Volatile matter and stoichiometry

Assuming that coal is represented by CH08O01 and DB by CH1 л O0 4, Figure 3.12 and Figure 3.13 plot the fraction of C atom remaining as volatile matter in coal and DB and the stoichiometric oxygen per kg volatile matter for coal and DB; if coal releases less VM for given H/C and O/C, then less C atoms leave with VM and more remain in char; typically coal has 40-50% as VM and as such, the stoichiometric O2 is about 2.5 kg per kg VM released; the DB has 80% VM and as such the stoichiometric O2 is only about 1.5 kg/kg of VM and lower compared to coal volatiles due to high O/C ratio in DB. At the same time, the % O2 consumed by coal during combustion of VM is only 50% while DB with VM consumes 78% of oxygen by the time all the volatile mater is burnt indicating that there is rapid depletion of O2 when DB is blended with coal resulting in lower O2 in the furnace which may lead to lesser NOx. In general AgB and AnB fuels with high VM may result in lesser NOx compared to coals.

3.2.2.4 Stoichiometric A:F

The mass based stoichiometric A:F ratio is simply the ratio of the minimum amount air required by fuel for complete combustion on a mass basis. This is calculated based on the empirical chemical formula derived from the ultimate analysis neglecting the moisture and ash in the fuel.

3.5.2.6 Flue gas volume

One can determine the volume of flue gas in m3/GJ from the knowledge of ultimate analysis, Boie equation and reaction equations of biomass with O2 supplied from air (Chapter 4, Annamalai and Puri, 2007). If the SATP is at 25°C, and 1 bar, then for any arbitrary 0 < O2% < 9% (volume), a fit is given as:

Fluegas STP volume, m3/GJ = {3.55 + 0.131O2% + 0.018 x (O2%)2}(H/C)2

— {27.664 + 1.019O2% + 0.140 x (O2%)2}(H/C)

+ {279.12 + 10.285O2% + 1.416 x (O2%)2} (3.16)

 Figure 3.12. Fraction of C atoms remaining with VM fraction, coal.

 Figure 3.13. Fraction of C atoms remaining with VM fraction, Dairy Biomass (DB).

Research status

1.2.1.1 Different biomass for co-combustion

Biomass includes forest wastes, agricultural wastes, animal wastes and anthropomorphic wastes. Considering co-combustion with straw and coal could achieve large-scale and efficient utilization, it is attracting more and more attention and research. Most methods for research are concentrated in the laboratory using thermal gravimetric analysis, or measuring the combustion characteristics of mixtures of pollutants (including toxic gases and heavy metals, etc.) emission characteristics and ash melting characteristics through combustion or pyrolysis of different coals and biomass. The conclusions gained through these methods are an important reference for the design calculations and material choices of biomass-fired boilers, but the site condition is quite different from the experimental condition.

(1) Co-combustion ofcoal and agricultural wastes

In the Northeast Institute of Electric Power Engineering, experimental research on co-combustion of coal and corn stalk were carried out (Lu et al., 2005). The results showed that the co-combustion of coal and corn stalk was helpful for coal burnout. With the increase of co-combustion rate from 20% to 80% (mass biomass to coal ratio), the burnout efficiency was increased, the burnout time was shortened and the burnout temperature was decreased. In Shandong Univer­sity, Zhang et al. (2006) researched on the characteristics of straw co-combustion with coal by the thermal gravimetric analysis method. Cotton stalk, cornstalk, wheat-straw were chosen for co-combustion with coal at different heating rate (30, 50, 75 and 100K/min) and different co-combustion rate (1:20,1:10, 3:20,1:5,1:4, 3:10). The results showed that the co-combustion of coal and straw was helpful for coal burnout. With the rise of heating rate, the ignition temperature of straw mixed coal was decreased and the rate of combustion was increased.

