Category Archives: Technologies for Converting Biomass to Useful Energy

Energy conversion through combustion of biomass including animal waste

Kalyan Annamalai, Siva Sankar Thanapal, Ben Lawrence, Wei Chen, Aubrey Spear & John Sweeten


Coal in the power generation industry is the norm since it represents a steady supply in lieu of the vast reserves in the USA and it is the cheapest available fossil fuel. According to the US Energy Information Administration (EIA), coal accounts for 43.1% of the total energy consumed for power generation. In the year 2010 coal consumption in the power sector was to the tune of

1085.3 million short tons, which is around 92% of the total coal consumption in the USA (Watson et al., 2011).

The combustion of fossil fuels particularly coal, a solid fuel, poses many challenges due to the pollution it creates. Coal combustion releases carbon dioxide (CO2) of about 90 kg/GJ, which aids in the phenomena of global warming. The US Environmental Protection Agency (EPA) reports that nitrogen oxides are one of the major pollutants generated in the USA and a large fraction of it comes from coal-fired power plants. As opposed to fossil fuels, the biomass fuels are CO2 neutral. Thus, extensive research is being conducted to reduce CO2 emission by using renewable fuels such as wind, solar, agricultural biomass fuels (AgB) and hydrogen generated from fossil fuels and splitting water into hydrogen (H2) and oxygen (O2).


Figure 3.1 shows a comparison between AgB energy and hydrogen energy cycles. In the biomass cycle, photosynthesis is used by autotrophs (photosynthesizing organisms, Fig. 3.2) to split CO2 into carbon (C), O2, and water (H2O) into H2, O2, produce hydrocarbon (HC) fuel (e. g. leaf) and release O2. The O2 released is used back to combust HC and produce the CO2 and H2O, which are returned to produce wood and AgB and release O2. On the other hand, in the hydrogen cycle


Figure 3.2. The Pyramid of Life; The base of this pyramid is occupied by organisms called autotrophs which are utilizing photosynthesis for extraction of energy and negative entropy from the sun’s radiation. Other organisms (herbivores and carnivores) are utilizing energy and negative entropy stored in food, during the process of digestion. (Mieszkowski et al., 1992).

H2O is dissociated using the photo-splitting process to produce H2 and O2, and then use H2 and O2 for the combustion process. Photosynthesis is water intensive; most of the water supplied to plants evaporates through leaves and is highly inefficient for conversion to electrical power.

The AgB is consumed by herbivores and processed into solid waste called manure or animal waste based biomass (AnB) as a byproduct of digestion and this biomass is almost a chemical replica of foods they consumed. As a matter of fact, Sweeten et al. (2003) had shown that the dry ash-free (DAF) gross or higher heat value (HHV) of cattle manure or cattle biomass (CB) is almost the same as the DAF HHV of agricultural ration fed to the cattle. The heat values of CB are comparable to low quality TX lignite coal.

This chapter gives an overview on energy conversion from animal wastes, fuel properties and TGA analyses, and various thermal energy conversion processes including co-firing, rebum and gasification.

. HHV of gases and energy conversion efficiency

The heat content of the combustible gases is computed on a dry tar-free basis. The energy density [kJ/m3] of the gases is represented in Table 3.10 for several ER and S:F ratios. Increased ER or S:F tends to increase the energy density of the gases; this is due principally to the increase in the production of hydrocarbons (HC) and H2. At constant S:F, increasing the ER tends to increase the HHV, due to more H2 and HC, until a certain ER beyond which the HHV starts to decrease. The energy density of the gases is strongly affected by the production of hydrocarbons such as CH4 and C2H6, which have a high HHV as compared to the other gases (CO and H2). For example, the HHV or energy density of the CH4 is 36264 kJ/SATP m3 while the HHV of CO and H2 are 11550 and 11700 kJ/SATP m3 respectively. Although the HHV of the H2 (141800 kJ/kg) ona mass basis is very high, its energy density is almost comparable to that of CO (only 1.08% higher) due to its low density (~0.0857kg/m3). At constant ER, increased S:F increase the H2/CO of the species produced (Fig. 3.49 and Fig. 3.50), which implies increasing the energy density slightly. For the set of operating conditions investigated the HHV of the gases ranged between 3268 and 4285 kJ/SATP m3, which correspond to a range between 9 and 12.6% of the energy density of the CH4 on a volume basis. Even though the energy density of the gases gives an idea of the energy content of the gases produced, it does not give information about the degree of energy conversion from biomass gasified.

