Waste and By-product Utilization 2(1+1)

Lesson 3.

Biomass (solid waste) as fuel

Proximate analysis

It is a way of categorizing the organic composition of biomass. It is the breakdown of the fuel in volatiles, fixed carbon and ash. The proximate analysis is typically on dry basis. Most biomass will have a higher volatile content than coals. A general trend would be that the lower the lignin content, the higher the volatiles in the biomass. With waste biomass, depending on the fraction, the volatile content can be as high as 90 % of dry and ash free (daf).

                                      Proximate analysis of common biomass (% dry)

                       

Biomass	Volatile Matter	Fixed Carbon	Ash
Paddy Straw	69.7	11.1	19.2
Wheat straw	71.3	19.8	8.9
Cotton stalks	72	24	4

 

                         

Ultimate analysis

The elemental or ultimate analysis is the way to present the components in the organic part of fuels. It presents directly the main elements present in the organic part of biomass. The main elements of carbon, hydrogen and oxygen are indicated indiviually while secondary elements like nitrogen, sulphur, chlorine etc are all grouped together in “other” since they typically amount to much less than the three major elements.

Carbon (C) is obviously the most important constituent of biomass fuels. It mostly comes from the atmospheric CO₂ that became part of the plant matter during photosynthesis. It is the major contribution to the overall heating value and during combustion it is transformed back into CO₂, and released in the atmosphere.

Hydrogen (H) is another major constituent of biomass, as can be expected from the chemical structure of the carbohydrate and phenolic polymers. During combustion, hydrogen is converted to H₂O.

Nitrogen (N) is the most important nutrient for plants. It is absorbed via the soil or the applied N-fertilizers by the plant during its growth. In some waste fractions, the content of nitrogen may be quite high which contributes significantly to the degradability in biochemical processes like digestion or fermentation. During combustion and for all practical purposes, nitrogen does not oxidize in any significant quantities and is released in the gas phase as N₂ – therefore, its contribution to the overall heating value is zero.

Sulphur (S) is incorporated in several organic structures like amino-acids, proteins and enzymes. With waste fractions, where a mixture of organic substances forms the main part of the fuel, the content of sulphur may in some cases be significant. During combustion, sulphur is typically oxidized and has a minor contribution to the overall heating value. However, its most important impact relates to gaseous emissions syngas cleaning in gasification processes and corrosion issues

Chlorine (Cl)It is typically found in negligible amounts in coals and in wood (< 0.05 % on a dry basis), herbaceous biomass species have a chlorine content ranging from less than 0.1% to 2% or more. During combustion, chlorine is almost completely vaporized, forming HCl, Cl₂ and alkali chlorides. The problems associated with chlorine stem from issues related to emissions and operation issues, namely fouling and corrosion of metallic surfaces.

Oxygen (O) is a major element in all biomass fuels, as is evident from the nature of the photosynthetic process and the chemical composition of the biomass constituents. Fuel oxygen reduces the amount of air needed for combustion and is found in the combustion products chemically bound in the molecules of CO₂ and H₂O.

Table 3.1:  Ulimate analysis of selected dry biomass (%)

Element	Paddy straw	Wheat straw	Cotton Stalks	Wood	Bagasse
C	39.2	43.2	39.5	50	47
H	5.0	5.7	5.6	6.1	6.5
O	36.5	43.3	42.8	45	44
N	0.59	0.6	0.7	0.2	Ash 2.5
S	0.1	0.1
Cl	0.3	0.3	0.5	<0.1

 

The heat value (Calorific value) is the quantity of heat which is released by combustion of unit weight of fuel. It can be expressed in one of two ways: the higher heating value (Gross calorific value) or the lower heating value (net calorific value).

The higher heating value (GCV) is the total amount of heat energy liberated by the combustion of one kg of fuel taken at 0⁰C under one atmosphere of pressure. The water present in the fuel as well as the water formed by combustion of the hydrogen being reduced to same condition.

The lower heating value (NCV) does not include the energy embodied in the water vapor i.e. water formed by combustion and the water of constitution of the fuel remains in vapour form. Generally, the GCV is the appropriate value to use          for biomass combustors, although some manufacturers may utilize the NCV instead.

For most agricultural residues, the heating values are in the range of 15 – 17 MJ/Kg

Theory of combustion

Burning is a chemical procedure through which a material rapidly reacts to the oxygen in the air producing intense heat and light; in the case of biomass burning, it involves three stages: ignition, flaming (burning and smoking with flame), smoldering (burning and smoking without flame). This burning is responsible for producing the main source of toxic gas, particulate matter and greenhouse-effect gases in the planet influencing the atmospheric physics and chemistry, producing rains with slightly changed the pH.

In practice the theoretical quantity of air is inadequate to mix with the entire quantity of fuel intimately. So air in excess of stoichiometric combustion is needed. By measuring CO₂ and O₂ in the flue gases by Orsat apparatus, the excess air levels can be estimated. However, the excess air to be supplied depends upon type of fuel and the firing system.

