Biomass Cogeneration Systems

Biomass fuels are typically used most efficiently and beneficially when generating both power and heat through biomass cogeneration systems (also known as combined heat and power or CHP system). Biomass conversion technologies transform a variety of wastes into heat, electricity and biofuels by employing a host of strategies. Conversion routes are generally thermochemical or biochemical, but may also include chemical and physical.

The simplest way is to burn the biomass in a furnace, exploiting the heat generated to produce steam in a boiler, which is then used to drive a steam turbine. Advanced biomass conversion technologies include biomass integrated gasification combined cycle (BIGCC) systems, cofiring (with coal or gas), pyrolysis and second generation biofuels.

Biomass Cogeneration Systems

A typical biomass cogeneration (or biomass cogen) 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 into existing building infrastructure.

Biomass cogeneration systems consist of a number of individual components—prime mover (heat engine), generator, heat recovery, and electrical interconnection—configured into an integrated whole. The type of equipment that drives the overall system (i.e., the prime mover) typically identifies the CHP unit.

Prime Movers

Prime movers for biomass cogeneration units include reciprocating engines, combustion or gas turbines, steam turbines, microturbines, and fuel cells. These prime movers are capable of burning a variety of fuels, including natural gas, coal, oil, and alternative fuels to produce shaft power or mechanical energy.

Key Components

A biomass-fueled cogeneration facility is an integrated power system comprised of three major components:

  • Biomass receiving and feedstock preparation.
  • Energy conversion – Conversion of the biomass into steam for direct combustion systems or into biogas for the gasification systems.
  • Power and heat production – Conversion of the steam or syngas or biogas into electric power and process steam or hot water

Feedstock for Biomass Cogeneration Plants

The lowest cost forms of biomass for cogeneration plants are residues. Residues are the organic byproducts of food, fiber, and forest production, such as sawdust, rice husks, wheat straw, corn stalks, and sugarcane bagasse. Forest residues and wood wastes represent a large potential resource for energy production and include forest residues, forest thinnings, and primary mill residues.

combined-heat-and-power

Energy crops are perennial grasses and trees grown through traditional agricultural practices that are produced primarily to be used as feedstocks for energy generation, e.g. hybrid poplars, hybrid willows, and switchgrass. Animal manure can be digested anaerobically to produce biogas in large agricultural farms and dairies.

To turn a biomass resource into productive heat and/or electricity requires a number of steps and considerations, most notably evaluating the availability of suitable biomass resources; determining the economics of collection, storage, and transportation; and evaluating available technology options for converting biomass into useful heat or electricity.

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.