In order to making clear the effect of the alkali metal K on nitrogen conversion in co-combustion of coal and straw, a series of experiments were carried by Yang et al. (2009). The results indicate that it is effective to inhibit the release of NO to add a certain proportion of straw. When the content of K increases in the de-ashed coal samples mixed with a low proportion of straw and KOH, it has a stronger catalytic effect on the reduction reaction of NO, and when the content of K reaches a certain value, the catalytic effect does not increase. The lower the O2 content in the combustion atmosphere, the better the reduction of NO. Dong etal. (2010) have taken some experimental tests. The experiments were carried out at a 400 t/h power station boiler to test its economy and emission characteristics. Considering each operation controllable factor, the best running condition were optimized, which could keep better economy and emission performance. The optimized condition consisted of oxygen content 3.6%, combustion temperature 1278K, pulverized coal fineness R90 = 20%, straw particle size 15 mm, primary air with average coordination, secondary air with waist type, and co-combustion ratio 20% (a heat ratio value). [3]

Table 4.2. Volume of garbage disposal and treatment plants in China (2010).

 Region Area under cleaning program 10000 m2 Volume of garbage disposal 10000 tonnes Number of treatment plants/grounds unit Sanitary landfill Compost Burning National total 485033 15804.8 628 498 11 104 Beijing 13804 633 20 15 3 2 Tianjin 7322 183.7 8 6 2 Hebei 20050 589.3 26 20 4 Shanxi 10609 361.2 17 14 3 Inner Mongolia 9674 334 18 17 1 Liaoning 28122 837.3 25 24 1 Jilin 13037 499.4 7 5 2 Heilongjiang 14937 782.4 20 17 2 Shanghai 15879 732 12 4 2 3 Jiangsu 44088 1017.1 44 30 14 Zhejiang 27805 959 52 30 22 Anhui 17339 435.3 16 13 3 Fujian 11433 417.3 20 14 6 Jiangxi 9911 284 13 13 Shandong 48528 992 55 46 8 Henan 20892 694.6 38 35 1 2 Hubei 16941 711.1 23 21 1 Hunan 12331 505.2 21 21 Guangdong 62768 1938.6 41 25 16 Guangxi 11005 245.1 20 16 1 3 Hainan 4076 97.7 3 2 1 Chongqing 6136 256.7 13 12 1 Sichuan 15173 656 30 23 5 Guizhou 3405 213.3 12 12 Yunnan 9726 265.5 19 12 2 3 Tibet 539 16.3 Shaanxi 10546 388.3 15 11 1 Gansu 5816 278.3 13 13 Qinghai 1951 86.3 3 3 Ningxia 3347 91.9 7 7 Xinjiang 7843 303.3 17 17

Note: Data from National Bureau of Statistics of China.

is 0.3876 million tonnes, the proportion of treated garbage (%) is 77.9%, all which are shown at Table 4.2, Table 4.3 and Table 4.4.

As shown in Figure 4.2, garbage disposed by burning isn’t the most important way in China, which takes 22% and less than other disposing ways e. g. sanitary landfill. Incineration is a waste treatment process that involves the combustion of organic substances contained in waste materials (Andrew, 2005). Incineration can reduce the solid mass of the original waste by 80-85% and the volume (already compressed somewhat in garbage trucks) by 95-96%, depending on composition and degree of recovery of materials such as metals from the ash for recycling (Ramboll, 2006).

In China, researchers have focused on co-incineration performance tests and experiments of coal and different types of MSW Gu et al. (2003) focused on the co-combustion research of municipal sewage sludge and coal. With a thermogravimetric method, the research results showed that co-combustion could enhance activation energy with a lowering of the ignition temperature.