Table 3.10. Energy density of the gases (kJ/Standard temperature and pressure (SATP) m3) for several ERs and S:Fs, adopted from Gordillo (2009).


S:F (mole ratio)






























Table 3.11. Heating value of the syngas obtained using enriched air, adopted from Thanapal et al. (2012).











ER = 2.1

ER = 2.8

ER = 4.2

ER = 2.8

ER = 4.2

ER = 2.1

ER = 2.8

ER = 4.2

ER = 2.8

ER = 4.2

S:F = 0

S:F = 0

S:F = 0

S:F = 0.33

S:F = 0.33

S:F = 0

S:F = 0

S:F = 0

S:F = 0.33

S:F = 0.33
























[kJ/Nm3] HHV with

N2 & CO2 [% CH4]













N2 & CO2 [% CH4]











Variation of HHV with ER in the presence and absence of steam for the case of air (21% O2) and enriched air mixtures (28% O2) is shown in Table 3.11. The enriched-air medium results in gas with higher HHV. The amount of hydrogen produced increases in the presence of steam, but the HHV based on mass is less even with H2 due to lower molecular weight of H2. For both air gasification and enriched-air gasification, we observe a decrease in HHV with ER. Table 3.11 also gives the HHV of the gas mixture with inerts (N2 and CO2) and without inerts (N2 and CO2) and these values are expressed in terms of percentage HHV of natural gas (Thanapal et al., 2012).

Ultimate analyses of samples of tar collected in the sample unit were obtained and were used to derive an empirical formula (CH2O0 48N0.064S0.0017). Because it was impossible to measure the mass of tar and H2O produced during the experiments, the volumetric flow of gases, required to calculate the energy recovery was estimated by mass balance using tar and gas compositions and with the knowledge of the char produced and the flows of the air and steam. Table 3.12 presents the energy conversion efficiency (ECE) estimated by atom balance and assuming gas composition on a dry tar free basis. Although, the energy density of the gases tends to increase with increased ERs, the ECE decreases, because increased ERs produce more mass of tar and char but less mass of gases per kg of DB gasified.

For the range of the operating conditions studied, the ECE ranged from 0.24 to 0.69; the remaining fraction corresponds to the energy in char, tar, and sensible heat of gases leaving the gasifier. This agrees with the fact that in a fixed bed gasifier the gases leave the gasifier at a lower temperature as compared to that of gases leaving a fluidized bed gasifier. Lower sensible heat of gases leaving the reactor implies higher gasifier efficiency, and hence more energy recovered in the gases.

Table 3.12. Energy conversion efficiency (ECE) for severalERs and S:Fs estimated by atom balance, adopted from Gordillo (2009).


S:F (mole ratio)






























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Density (EN15103, 2010)

This represents the weight of biomass per unit of volume and can be expressed on dry basis and on wet basis, indicating the value of moisture content. Another important parameter is the bulk density that expresses the volumes required for storage. Typical values of the bulk density range from 150-200 kg/m3 for straw and wood chips, to about 600-900kg/m3 for wood. Table 5.1 shows typical values for bulk density of selected biomasses. Together with heating value the bulk density identifies the energy density of biomass, that is the energy available per unit of volume. This information is necessary to design the storage facilities and to evaluate transportation costs. In general the energetic density of biomass is about 1/10 that of fossil fuels. Sulfur content analysis (EN 15289, 2011)

This represents the quantity of inorganic and organic sulfur contained in biomass. The measure is realized eliminating all the organic substances present in the fuel and operating the complete transformation of sulfur compounds in soluble sulfates, through calcination at elevated tempera­tures in oxidizing and basic environments. The content of sulfur in the fuel is responsible for the presence and quantity of sulfur oxides (and so acid gases of sulfur: sulfuric acid and hydrogen sulfide) in the emissions deriving from energy conversion processes.

Gas cleaning and upgrading

The gases formed by gasification are more or less contaminated depending on the process and the feedstock. Depending on the gas utilization, some kind of gas cleaning must thus be applied to prevent eventual problems in downstream process equipment such as plugging, erosion, corrosion, and catalyst poisoning, but also to prevent pollution of the environment. Table 6.3 shows the main requirements for a general syngas.

