Biomass power generation is a proven, mature technology and is the single largest source of non-hydro renewable electricity. Biomass installations range from very small units of 5-10 kW capacity to large facilities 50 MW and larger in size. Commercial scale power is considered 10MW and larger.

Biomass fuel from forested areas comes from residues of non-commercial wood. Residues can be gathered during milling, logging, thinning, and other forest management activities. With a biomass plant, efficient generation of forest residues is helpful for providing sustainable fuel supplies.

Electricity produced from biomass is considered to be carbon neutral and therefore helps to combat global warming. The CO₂ that the facility will release would have been produced as the plants and trees naturally decomposed in the forest without the benefit of electricity production. So, it is like harnessing energy that would otherwise be wasted, just like wind energy and solar energy. So, there is a potential advantage of reducing Green House Gas (GHG) emissions.

Biomass energy is the utilization of energy stored in organic matter. Examples of biomass include wood, leaves, animal waste, crops, bones, and scales. The abundant plant life in our planet is natures store house of solar energy and chemical resources. Whether cultivated by man, or growing wild, plant matter represents a massive quantity of a renewable resource that we call biomass. Put another way, biomass is stored solar energy that can be converted to electricity or fuel. Biomass is a renewable resource.

The other major reasons for consideration of the biomass power option are:

• Disposal of biomass residues combined with the production of electricity and heat
• Power production from abundant indigenous biomass resources
• Power for remote locations rich in biomass resources
• Pollution-free: Lower NOx, SOx, CO emissions
• No bed make-up material required
• Helps reducing unemployment in specific regions

The area requirements are not much and are comparable to Thermal power plants, if built nearby sources of rich biomass.

 

A variety of conversion processes are used to convert biomass to either thermal energy, liquid, solid or gaseous fuels. These processes include thermal conversion via combustion or pyrolysis (Torrefaction) , chemical conversion, microbial conversion or fermentation, and physical conversion to pellets or cubes, i.e. densified fuels. Electrical energy and heat generation is most commonly accomplished through direct combustion of biomass in a boiler. In the combustion process, energy content, moisture content and chemical composition are the most important biomass characteristics affecting combustion processes.

 

Co-firing

Biomass can be used to produce energy either by setting up Biomass power plants that utilize only biomass to produce energy or by co-firing biomass with coal in existing coal power plants.

Co-firing biomass with coal is one of the most attractive and easily implemented biomass energy technologies that help in reducing GHG emissions. Biomass can substitute up to 20% of the coal used in the boiler. The biomass and coal are combusted simultaneously. Overall production cost savings can be achieved by replacing coal with inexpensive biomass fuel sources. A proven technology, it is also proving to be life-cycle cost-effective in terms of installation cost and net present value. It is particularly well suited to a stoker boiler. However, it has been successfully demonstrated and practiced in all types of coal boilers. Some tests conducted in power plants have shown that biomass can be co-fired successfully without any major modifications to the coal burner.

Currently the most popular option for co-firing applications is direct co-firing, where biomass and coal are utilized together in the same boiler. This is mainly due to relatively low investment cost of turning an existing coal power plant into a co-firing plant. Parallel co-firing units, also called hybrid systems (where biomass and coal are fed into separate boilers jointly producing steam for power generation) are popular too, especially in pulp and paper industrial power plants. The indirect option for co-firing i.e. involving the installation of separate biomass gasifier is expensive and applied only in few European power plants.

 

Options for biomass gasification

   

Gasification is an important process to be mentioned with relation to co-firing. Gasification is a thermochemical process converting solid or liquid carbon-based fuels into a gas (syngas, which in case of gasification of biomass can be called bio-syngas). Syngas is rich in a number of components like CO, H₂, CO₂, H₂O, and CH₄, whose proportions depend on the raw material composition and on the gasification conditions such as pressure and temperature. Gasification takes place in the presence of an external oxidizing agent (oxygen, air or steam) and heat. The addition of heat can occur directly (by partial oxidation of the fuel) or indirectly by heat transfer. Unlike combustion, in gasification the amount of the external oxidizing agent is lower, hence only a relatively small portion of fuel is used to generate the required heat. There are two basic types of gasification – direct and indirect. There are three basic types of gasifiers – fixed bed, fluidised bed, and entrained flow gasifiers.

