**3.2.3 Biomass combustion**

One of the most common methods of biomass conversion is by direct combustion or burning. The simplest units include numerous cookstoves already developed in rural areas of developing countries. Much improved and continuous flow designs include the Spreader-Stoker system (similar to that shown in Figure 13) used in many refuse derived fuels (RDF) facility for converting solid wastes, and the fluidized bed combustion units (similar to that shown in Figure 12). The number component parts of this system is listed below:


In a spreader-stoker system, the fuel is introduced into the firebox above a grate. Smaller particles will tend to burn in suspension and larger pieces will fall onto the grate. Most units, if properly designed, can handle biomass with moisture content as high as 50-55%. Moisture contained in the fuel is driven off partially when the fuel is in suspension and partially on the grate. The feed system should provide an even thin layer of fuel on the grate.

In a fluidized bed combustor (FBC), the fuel particle burns in a fluidized bed of inert particles utilizing oxygen from the air. Advantages of fluidized bed combustion include: (1) high heat transfer rate, (2) increased combustion intensity compared to conventional combustors and, (3)

Type of Gas Percent Composition

Table 3. Typical gas composition of a fluidized bed gasifier using wood as feedstock.

Type of Gas Percent Composition 1. Carbon dioxide (CO2) 18.25% 2. Carbon monoxide (CO) 13.44% 3. Hydrogen (H2) 14.68% 4. Methane (CH4) 3.21% 5. Nitrogen (N2) 47.31% 6. Ethylene (C2H4) 1.83% 7. Ethane (C2H6) 0.36%

One of the most common methods of biomass conversion is by direct combustion or burning. The simplest units include numerous cookstoves already developed in rural areas of developing countries. Much improved and continuous flow designs include the Spreader-Stoker system (similar to that shown in Figure 13) used in many refuse derived fuels (RDF) facility for converting solid wastes, and the fluidized bed combustion units (similar to that

In a spreader-stoker system, the fuel is introduced into the firebox above a grate. Smaller particles will tend to burn in suspension and larger pieces will fall onto the grate. Most units, if properly designed, can handle biomass with moisture content as high as 50-55%. Moisture contained in the fuel is driven off partially when the fuel is in suspension and partially on the grate. The feed system should provide an even thin layer of fuel on the

In a fluidized bed combustor (FBC), the fuel particle burns in a fluidized bed of inert particles utilizing oxygen from the air. Advantages of fluidized bed combustion include: (1) high heat transfer rate, (2) increased combustion intensity compared to conventional combustors and, (3)

shown in Figure 12). The number component parts of this system is listed below:

Table 4. Typical gas composition of the TAMU fluidized bed gasifier.

**3.2.3 Biomass combustion** 

1. Refuse charging hopper 2. Refuse charging throat

6. Hydraulic power cylinders and control valves

10. Automatic sifting removal system

3. Charging ram 4. Grates

5. Roller bearings

7. Vertical drop-off 8. Overfire air jets 9. Combustion air

grate.

1. Carbon dioxide (CO2) 10% 2. Carbon monoxide (CO) 20-22% 3. Hydrogen (H2) 12-15% 4. Methane (CH4) 2-3% 5. Nitrogen (N2) 50-53% absence of fouling and deposits on heat transfer surfaces. The schematic diagram of a fluidized bed combustor is similar to that of a fluidized bed gasifier. The only difference is the use of excess air for combustion processes and starved air for gasification processes.

So far FBC has been used mostly for coals. A number of wastes, e.g. wastes from coal mining and municipal wastes, are also sometimes incinerated in fluidized beds. It has been suggested that certain quick-maturing varieties of wood could be combusted in fluidized beds for generation of steam. There is indeed a global search for suitable varieties of wood for this purpose and FBC is likely to play an important role in supplying energy requirements in certain countries in the future.

Fig. 13. Schematic diagram of a reciprocating grate combustor (Courtesy of Detroit Reciprogate Stocker).

Granular biomass fuels, e.g. paddy husk and chips of wood up to 2cm x 2cm x 2cm in size have been successfully combusted in fluidized beds of sand particles. Conventional combustion of paddy husk is slow and inefficient. Nearly complete combustion and high combustion intensities of paddy husk can be achieved in a fluidized bed combustor. The same combustor can also be used for burning wood. Combustion intensities up to about 500 Kg/hr-m2 have been achieved in fluidized bed combustors using biomass fuels.