The product gas from biomass gasification will contain fine carbon-containing ash particles, which are difficult to remove only by cyclones. Barrier filtration methods using e. g. sintered metal or ceramic filters are therefore employed. The differential pressure over the filter tends to increase as deposits build up over time. Problems with filter clogging by soot resulting from the thermal cracking of tars both in the gas phase and on the filter surface. This problem can partly be handled by cooling the gas to <500°C and lowering of the gas velocities through the filter.

Table 6.3. Purification level of main syngas impurities (Boerrigter, 2005).


Removal level

Sum of sulfur compounds (H2S + COS + CS2)

< 1 ppmV

Sum of nitrogen compounds (NH3 + HCN)

< 1 ppmV

HCl + HBr + HF


Alkaline metals


Solids (soot, dust and ash)

Essentially completely

Organic compounds (hydrocarbons, tars)

Below dewpoint

However, temperatures should not be allowed to fall below 400°C due to the potential problem of tar condensation and subsequent clogging.

Hot gas cleanup is to be preferred if the sensible heat of the gas has to be retained and thus are low temperature scrubbing systems for tar removal often avoided.

Tar concentrations in the gas are mainly a function of the gasification temperature, the higher the temperature the lower the tar concentration. Tar levels and tar characteristics are not only a function of temperature though, but also of the feedstock, gasifier configuration and processing conditions. The tars formed in pyrolysis are thermally cracked in most environments to refractory tars, soot and gases. However, the problems associated with tar during gasification of biomass differ from those when gasifying coal or peat and thus are the methods for tar handling developed for coal gasification not directly transferable to biomass gasification applications.


Thermo gravimetric analyses on the coal and other biomass were performed using a TA SDT Q600 TGA-DSC instrument. Around 10 mg fuel sample was heated at a rate of 20 K/min from ambient to high temperature (above 1000°C) in an inert (nitrogen) environment. The mass of the sample as a function of temperature was recorded. All fuels were analyzed as received. A reference pan was also heated in the same furnace at the same rate. The temperature of the reference pan was recorded with the temperature of the sample pan. The difference in the temperatures between the two pans can be used to create a DTA trace (Martinez, 2012 and Lawrence, 2007)). Figure 3.15 gives the TGA (ABCDEWF) and DTA traces (A’B’C’D’E’F’) for the fuels considered.

Point A marks the beginning of the traces. Point B (B’ on DTA) marks the peak of the drying (endothermic) process. Point C marks the beginning of the pyrolytic exothermic. Point D marks the peak of the pyrolytic exothermic. Point E marks the end of the pyrolytic exothermic. Following pyrolysis, only fixed carbon and ash remained in the pan. Point F marks the end of the trace. Of par­ticular interest are the temperatures at which pyrolysis began, ended, and the percentage of mass lost due to pyrolysis. The portion between points A and B on the TGA trace defines the amount of mass lost due to drying (moisture loss). The portion between points C and E on the TGA trace defines the amount of mass lost due to pyrolysis. The temperature and remaining mass at this point have been marked in Figure 3.15. The pyrolysis data for several fuels are summarized in Table 3.4.

The influence of biomass gasification gas

Considering the advantages of indirect co-combustion technology, many researchers focused on the reduction of NOx emissions with co-combustion coal and biomass gasification gas.

Fan et al. (2006) performed experiments with simulated biomass gasification gas (CO/H2/CH4/C3H6) and found that the reduction effect on NO emissions was decreased with the concentration of H2, CO in biomass gas increased; the proportion of CH4, C2H6, C2H4 increased, the NO reduction efficiency increased, where the reaction CH; + NO ^ N2 + M played a leading role.

Duan et al. (2006), performed experiments with simulated biomass gasification gas (CO/CO2/CH4/H2/N2) co-combustion for reduction of NO in an electrical heating corundum tube flow reactor. It is confirmed that the mixing of biomass gasification gas can improve NO reduction rate when the oxygen content changed in a range of 0-5% in reactor entrance, temperature changed from 1000 to 1400°C

Dong and Hu et al. (2009) performed some experiments to study the influence biomass simu­lation gas played on emissions of N2O in a small fluidized bed reactor. Also, N2O reduction was effected by the co-combustion ratio of biomass gas (0-1.4%), co-combustion temperature (800- 1000°C), residence time in co-combustion zone (0.16-0.32 s), O2 initial concentration of gas (4-8%), bed material height (0-50 mm), and other conditions changed. The results showed that the higher the co-combustion temperature was, the higher the thermal decomposition rate of N2O was. With the biomass gas content of 1.0%, when the co-combustion temperature was 850°C, N2O decomposed absolutely; the oxygen concentration of flue gas plays an inhibitory action on N2O decomposition, but the injection of biomass gas can effectively avoid this problem. Based on