 

Pre-treatment of Biomass

Most of the challenges that co-firing poses to boiler operation originate from fuel properties (the differences in characteristics of coal and biomass) and can be summarized as follows: 

• Pyrolysis starts earlier for biomass than for coal
• The volatile matter content of biomass is higher than in coal
• The fractional heat contribution by volatile substances in biomass is approximately 70% compared with 30-40% in coal
• The specific heating value [kJ/kg] of volatiles is lower for biomass compared with coal
• Biomass char has more oxygen compared with coal and it is more porous and reactive
• Biomass ash is more alkaline in nature, which may aggravate the fouling problems
• Biomass can have high chlorine content.

Biomass has usually high moisture content resulting in a relatively low calorific value of the fuel, which might negatively affect its combustion properties. Additionally biomass feedstocks have low bulk density and potentially high chlorine content as well as hydrophilic character. Biomass properties which set demanding requirements for power plant include total ash content, its melting behavior and chemical composition - typically biomass contains fewer ashes than coal but alkaline metals that are usually  responsible for fouling of heat transfer surfaces are high in biomass ashes.

Raw biomass has much lower bulk density than coal resulting in expensive storage, transport and handling, and has non-friable character resulting in problems with biomass grindability. Most of the challenges that co-firing poses to boiler operation originate from biomass properties and therefore improving the properties e.g. by pre-treatment can be applied as one of the measures to avoid or  reduce these challenges.

 

Biomass pre-treatment options  

• Drying
• Sizing
• Baling
• Pelletizing
• Briquetting
• Washing/Leaching
• Torrefaction
• Torrefaction with pelletizing
• Pyrolysis
• Char wash combined with pyrolysis

 

Co-firing Project Phases

In general, the following are the steps to considering and implementing a co-firing project:

Fleet/multiple unit screening – The generation portfolio is reviewed, including conventional and renewable energy sources, to benchmark GHG emissions. Units that can most contribute to the company’s climate action plan when co-firing with RPS eligible biomass are identified.

Fuel supply study – Reliable co-firing is contingent upon the supply and characteristics of biomass. This study identifies options for a secure, predictable supply from source to the power plant stockpile. Also, pre-processing of woody biomass (such as pelletizing) can strengthen the supply infrastructure and reduce transportation costs; these options are investigated.

Preliminary technical/economic assessment – The preliminary assessment looks at the power plants suitable for co-firing. An initial ranking is performed – based on location, transport and storage infrastructure, boiler design layout and size – while considering economics.

Detailed technical/economic assessment – The detailed assessment further analyzes the ranked set of power plants technically and economically, taking into consideration the detailed design, characteristics, operating experience, and configuration to further refine the return-on-investment and technical changes required.

Conceptual design – This phase provides a high level design for a specific plant, including the overall plant architecture and layout plans. The biomass injection route is selected during this phase.

Detailed design – The detailed design includes an analysis of technology and equipment suppliers, details all interface points and provides work scope packages to perform the adaptation to co-firing.

Implementation – This phase provides the project management and governance programs to ensure a successful project.

Operation and monitoring – In this phase, the project is fully implemented, and now the fine-tuning is performed with appropriate diagnostics and calibration. Also, a corrosion monitoring and prevention program is established.

 

Technical Considerations

In general, there are four alternative approaches to injecting biomass into the generation process. Each approach targets different parts of the process, as shown in Figure.

These approaches can be summarized as follows:

Approach 1 blends the coal/biomass mixture on a conveyor belt and co-mills the fuel mixture in the existing coal pulverizers, then combusts in the existing coal burners.

Approach 2 separates biomass pre-processing (milling and drying), followed by injection of the biomass in the pulverized fuel lines (after the pulverizers) and simultaneous biomass/coal combustion in the original or modified coal burners.