 Table 4.3. Quantity of waste treated in China (2010) (Unit: million tonnes). Region Quantity of waste treated Sanitary landfill Compost Burning City sanitation special vehicles (unit) National total 12317.8 9598.3 180.8 2316.7 90414 Beijing 613.7 445.4 79.3 89.1 7461 Tianjin 183.7 125.4 58.3 1951 Hebei 411.5 311.6 58.9 3306 Shanxi 265.8 213.5 52.3 3689 Inner Mongolia 276.5 251.7 24.9 1480 Liaoning 593.5 571.6 21.9 4998 Jilin 222.3 172.4 49.9 2608 Heilongjiang 315.7 284.6 16.6 3814 Shanghai 599.2 416.5 21.2 108.1 5560 Jiangsu 951.7 488.5 458.7 7481 Zhejiang 942.7 504.9 437.8 4685 Anhui 281 231.1 49.9 1508 Fujian 383.8 241.7 142 2036 Jiangxi 243.9 243.9 899 Shandong 911.6 751.9 131.4 5983 Henan 573.7 501 6.9 65.7 3025 Hubei 436.9 405.8 18.4 3077 Hunan 399.1 399.1 1998 Guangdong 1398 1031.6 366.4 8535 Guangxi 223.3 203.5 9.3 10.5 1748 Hainan 66.4 61.6 4.8 1525 Chongqing 253.7 216.3 37.4 1786 Sichuan 569.8 464.3 7 80.8 3301 Guizhou 193.3 193.3 882 Yunnan 234.4 123.2 10.4 77.7 1783 Tibet 20 Shaanxi 310 281.2 2.2 1719 Gansu 105.6 105.6 1045 Qinghai 58.1 58.1 330 Ningxia 85 85 554 Xinjiang 214 214 1627
 Note: Data from National Bureau of Statistics of China.

The fuels have basically attained devolatilization characteristics in the co-combustion process. Liu’s (2006) experimental research showed that the reactivity of the blend with 20 wt. % of sludge is similar to that of coal. When the blend is with 50 wt. %, there are two temperature zones with obviously different reactivity trends. In the lower temperature zone (less than 430°C), the reactivity of the blend is similar to that of the sludge, and in the higher temperature zone (greater than 430°C), the reactivity of the blend is close to that of the coal. Zhao etal. (2005) researched on co-combustion of sludge/residue in a paper mill with high moisture content and low heating value coal at the hot circulating fluidized bed test facility. His research showed that when the secondary air rate increases, temperature in the dense bed decreased slightly and temperature in the dilute phase region declined, while the combustion efficiency was increased. When the excess air coefficient was increased, temperature in the dense bed increased, temperature in the dilute phase region increased at first and then declined forming an optimum value corresponding to the highest combustion efficiency. When the ratio of paper mill waste to coal was increased, the decline in temperatures in both dense bed and dilute phase region was decreased, and the combustion efficiency was decreased. Lu et al. (2004a) indicated that co-combustion of sewage

 Table 4.4. The waste treatment capacity (tonne/day) in China (2010). Region Treatment capacity (tonne/day) Sanitary landfill Compost Burning Proportion of treated garbage (%) National total 387607 289957 5480 84940 77.9 Beijing 16680 12080 2400 2200 97 Tianjin 8000 6200 1800 100 Hebei 13614 10064 2450 69.8 Shanxi 10568 7968 2600 73.6 Inner Mongolia 9167 8367 800 82.8 Liaoning 17247 16647 600 70.9 Jilin 6496 4456 2040 44.5 Heilongjiang 10969 9869 500 40.4 Shanghai 10545 5750 520 2575 81.9 Jiangsu 37637 22445 15192 93.6 Zhejiang 33323 16438 16885 98.3 Anhui 9420 7670 1750 64.6 Fujian 12747 7197 5550 92 Jiangxi 6066 6066 85.9 Shandong 35225 26425 8200 91.9 Henan 20416 17616 400 2400 82.6 Hubei 12800 11400 1000 61.4 Hunan 11818 11818 79 Guangdong 33956 22213 11743 72.1 Guangxi 8191 6871 400 920 91.1 Hainan 1764 1539 225 68 Chongqing 6465 5265 1200 98.8 Sichuan 16974 13334 2340 86.9 Guizhou 5697 5697 90.6 Yunnan 7749 3849 360 2870 88.3 Tibet Shaanxi 10707 9347 500 79.8 Gansu 3355 3355 38 Qinghai 931 931 67.3 Ningxia 2785 2785 92.5 Xinjiang 6295 6295 70.6
 Note: Data from National Bureau of Statistics of China.