Figure 4.8. Circulating fluidized bed system.

this, the further research showed that in the process of biomass gas reducing N2O, the following reactions took the more important influence on N2O decomposition:

N2O + H< = >N2 + OH (4.1)

N2O(+M)< = >N2 + O(+M) (4.2)

N2O + CH3< = >CH3O + N2 (4.3)

A integrated system, including a circulating fluidized bed subsystem and a fixed bed biomass gasifier subsystem, was built by Zhang (2011). The circulating fluidized bed subsystem mainly includes a circulating fluidized bed reactor, a hot air ceramic electric heater, a fluidized bed start heating furnace, a spiral feeder, a spray desuperheating tower, a tubular heat exchanger and a mechanical vibration type bag dust extraction, which is shown in Figure 4.8. In order to describe different nozzles, Rh is defined as the ratio of its height away from air distributor to the furnace diameter. Corresponding to nozzle A, B, C, D, E and F, the value of Rh was 4.3, 6.3, 8.3, 10.3,

12.3 and 14.3 respectively. Six temperature probes were distributed at the nozzles.

Figure 4.9 is the flow diagram and photograph of the fixed bed biomass gasification subsystem, which mainly includes fixed bed gasifier, catalytic tower, spraying tower, purification tower, water ring type vacuum pump, drying tower and connection pipes. Rice husk, the raw material for gasification, was fed into the gasifier from the hopper. The produced gas flowed into the catalyzing tower from the bottom of the gasifier. Some activated carbon particles were added to the catalyzing tower for adsorbing and catalyzing decomposition of the tar produced. Then the gases were purified further through the water tower for protecting other equipments and pipes, and were dried by the drying tower with a water-ring vacuum pump. Finally, the dried gas was used for co-firing with coal in the circulating fluidized bed, which helps to reduce N2O and NO emissions. The air used in the gasifier was supplied from the medium of the gasifier at the top of the hopper. After gasification, the ash fell into the ash hopper at the bottom of the gasifier. At the end of experiments the ash was cleaned. During operating the pilot plant system, firstly the water ring vacuum pump is run before starting the gasifier system to ensure the gasifier system operated with a slightly negative pressure, and then starting the circulating water pump in the water tower. After checking all the relevant components, the gasifier was ignited with a electric igniting torch.


Figure 4.9. Process diagram of flue gas analysis.

From an analysis of the results, it is concluded that:

1. With the increase of the proportion of the reburning, the theoretical air requirement was decreased, and in contrast the theoretical flue gas was increased accordingly, as was the furnace temperature and exhaust temperature. However, the boiler efficiency was decreased with the increase of exhaust volume and exhaust temperature.

2. By injecting gasified biomass from the nozzle with a length to diameter ratio of 8.3, the highest N2O removal rate of 99% was achieved, while its NO removal rate was 44%.

Grate burners

Grate burners (Fig. 5.15) achieve a higher combustion velocity and efficiency, with respect to pile burners, because the solid fuel is evenly spread (not piled) on a grate by a dedicated device called a stoker, and this allows a better mixing between air and fuel. Moreover the fuel moves across the grate, from the inlet section to the ash discharge section, mechanically driven by the movement of the grate or by gravity; this increases the mixing of fuel and air and it facilitates the disruption of the solid char, which then burns more quickly. Moreover the movement of the fuel inside the combustion chamber allows a certain degree of differentiation in the air supply to different areas of the combustion chamber, which can then be adjusted according to the typical heating-drying-pyrolysis-oxidation sequence.

A scheme of a modern grate-fired boiler is composed of: a fuel feeding system, a grate assembly, a secondary air (including over-fire air or OFA) system and an ash discharge system (Yin etal., 2008).