Composition of atmospheric air

Oxygen = 23%, Nitrogen =77% (by mass)

Oxygen = 21%, Nitrogen = 79% (by volume)

Chemical Reactions: C + O2 = CO2 12 + 32 = 44 i.e. 12 kg of carbon requires 32kg of oxygen to form 44 kg of carbon dioxide or 1 kg of carbn dioxide therefore 32/12 = 2.67 kg of oxygen. 2H2 + O2 = 2H2O 4 + 32 = 36 i.e. 4 kg of hydrogen requires 32 kg of oxygen to form 36 kg of water. Therefore 1 kg of hydrogen requires 32/4=8 kg of oxygen. S + O2 = SO2 32 + 32 = 64 i.e. 32 Kg of sulphur requires 32 kg of oxygen to form 64 kg of sulphur dioxide. Therefore 32/32kg= 1 kg of oxygen.

Element                 Molecular Wt. C                                             12 O2                                                          32 H2                                                          2 S                                              32 N2                                                          28 CO2                                                       44 SO2                                                       64 H2O                                         18

 

Direct combustion of biomass as fuel in furnaces

There are four principal types of furnaces used to burn the biomass

• Step grate furnace
• Horseshoe furnace
• Ward furnace
• Spreader stoker furnace

Mostly step grate furnace is used.

Step grate furnace

Grate furnace combustion is a widely used conversion method to obtain heat and power from biomass. It is typically used for applications with a nominal thermal capacity of roughly 0.1−100MW. Grate furnaces can deal with a wide range of biomass fuel types (e.g. sawdust, wood pellets, bark, and fiber board) and are flexible regarding fuel size and moisture content. Grate furnace combustion is also applied to convert solid municipal waste.

The grate consists of small plates of cast iron arranged in steps. Its inclination to the horizontal should be 52⁰. The grate consists of three parts:

1. The upper part or dead plate, without steps or opening for passage of air on which biomass is dried before passing on to the proper grate.
2. The grate proper corresponding to the steps. This helps to proportion the quantity of air passing through the biomass to the degree of combustion required.
3. The portion of slight slope or ash grate, at the lower end of the grate on which combustion of biomass is completed, leaving ash which falls into ash pit.

Advantages

• Different size of biomass can be burnt
• Biomass with high moisture content can be burnt.
• Low emission of fly ash

Disadvantage

• High emission of NO₂, CO etc.         

Operating conditions affecting design of the furnace

All the furnace dimensions are fixed by the necessity to observe the following conditions:

i) Length of flame

      The length of the passage for the burning gases, between the grate and boiler tubes, should be at least 5 m, and preferably 7-8 m. It should not exceed 10 m.

Below 7 m and particularly below 5 m, the gases would not be completely burnt on reaching the cold water, tubes, and the sudden cooling caused by their passage between the tubes would to a great extent arrest the combustion, thus increasing the proportion of CO, and decreasing the efficiency. Furthermore, below 5 m, the ash entrained with the gases will not be completely burnt, and will thus tend to adhere to the tubes, thus becoming harmful and dangerous. On the other hand, if the length of path for the gases is unnecessarily increased, there will be increased losses by radiation and by air leakage, as well as an increase in the space required and in the cost of the installations.

ii) Width of the boiler

 The various types of water tube boilers generally have a given heating surface per unit width of furnace. For example:

Cail – Steinmuller boilers with headers: 110m²/m (360 sq.ft./ft) width

Fives – Stirling boilers with drums: 135 m²/m (443 sq.ft./ft) width

With recent installations, where the capacity is expressed in tones of steam per hour rather than in heating surface area, we have:

Fives Cail – Babcock boilers with BC1 type furnace: 5 t/h/m (1.5 t/h/ft.) of width   

Fives Cail – Babcock boilers with spreader stokers, type BR1: 6 t/h/m (1.8 t/h/ft.) of width

To avoid the drawbacks of a complicated shape, the combustion chamber should have the same interior width as the boiler, and the total width of the furnace or furnaces should be atleast equal to the latter.

iii) Volume of combustion chamber

The volume of the combustion chamber should be proportioned to the volume of gases necessary for combustion. This volume is therefore fixed in relation to the quantity of heat liberated per hour by the fuel used. Since a certain ratio exists between the heating surface of the boiler and the quantity of steam which it can produce, the combustion chamber volume may be related to the heating surface of the boiler.

The length, width and volume of the combustion chamber must conform to certain conditions, leaving little liberty to the designer of the furnace. The least imperative condition and the most elastic of the three is, however, that of the volume, which can, without great inconvenience, differ appreciably from the values given.

In all modern furnaces, there is provided, in addition to the normal air or primary air entering directly through the grate or by the tuyers ( pipe through which air is blown into a furnace) of the hearth furnace, a complementary air supply for secondary air behind the bridge wall and consequently after the furnace proper. This air is introduced by a small duct built into the bridge wall. The introduction of this supplementary air for combustion has the object of ensuring complete combustion by changing to CO₂ the CO which may remain after combustion in the furnace. Secondary air is generally made 5-15% of the total air supplied, averaging 10%. There is no advantage in exceeding this amount; if combustion is good in the furnace, i.e. if the combustion temperature is high, it forms very little CO, and there would be risk of causing a useless increase in excess air.