 ■ Burning; Figure 4.2. Percentage of different garbage disposal methods in China (2010).

sludge with coal on a circulating fluidized bed was stable at water contents of 30-60% in sewage sludge and co-combustion rates of 25-100%.

Co-combustion of coal and refuse derived fuel (RDF) were carried out in a bubbling fluidized bed combustor by Sun et al. (2006). The feasibility of solidification and co-combustion of waste

 Figure 4.3. Biomass co-combustion system.

products in oily wastewater with coal was analyzed by Liu et al. (2005). The combustion process, ignition and burnout characteristics of waste tire and coal with a tire-coal ratio of 10%, 30% and 50% were investigated by means of thermogravimetric analysis (TGA), which were carried out by Li etal. (2007), whose research showed that co-combustion with waste tires could improve the burnout characteristics. Co-combustion of waste plastic and coal in fluidized bed were researched by Jin et al. (2001) and co-combustion of Medical Solid Waste and coal in a CFBC by Pu Ge et al. (2003).

Combustion rates, flame temperature and efficiency

Combustion time, defined as “total time for burnout of the particle” (Tillman, 1991) can be calculated with the following formula:

(5.19)

where:

tbo — total burnout of the particle td — time for heating and drying tp — time for solid particle pyrolysis and tco — time for char oxidation.

Table 5.5. Some relevant gas-phase homogenous reactions (Haseli et al., 2011).

 cH5o X exP 10-45 [s-1]

 K = 2.78 x 103exp(-1510/Tg) [kmol-1 m3 s-1] K* = 0.0265exp(3968/Tg) [-]

Source Reaction rate expression Kinetic parameters

Table 5.6. Thermochemical data for cellulose combustion (Di Blasi, 1993; Antal and Varhegyi, 1995; Branca and Di Blasi, 2004; Parker and LeVan, 1989).

 Activation Frequency Reaction Reaction energy [kJ/mol] factor [s-1 ] enthalpy [kJ/g] Product Reference Charring 110 6.7 x < 105 -1.0 Char formed Di Blasi (1993) Volatilization 198 3.2 > <1014 0.3 Volatiles Antal and formed Varhegyi (1995) Char oxidation 183 1.4 x < 1011 -33 Char Branca and Di oxidized Blasi (2004) Volatile oxidation 188 2.6 x <1013 -14 Volatiles Parker and oxidized LeVan (1989)

The evaluation of combustion times can be done also with single particle combustion models. Besides important effort in the research of modeling of single particle combustion has been done (Yang et al., 2008) and some models evaluate also particle burnout (Saastamoinen et al.,

2010) . The combustion rate or burning rate can be considered as the mass of fuel consumed per unit of time. This influences heat release and so this parameter is important for the design of the combustion system. Typical design heat release rates, expressed per unit grate area, are 2-4MWt/m2 (Brown, 2011), even though some fluidized beds can reach 10MWt/m2 (Jenkins, 1998). Combustion rates can be calculated starting from some thermochemical data, such as those shown for cellulose in Table 5.6 (Sullivan and Ball, 2012).

The reaction of charring starts with hydrolysis of cellulose and then continues with dehy­dration, decarbonylation, decarboxylation cross-linking and aromatization reactions to produce primary char.

The volatilization reactions of cellulose can be thought as the formation of levoglucosan, while char oxidation and volatile oxidation complete the combustion process. As has been previously discussed the more important reaction from the point of view of combustion velocity control is char oxidation influenced by oxygen penetration inside the particle. This is why for practical and engineering purposes the most adequate formula to express the burning rate is that proposed by Kuo (1998). Kuo’s method is based on the assumption that solid fuel bed-burning rate is proportional to the oxygen consumption rate as air flows through the fuel bed.