Typical fuel feeding and distribution systems are represented by mechanical stokers and spreader stokers which continuously propel fuel into the combustion chamber and above the grate. The smallest particles burn in suspension (if the fuel is very fine, i. e. 30% or higher then mass fraction has dimensions smaller than a few millimeters) while bigger pieces fall into the grate and form the fuel bed. The fuel feeding system regulates the amount of fuel which is fed to the furnace while the distributor evenly spreads the fuel on the grate. Typical feeding systems are positioned under a hopper and are either a conveyor belt, a piston, a screw conveyor or a rotary vane feeder, whose speed may be varied to regulate the fuel flow (dosing system). The rotary vane system provides also a physical isolation of the hopper from the combustion chamber preventing backfire and may also be used in combination with the other systems. From the feeding systems





Figure 5.15. Working principle of a sloping grate combustor.

the solid fuel reaches the distributors which can be either mechanical or pneumatic (Fig. 5.16). In the first case a revolving device throws the fuel into the furnace while in the second case a pulsating high pressure air flow blows the fuel inside the combustion chamber and on the grate.

Capacities of grate-fired boilers range from 4 to 300 MWt and are mainly grouped around 20­50 MWt in biomass fired CHP plants. The heat release per grate area can be around 4 MWt/m2 (U. S. Environmental Protection Agency, 2007). The grate that is at the bottom of the combustion chamber has two main functions: to guarantee a homogeneous distribution of the fuel and of the bed of embers over the whole grate furnace, and equal distribution of air entering from beneath over the whole grate areas.

As far as the primary and secondary air supply systems are concerned, they play a very important role in the efficient and complete combustion of biomass. For grate-firing the overall excess air is about 25-50% (Nor’West-Pacific Corporation, 1981) and it can be considered that excess air equals the moisture content of the fuel. The ratio between primary air and secondary air tends to be 40/60. Primary air supply must be divided into sections in order to operate the grate furnaces at partial loads (down to about 25% of the nominal furnace load) and control the primary air ratio needed (to secure a reducing atmosphere in the primary combustion chamber necessary for low NOx operation).

The fuel bed is composed of solid particles that are piled up with a characteristic porosity and it is heated by over-bed flames and refractory furnace walls until it ignites on the top surface of the fuel bed. The accepted combustion mechanism of cross current units considers that after ignition a reaction front propagates from the surface of the bed to the grate against the direction of primary air even if this traditional pathway may not be observed, depending on fuel properties and operating conditions (Saastamoinen et al., 2000; Thunman and Leckner, 2001; Zhou et al., 2005; Yang et al., 2004). A reaction front propagating from the grate upwards to the surface of the bed was also reported. The reaction front then moves downward against the primary air flux and a char and has layer remains in the surface. When the reaction reaches the surface of the grate the availability of oxygen increases and char can be oxidized as well; glowing combustion takes the place of flaming combustion.

Inhomogeneous air supply may cause slagging and higher fly-ash amounts and may increase the excess oxygen needed for a complete combustion, resulting in boiler heat losses. To avoid

Подпись: FUEL
Подпись: ~1 PLATE І

image247BIOMASS , FEED




Figure 5.16. Working principle of a mechanical (a) and pneumatic (b) spreader-stoker.

this problem continuously moving grates, a height control system of the bed of embers (e. g. by infrared beams) and frequency-controlled primary air fans for the various grate sections can be used. Gases released by biomass conversion in the grate and a small amount of entrained fuel particle continue to burn over the bed and secondary air plays an important role in mixing, burnout and emissions.

An advanced secondary air supply system is one of the most important elements in the opti­mization of the gas combustion in the freeboard and it is an important retro-fit to improve burnout in old grate-fired boilers and reduce pollutant emissions.

Grates can be water-cooled (more indicated for dry biomass fuels with low ash-sintering temperatures) or air-cooled (more indicated for wet bark, saw dust and wood chips). Grate burners may be divided in two main categories (see Table 5.7) (Yin et al., 2008; Van Loo and Kopperjan, 2002):

• stationary grates, where the grate is fixed with respect to the combustion chamber and fuel motion is driven by gravity; they are also called sloping grates;

• moving grates, where the grate moves with respect to the combustion chamber and the mechan­ical motion induced (vibration, alternate or translational) moves the solid fuel. Depending on the motion they are called vibrating grates, reciprocating grates and traveling grates.

In stationary sloping grates fuel is introduced on the top of the slope with mechanical or pneumatic distributors and it tumbles down the slope, which angle may be constant or varying according to the different velocity of combustion required at different stages. The grates consists

Table 5.7. Grate burners typologies and characteristics (Yin et al., 2008; van Loo and Kopperjan, 2002).