A number of thermo-chemical conversion processes exist for converting biomass into liquid fuels. These can be crudely divided into direct liquefaction and indirect liquefaction (in which the biomass is gasified as a preliminary step) processes. While all these techniques are relatively sophisticated and will generally be suitable for large scale conversion facilities,

Biomass Energy Conversion 225

popularity. In the implementation of new and emerging technologies, lessons learned from past experiences must be taken into consideration. Many of these technologies require highly qualified and skilful manpower, more advanced monitoring techniques and equipment and materials that many developing countries may not have. The government of each country should have an active role to support the developing of such technologies including massive information campaign and training and improvement of local expertise in the use of advanced materials and process equipment for biomass conversion into energy

Finally, to reverse the trend in the depletion of agriculture and forestry resources, massive reforestation program must be made together with developing technologies for harvesting, pre-processing and storage of biomass. This should be implemented together with infrastructure development for efficient transport of biomass to where it is needed or

Annamalai, K, J. M. Sweeten and S. C. Ramalingam. 1987. Estimation of Gross heating

Barret, J.R., R.B. Jacko and C.B. Richey. 1985. Downdraft Channel Gasifier Furnace for

Callander, I.J. and J.P. Barford. 1983. Recent Advances in Anaerobic Digestion Technology.

Carlin, N. T. 2009. Optimum Usage and Economic Feasibility of Animal Manure-Based

Coovattanachai, N. 1991. Gasification of Husk for Small Scale Power Generation. RERIC

Eisenstat, L., A. Weinstein and S. Wellner. 2009. Biomass Co-firing: Another Way to Clean

Energy Information Administration. 2002. Annual Energy Outlook. DOE/EIA-0383 (2002).

Gupta, S. C. and P. Manhas. 2008. Percentage Generation and Estimated Energy Content of

Haq, Zia. 2002. Biomass for Electricity Generation. EIA, US Department of Energy, 1000

LePori, W.A. 1985. Thermo-chemical Conversion of Biomass Using Fluidized Bed

LePori, W. A. and E. J. Soltes. 1985. Thermochemical Conversion for Energy and Fuel. In :

Lettinga, G., A.F.M. Van Velsen, S.W. Homba, W. de Zeeuw, and A. Klapwijk. 1980. Use of

Technology. ASAE Paper No. 85-3701, ASAE, St. Joseph, MI 49085.

Municipal Solid Waste at Commercial Area of Janipur, Jammu. Environmental

Biomass Energy : A Monograph. E. A. Hiler and B. A. Stout : Editors. Texas A&M

the Upflow Sludge Blanket (USB) Reactor Concept for Biological Wastewater Treatment, Especially for Anaerobic Treatment. Biotech and Bioengineering. 22:699-

Engineering, Texas A&M University, College Station, Texas.

Your Coal. Power Vol. 153 Issue 7, 68-71 (July 2009).

Independence Ave., SW, Washington, DC. USA.

University Press, College Station, Texas, USA.

Values of Biomass Fuels. Transactions of the ASAE, American Society of

Biomass Fuels. Transactions of the ASAE, American Society of Agricultural

Biomass in Combustion Systems. Ph.D. Dissertation, Department of Mechanical

develop technologies that will be brought to where biomass resources are abundant.

Agricultural Engineers, Vol. 30(4): 1205-1208.

Engineers. Vol. (32): 592-598. St. Joseph, MI.

Proc. Biochem. 18(4):24-30 and 37.

Washington, DC. USA.

734.

Conservation Journal 9(1): 27-31.

International Energy Journal. 13(1):1-17.

and fuels.

**5. References** 

they do represent an important energy option for the future because the heavy premium that liquid fuels carry.

The steam produced from heat of combustion of biomass may power a steam turbine to produce electricity. However, because of the high ash contents of most biomass resources, direct combustion of these biomass resources is not practical and efficient due to slagging and fouling problems. Because of these problems, some biomass with high ash are often mixed with low ash biomass such as coal, also termed co-firing.

### **3.2.4 Biomass co-firing**

*Co-firing* refers to mixing biomass and fossil fuels in conventional power plants. Significant reductions in sulfur dioxide (SO2 – an air pollutant released when coal is burned) emissions are achieved using co-firing systems in power plants that use coal as input fuel. Small-scale studies at Texas A&M University show that co-firing of manure with coal may also reduce nitrogen oxides (NOx- contribute to air pollution) emissions from coal (Carlin, 2009). Manure contains ammonia (NH3). Upon co-firing manure and coal, NH3 is released from manure and combines with NOx to produce harmless N and water.

Biomass co-firing has the potential to cut emissions from coal powered plants without significantly increasing the cost of infrasructure investments (Neville, 2011). Research shows that when implemented at relatively low biomass-to-coal ratios, energy consuption, solid waste generation and emissions are all reduced. However, mixing biomass and coal (especially manure) does create some challenges that must be address.

There are three types of co-firing systems adopted around the world as follows:


Direct co-firing is the simplest of the three and the most common option especially if the biomass have very similar characteristics with coal. In this process, more than one type of fuel is injected into the furnace at the same time. Indirect co-firing involves converting the biomass into gaseous form before firing. The last type has a separate boiler for the co-fired fuel.

It was reported that the carbon life cycle and energy balance when co-firing 15% biomass with coal is carbon neutral or better (Eisenstat, et al., 2009). In this research, carbon emissions are reduced by 18%.
