be based on a combined cycle that incorporates a fuel cell, gas turbine  and possibly a Rankine bottoming cycle.

Integrated Gasification Combined Cycle

The most attractive means of utilising a biomass gasifier for power generation is to integrate the gasification process into a gas turbine combined cycle power plant. This will normally require a gasifier capable of producing a gas with heat content close to 19 MJ/Nm3. A close integration of the two parts of the plant can lead to significant efficiency gains.

The gas from the gasifier must first be cleaned to remove impurities such as alkali metals that might damage the gas turbine. The clean gas is fed into the combustor of the gas turbine where it is burned, generating a flow of hot gas which drives the turbine, generating electricity.

Hot exhaust gases from the turbine are then utilised to generate steam in a heat recovery steam generator. The steam drives a steam turbine, producing more power. Low grade waste heat from the steam generator exhaust can be used within the plant, to dry the biomass fuel before it is fed into the gasifier or to preheat the fuel before entry into the gasifier reactor vessel.

Schematic of integrated biomass gasification combined cycle

The gas-fired combined cycle power plant has become one of the most popular configurations for power generation in regions of the world where natural gas is available. The integration of a combined cycle power plant with a coal gasifier is now considered a potentially attractive means of burning coal cleanly in the future.

Biomass Fuel Cell Power Plant

Another potential use for the combustible gas from a biomass gasification plant is as fuel for a fuel cell power plant. Modern high temperature fuel cells are capable of operating with hydrogen, methane and carbon monoxide. Thus product gas from a biomass gasifier could become a suitable fuel.

As with the integrated biomass gasification combined cycle plant, a fuel cell plant would offer high efficiency. A future high temperature fuel cell burning biomass might be able to achieve greater than 50% efficiency.