Approach 3 separates biomass pre-processing and feeding and combustion in separate, dedicated biomass burners.

Approach 4 provides for indirect co-firing of biomass; for example, through an upfront gasifier with co-combustion of the (cleaned) fuel gas in the main coal-fired boiler.

 

Advantages of Biomass

The most important advantage of biomass is that it is everywhere and very easily available. In the agriculture industry, residuals like bagasse (fibers) from sugarcane, straw from rice and wheat, hulls and nutshells, as well as manure lagoons from cattle, poultry and hog farms are usable. Similarly, the timber industry has a lot to offer. Wood wastes like sawdust, timber slash and mill scrap are considered organic materials. Even in cities, paper and yard wastes are usable. Fully utilized biomass reduces pollution in underground water bodies by offsetting the amount of waste in landfills. Methane and other poisonous gases that form from dead organic matters can be found in landfills and water treatment plants. These can be captured and converted in to fuels suitable for generating electricity.

Economic benefits:

Rural economies will grow because of the development of a local industry to convert biomass to either electricity or transportation fuel.  Because biomass feedstocks are bulky and costly to transport, conversion facilities will be located where the crop is grown.  That means more people have chances of getting employed. Farmers will see their income rise thanks to these new markets -- for both agricultural wastes and crops that can be grown sustainably on marginal land. As new markets are created, the rural economy will become more diversified.

Energy benefits:

Energy producers and consumers will have available a renewable energy option with uniquely desirable characteristics.  Biomass has the greatest potential of any renewable energy option for baseload electric power production.  It is also the renewable resource with the most promise for producing economically competitive liquid transportation fuels.  Co-production facilities will allow the production of electricity when it is needed and ethanol when it is not -- acting, in effect, as "seasonal peaking" facilities. The energy security of a nation will be significantly enhanced.  With sustainable agricultural practices, biomass fuels could replace half or more of the nation's entire current level of gasoline consumption. Burning new biomass contributes no new carbon dioxide to the atmosphere because if we replant harvested biomass, carbon dioxide is returned to the cycle of new growth. Bioconversion and thermal conversion techniques for transforming biomass into fuels are currently under development at NREL and other research laboratories. These new technologies will reduce our reliance on oil and coal with no net addition of carbon dioxide to the atmosphere. New thermal conversion techniques coupled with chemical catalysis are making it possible to exploit the previously discarded lignin fraction by converting it into valuable chemicals that we now get from non-renewable fossil sources.

Environmental benefits:

Agricultural land that might otherwise be converted to residential or industrial use -- because we will need fewer and fewer acres to meet the market demand for food -- can be used to grow biomass crops that will restore soil carbon, reduce erosion and chemical runoff, and enhance wildlife habitat.   Perennial energy crops can be harvested without damage to the root structure and thus continue to serve as a soil stabilizer and stream buffer and habitat for wildlife. The use of biomass will greatly reduce greenhouse gas emissions.  Fossil fuels remove carbon that is stored underground and transfer it to the atmosphere.  In a combustion system, biomass releases carbon dioxide as it burns, but biomass also needs carbon dioxide to grow -- thus creating a closed carbon cycle.  In a gasifier-fuel cell combination, there is a net reduction of carbon dioxide. In addition, substantial quantities of carbon can be captured in the soil through biomass root structures, creating a net carbon sink.

 

Disadvantages of Biomass 

• The burning method of biomass is not clean. It is similar to the burning of fossil fuels and produces large amounts of carbon dioxide. However, it produces much less harmful pollutants (e.g. sulfur), as the main elements found in organic materials are hydrogen, carbon, oxygen and nitrogen.
• It takes considerable energy to produce biofuels from certain feedstock, resulting in less than desirable energy returns on energy invested (EROEI).
• Biomass collection is difficult.
• Biomass crops not available all year.
• Still an expensive source, both in terms of producing the biomass and converting it to alcohols
• On a small scale there is most likely a net loss of energy--energy must be put in to grow the plant mass

For a list of Biomass Co-firing power plants around the world, please refer:  http://www.ieabcc.nl/database/cofiring.php