Type of grate

Main characteristics


Sloping grate MOVING GRATES

— the grate does not move;

— the fuel burns as it slides down the slope under gravity;

— the control of combustion is difficult;

— there is risk of avalanching of the fuel.

Traveling grate

— the fuel is fed on one side of the grate and is burned while it is transported to the ash pit;

— the control and carbon burnout efficiency are improved.

Reciprocating grate

— the grate tumbles and transports fuel by reciprocating (forward-reverse) movements of the grate rods as combustion proceeds;

— finally the solids are transported to the ash pit at the end of the grate;

— carbon burnout is further improved due to better mixing.

Vibrating grate

— the grate has a shaking movement that spreads the fuel evenly;

— this type has less moving parts than other movable grates;

— carbon burnout efficiency is further improved;

— vibrating grates have the longest life.

Underfeed rotating

— conical grate sections rotate in opposite directions and are supplied with


primary air from below;

— wet and burning fuels are well mixed (the system can work with very wet fuels);

— combustible gases formed are burned out with secondary air in a separate horizontal or vertical combustion chamber;

— the fuel is fed from below by screw conveyors (similar to underfeed stokers);

— the fuel moves to the periphery of the circular grate, at the edge of the grate ash falls into a water-filled ash basin underneath the grate.

of steel cast alloy blocks, which may be water-cooled, and evenly spaced to form either a flat surface or a staircase. Water cooling may be used to increase the life length of the grate and also to reduce the local temperature where ash is formed to avoid ash melting and slagging problems.

Primary combustion air is provided in excess with respect to stoichiometric conditions through pin holes in the grate blocks and may represents up to 75% of the overall combustion air. Alternative solutions for air supply consider integral fins where holes are protected by deflector plates welded to the fins to prevent fines from plugging the holes and to direct the air down the slope and help the advancement of ashes towards the ash pit. Stationary grates (Fig. 5.17) require periodical manual cleaning of the grate from ashes, since the slope and air jets are not sufficient to move all the ashes to the pit, however, much less than in pile burners. The remaining 25% of combustion air is provided as secondary over-fire air from ports in the brick wall.

Moving grates have a different configuration according to the different mechanical principle that moves the bars of the grate.

In vibrating grates a rapid oscillatory motion of the elements facilitates the movement of the solids throughout the combustion chamber and the sliding across the grate which is usually a slope with a fixed angle. The compacting action of the vibrations allows a wide range of fuels to be burnt with a maximum thermal load around 2500 kW/m2.

In reciprocating grates it is the alternate motion of the moving segments, either forward- backwards on sloping grates or an angular tilting on horizontal grates, that pushes solids throughout the grate and also the ashes to the ash pit. The moving bars are positioned between fixed ones and are attached to a frame which is activated by a hydraulic system slowly and inter­mittently. The frequency and width of the strokes allow adjustment of the fuel layer to optimize combustion and emissions. Under-fire primary air is provided through the gaps between movable and fixed bars.


Figure 5.17. Cross section of a stationary sloping grate (Tillman, 1991).


Figure 5.18. Working principle of a reciprocating horizontal grate (a) and traveling grate (b).

In a traveling grate the elements (Fig. 5.18) are linked together to form a chain like structure which is continuously moved in one direction around running wheels of which one is motive and the other is idle. It enters one side of the combustion chamber, where it receives the fuels, it runs inside the combustion chamber carrying the fuel bed until it reaches the far end where ashes are

Подпись: Working principle of a suspension burner with auxiliary pilot flame.

Figure 5.19.

discarded and eventually it exits the chamber to renter from the other side, after traveling the whole distance below and outside the combustion chamber. A proper grate tension is essential to guarantee the proper functioning, among the different solutions a movable shaft of the idle wheel and the free hanging of the return side of the grate to form a catenary are used.

Simulations of combustion and emissions characteristics of biomass-derived fuels

Suresh K. Aggarwal


There is worldwide interest in developing renewable energy sources in a sustainable manner. This is motivated by our excessive reliance on finite fossil energy sources, environmental concerns due to greenhouse gas emissions, and ever-growing energy needs especially due to emerging economies and population growth. A sustainable and carbon-neutral energy future will require a significant broadening of our energy portfolio and reducing reliance on non-renewable sources. While multiple renewable energy sources and technologies will be needed to attain this goal, non-food and regional fuel sources, especially biomass, are expected to play a major role in this effort. Biomass represents one of the primary energy resources in the world after coal and oil, particularly in developing countries (Hall et al., 1991). It refers to a broad variety of feedstock ranging from agricultural waste, such as straw, bagasse, rice husks, olive pits, and nuts, to energy crops such as miscanthus and sorghum (Werther et al., 2000). It also includes algae, forestry waste such as wood chips, bark and thinning, and other solid wastes including sewage sludge, as well as municipal waste. The use of biomass would not only reduce our dependence on fossil energy sources, but also provide energy in a sustainable and carbon neutral manner.