(5.23)

where:

mbio = biomass mass flow [kg/h]

qLHV = lower heating value of biomass [kJ/kg]

mfuel = auxiliary fuel mass flow [kg/h]

qfuel = heating value of auxiliary fuel [kJ/kg]

mpa = primary air mass flow [kg/h]

hpa = specific enthalpy of primary air [kJ/kg]

msa = secondary air mass flow [kJ/h]

hsa = specific enthalpy of secondary air [kJ/kg]

tnfwat = boiler feed water mass flow [kg/h]

hfwat = specific enthalpy of boiler feed water [kJ/kg]

mst = steam mass flow [kg/h]

hst = specific enthalpy of steam [kJ/kg]

tnfgas = exhaust gas mass flow [kg/h]

hfgas = specific enthalpy of exhaust gas [kJ/kg]

niash = ashes mass flow [kg/h]

hash = specific enthalpy of ashes [kJ/kg]

m curb, ioSS = unbumed carbon mass flow [kg/h] qcorb = heating value of unburned carbon [kJ/kg]

Qloss = heat loss outwards from the furnace/boiler system [kJ/h].

mbio = (pairMB/Mair) (Fpa + Fsa)/(4-76 (1. + є) rof)

where:

Fpa = primary air flow rate [Nm3/h]

Fsa = secondary air flow rate [Nm3/h] pair = air density [kg/m3]

MB = molecular weight of biomass [-]

Mair = molecular weight of air [-] є = excess air ratio [-]

rf = stoichiometric oxygen-to-fuel mole ratio [-].

5.2 COMBUSTORS

GASIFICATION APPLICATIONS

There is a large number of different gasification techniques, developed and under development, to select from when planning for a gasification based conversion processes. Each technique has there advantages and disadvantages depending on factors such as:

• The scale of the conversion process considered. For example, large-scale production of trans­portation fuels or chemicals, pressurized gasification systems are economically more feasible (Hamelinck, 2004).

• The biomass feedstock that will be used. Here we also have the question of the availability of the feedstock as well as the needed feedstock flexibility of the gasifier.

• The requirements from the application, such as power generation with gas turbine, FT-diesel or methanation, concerning the quality of the synthesis gas or product gas.

There are also other factors that may influence the choice of gasification technology.

R&D activities in the area gasification of biomass to commercialize the technology started already in the 1970s and since then several projects to demonstrate the technology have been executed. So far, due to economic and political incentives, there are no advanced large-scale plants, such as IGCC for heat and power as well as syngas production, in operation today. In the sections below a few selected examples of recent ongoing activities within the field is presented.

TECHNOLOGIES FOR CONVERTING BIOMASS TO USEFUL ENERGY — COMBUSTION, GASIFICATION, PYROLYSIS, TORREFACTION AND FERMENTATION

A good environment and at the same time good economic living conditions—that is the goal for us as well as for our children and their children. To achieve that we need sustainable energy resources that do not harm the environment through pollution of water, air and food. At the same time we need food and thus should not compete between food and use of resources for other purposes.

Biomass resources is one of the key resources we have that can both give us the energy we need, the food we demand and also be a feed stock for many kind of products we use daily like paper, packages, furniture, plastic, chemicals etc. Estimates made from statistics on use of land area for agriculture, forestry or just more extensive use indicates a biomass production of approximately 270,000TWh/year, which should be compared to the total global energy use of approximately 140-150,000 TWh/year. As we also have huge amounts of solar and wind power potential, and already have explored a lot of our hydro power resources, there should principally be no problem to build a sustainable society without fossil fuels, although the distribution of resources is not always matching the demands locally or even regionally.

The major concern thus would be to use the biomass resources we have in best possible way. Conversion methods thus are important to refine. We could just burn the wood over an open fire, and then have a net efficiency of less than 10% between higher heating value of the wood compared to the energy taken up by the water you want to boil. Or we could use the biomass as fuel in a combined heat and power plant with exhaust gas condensation, where the corresponding efficiency as heat plus electricity would be 117%, which is common in Scandinavia.