Biomass can be converted to more valuable energy forms via a number of processes includ­ing biological, thermal, and mechanical or physical processes. Figure 2.1 from Gill et al.,


Figure 2.1. A schematic of various conversion methods and major fuels produced from biomass (Gill et al., 2000).

(2000) provides a schematic of the various conversion processes and major products (fuels) from lignocellulosic biomass.

Biological conversion faces many challenges due to high cost and low efficiency, and is currently limited with regards to feedstock and products (Lin et al, 2006). In contrast, thermo­chemical methods have been extensively investigated for the conversion of biomass to a variety of products, such as energy, fuels, and chemicals. This chapter starts with an overview of thermo­chemical conversion processes, namely direct biomass combustion, pyrolysis and gasification. A brief discussion of various processes involved and fuels produced is provided. This is followed by a discussion of research dealing with biomass-derived fuels. The focus is on the combustion and emission characteristics of syngas and biogas. Both the fundamental and applied research is reviewed. Finally, some research needs are outlined.

NOx emissions

As the temperature of the reactor is insufficient to generate thermal NO, the source of NO is fuel nitrogen. Thus most NO comes from oxidation of fuel N. Correcting the NO emissions of 3% O2 is a common industry practice to prevent utilities from artificially diluting NOx emissions with O2. In the very lean regime, correcting caused the NOx emissions to increase. However, for

Подпись: Figure 3.28. Effect of fuel on BF for WYO and WYO:DB blended fuels. Note that the data points come close to overlapping for all equivalence ratios. Thus, BF was independent of fuel type (adopted from Lawrence, 2007).

all other equivalence ratios, correcting caused the NOX emissions to decrease because there is less than 3% O2 in the exhaust prior to correcting. Figure 3.29 and Figure 3.30 present the NOX emissions forTXL andTXL:DB blended fuels in ppm and corrected to 3% O2. With the exception of 95-5 TXL:HA-PC-DB-SoilS, all of the blended fuels produced more NOX in the lean region than the pure TXL even though DB contains more N. There are three possible reasons: (i) higher amount of fuel-bound nitrogen present in the biomass binding with the excess oxygen to form NOX, (ii) release of more N in the form of NH3 due to high urea in DB, and (iii) depletion of oxygen due to oxidation of higher amount of VM released at lower temperatures from biomass thus preventing the oxygen from bonding with fuel nitrogen.

Подпись: k [kg/GJ] Подпись: cXk Mk (XCO2 + XCO) MFHHVF [GJ/kg] Подпись: (3.35)

Instead of reporting NO at 3% O2, another method employed to prevent emission dilution is to report NOX levels on a heat basis. Annamalai and Puri (2007) presented the conversion formula from ppm of pollutant species k to k in kg/GJ, given that the fuel fired is CcHfcO0N„Ss. For pollutant species k:

Подпись: NO [kg/GJ] Подпись: cXNO MNO2 (XCO2 + XCO) MFHHVF [GJ/kg] Подпись: (3.36)

on an atom basis, where c is the carbon atoms in the fuel, Xk mole fraction of species k and MF is the molecular weight of the empirical formula for the fuel. With k = NO:

where the molecular weight for NO is expressed in terms of the molecular weight of NO2, as required by the EPA. Figure 3.31 presents the NOX emissions in kg/GJ of heat input.

Note that in the lean region, the blended fuels produce more NOX than the pure WYO. In the slightly rich region, the blended fuels produce less NOX than the pure WYO due to more release ofN in the form ofNH3 from DB.



Figure 3.29. Effect of fuel onNOj forTXL andTXL:DB blended fuels. Note that blended fuels have lower NOx values at stoichiometric and in rich combustion (adopted from Lawrence, 2007).