In China the majority of the energy used for electricity production comes from coal. Installed capacity for electricity production from biomass is forecasted to increase from 5500 MWe in 2010 to 13,000MWe in 2015. 8000MW should come from agricultural waste, 2000MW from biogas and 3000 MW from municipal solid waste. The total available resources of biomass still are much higher. They amount about 690 million tonnes of straw, 840 million tonnes manure from live-stock, 3 million tonnes food waste for biodiesel, and 950 million tonnes solid industry waste. If all this could be used for energy purpose it could replace about 1000 million tonnes coal, or some 7000TWh/y. The main question then is how to do this in a sustainable way. Many different technologies would be needed. Easy to decompose biomass could be fermented to give biogas, but at the same time also give good fertilizer back to the farm land. More difficult materials can be combusted or gasified thermally. Ash might be brought back to at least forestry. The same wood could first be used as building material, furniture or paper boxes, and then be used as an energy resource in a power plant when the function as building material is over. Food waste can be used where manure or house hold waste is fermented in a biogas plant, etc. There are many technologies available, and many of them are covered in this book on biomass conversion with examples from all over the world.

Concerning the biomass resources these differ between different climate zones and soil types. Still, there is a major potential to enhance the production everywhere by introducing good conditions like enough water, nutrients and new more resistant species of the crops with respect to insects, fungus etc. The potential can be seen by comparing the production as tonne product per hectare 1970 compared to today. In middle income economies and high income economies, the production has approximately doubled during this time period, while it has increased by some 50% in low income economies, according to statistics from the United Nations for 213 countries. Still, there is a major gap between both the middle income economies and the high income economies, and even larger compared to low

income economies. A review of resources and crops used in different climatic zones, as well as new possibilities to use crops efficiently in e. g. biorefineries are covered in this book.

Yang Yong-Ping,

Professor, Vice President of North China Electric Power University Director of National Engineering Laboratory for Biomass Power Generation Equipment

Member of National Energy Expert Advisory Committee

150 years ago the modern world was developing as a consequence of cheap and easily accessed energy from fossil fuels. Together with this resource engineers developed new technologies for converting the fuels into useful products like mechanical power, electricity and heat. Today we are facing a situation where cheap fossil fuel is becoming more scarce, and the oil price has gone from around e. g. US\$ 20/barrel in the mid-19th century to around US\$ 100-110/barrel in 2012. Coal is principally still relatively cheap, but environmental concerns with respect to global warming as well as other negative impacts from spreading dust and sulfur are alarming. In August 2012 we heared that the Arctic ice cap is smaller than it has been for several thousand years. It is as small as the previous smallest size in 2007 already in August, while the minimum takes place in September. Heavy storms are causing problems in the USA and East Asia. The previous stable weather patterns are becoming unstable and unpredictive, probably as a main consequence of the global warming caused by emission of primarily CO2 from fossil fuel combustion. To avoid this effect we need to use renewable energy instead, and this as fast as possible. Here we have hydro power, wind power and solar power, but first of all Bioenergy, which can be both stored over the seasons as well as converted into all energy forms we need for heat and power, transportation and as a base chemical for manufacturing of anything from plastics and soap to buildings. As biomass is also food needed for a growing population, we need to look at biomass from a holistic perspective, where e. g. the cereal grain should be used primarily for food while the straw and other agricultural waste should be used for the other applications. To do this a number of different conversion techniques are needed.

The purpose of this book is to give a concise overview of all major conversion techniques for biomass. We start with thermal conversion and follow with torrefaction, biogas production using biological methods and finally mechanical processes like briquetting and pelletizing.