Effect of Fuel on NO„ for TXL and TXL:DB Blended Fuels eTXL И95-5 TXL LA-PC-DB-SepS A 90-10 TXL LA-PC-DB-SepS Є 80-20 TXL LA-PC-DB-SepS


Подпись: NO, (ppm corrected to 3% Oj)

Figure 3.30. Effect of fuel onNOx forTXL andTXL:DB blended fuels corrected to 3% O2 (adopted from Lawrence, 2007).




Figure 3.31. Effect of fuel onNOj forTXL andTXL:DB blended fuels in kg/GJ (adopted from Lawrence, 2007).


The same explanation for TXL applies to the WYO fuels. Figure 3.32 and Figure 3.33 present the NOX emissions from WYO and WYO:DB blended fuels in ppm and corrected to 3% O2. Figure 3.34 presents the NOX emissions from WYO and WYO:DB blended fuels in kg/GJ of heat input.

Hydrogen production from biomass gasification

As a flexible energy carrier that can be produced from a variety of resources and with compre­hensive uses, hydrogen is one of the most promising substitutes for fossil fuels. It is certain that renewable-based hydrogen will be quite important in the future, especially hydrogen from biomass which has a series of unique merits. Technologies of hydrogen production from biomass mainly contains two kinds of processes. One is the thermo-chemical route, including biomass/waste gasification, biomass pyrolysis, hydrogen from biomass derived methane/methanol/ethanol; the other is the biological route, including direct bio-photolysis, indirect bio-photolysis photo fer­mentation, hydrogen synthesis via the water gas shift reaction of photo-heterotrophic bacteria, dark fermentation and microbial fuel cells, etc (Chen etal., 2006).

The catalytic hydrogen production technology route of Lu et al. (2004b) research is: CFB and fixed bed are used as reactors, and the catalyst is a mixture of dolomite and nickel-based powder. Dolomite is used as bed material and catalyst, and Ni-based catalyst is placed at outlet of the CFB. The results show that the volume content of hydrogen is more than 50%, and content of CO2 is lower than 1%, gas yield could reach 3.31 Nm3/kg, productivity of hydrogen is 130.28 g/kg biomass.

In the State Key Laboratory of multiphase flow in power engineering of Xi’an Jiao Tong Uni­versity, Yan etal. (2005) have done much work on supercritical water gasification and solar energy catalyzed biomass for hydrogen production. In their study, biomass was used as raw material and an Ni-based alloy tube was taken as reactor. Supercritical water-gasification processed at 650°C under a pressure of 25 MPa. Experimental results indicated that volume content of hydrogen is 41.28%, and the small size of biomass particles favor of the production of hydrogen. Besides, the wall of the reactor could enhance the production of hydrogen (Guo et al., 2005).

The Institute of Coal Chemistry, Chinese Academy of Science has studied the CFB conversion of biomass and supercritical water conversion for hydrogen production. The feasibility of total processing of biomass and coal was investigated in the CFB. Besides, hydrogen production from sawdust was processed under supercritical pressure at 773-923 K, and batch-type supercritical water reactor was used as reaction chamber. It was found that the molar ratio of calcium and carbon has a great influence on the conversion of sawdust. Gas conversion of carbon and the gas yield of hydrogen is doubled when the molar ratio of calcium and carbon equals to 0.48. Besides, reaction temperature also has great impact on gas yield of hydrogen.

Подпись: System image170
Подпись: power
Подпись: Preprocessing
Подпись: Carbonization Подпись: I oe
Подпись: Flam ash
Подпись: cotton stalk
Подпись: Solid molding system
Подпись: Gasification

image181Figure 4.10. Polygeneration scheme of cotton stalk system.

The Institute for Thermal Power Engineering of Zhejiang University has developed steam-gas co-production of biomass and coal experiment and a mechanism study which aim at obtaining combustion gas. The synthetic gas was tested in a double CFB circulating system and the calorific value can reach 2800kcal/Nm3, and conversion of fuel is 95%. Based on these results, biomass conversion for hydrogen production is under investigation at present, including the separation of CO2.

Tianjin University is famous for catalytic pyrolyzation of biomass for hydrogen-rich gas, and has proposed the technology route of fast pyrolysis-catalyst steam reforming. A two stage catalyzed gasification hydrogen production system, which includes CFB gasification reactor and fixed bed, was built. The impact of operation parameters, design parameters and catalyst type to the gas yield were investigated, and the results showed that the volume of hydrogen-rich gas can reach 50-65% (Chen etal., 2003b).