Combustion, biogas production using microbiological methods and polarization are already used extensively, while gasification, pyrolysis and torrefaction are still under development. Some countries are utilizing biomass a lot while others very little. In Sweden 1/3 of all primary energy used is as biomass, or 132TWh/y out of a total 400TWh/y 2010 (when we exclude the waste heat from nuclear power plants). That is one of the highest percentages in developed countries, while many still developing countries may have similar or even higher figures, at least if we include also biomass collected and used locally.

From a future perspective biomass could replace most of our energy needs if it was utilized in a most efficient way. Still, the use must be in a sustainable way. Here for instance it is important to see that organic material and nutrients like phosphorus and nitrogen are recirculated to farmland, and thus biogas production is good from a system perspective. The organic residues then will be a fertilizer to keep the production high in the long term, but we also have to see that we do not bring negative substances from anaerobic digestion into the food, and precautions have to be made. On the other hand some materials like wood are not very suitable for biogas production and here gasification and combustion are more suitable. We can also produce ethanol from pre-treated cellulosic material like straw, and then it is useful to combine it with biogas production of the residual brine. Also pyrolysis to replace oil and torrefaction to replace coal are new alternatives. All these aspects are highlighted in this book.

If we just look at EU27 I have tried to estimate the total annual biomass production from the data on crops grown and areas used for agriculture and forestry. The rough figures come to around 8500 TWh/y biomass produced. A very small portion of this is really utilized for our different needs. If we could use e. g. straw efficiently for biogas production or for production of ethanol using fermentation

we could produce most of the fuels needed for our vehicles. By introducing a series of hybrid electric vehicles the total energy for transportation could be decreased by roughly 70-80%, where half would be as electricity, and the rest as methane, ethanol or bio-diesel. The electricity then could be produced in CHP plants using biomass as the fuel, aside from wind power, hydro power and solar power.

To make this economically attractive still we need to have good conversion techniques, and these will be the focus of this book. If we can combine these conversion techniques with robust agriculture and forestry, and reuse materials in a most efficient way, we can see a bright future without fossil fuels. The advantage also would be a solution to the upcoming climate problems with global warming. This is the motivation for this book.

The book is using SI units as the standard. Still, SI units can have different forms as well. It is common to use MJ (million Joules) for energy in SI units, but as kWh, (kilo watt hours), MWh (mega watt hours), TWh (terra watt hours) and toe (tonne oil equivalents) are used by e. g. UN and the World Bank for energy these units have been used as well. One toe is approximately 10 MWh. For electric power usually MW has been used. China is using t. c.e (tonne coal equivalent) for energy as well, and in a few places this unit has been used relating to Chinese energy data. For surface area ha (10,000 m2) and km2 (100 ha) have been used concerning calculations related to production of different crops, yield, etc. Both m3 (1000 liters) and liters have been used for volume. Concerning pressure this is MPa in SI, but as bar is very common also this is used. Parts per million, or ppm is also used commonly, and thus also is used here and there in the book, although kg/kg is the SI sort. Where it is used in the book is just because it is difficult to change in some already produced diagrams and similar. Both kg and tonnes have been used as well, where we refer to metric tonne (tonne = 1000 kg). Sometimes also other units like Pg for weight (1015 g) and TJ (1012 J) for energy are used, and probably will be used even more in the future.

The authors are all well established in different fields of biomass conversion and also cover most parts of the world. This book is written in parallel with volume 3 in this book series on sustainable energy developments, where biomass resources are presented.

Erik Dahlquist January 2013

O2 and equivalence ratio

The air fuel ratio, and hence the equivalence ratio, can be estimated from measured flow rates of air and fuel. It can also be computed using the measured O2 percentage in the exhaust for lean mixtures. Using O2 percentage data the equivalence ratio of the exhaust stream was approximated by:

Vfiue ~ 1 — 4.76 * O2, <Pfiue < 1-0 (Annamalai and Puri, 2007)

The equation above assumes that all the fuel has been gasified. If large particles are not gasified, the O2 percentage will increase. This will cause the у based on exhaust gases to decrease. Figure 3.21 plots the yfue computed from flue gas analysis versus the ifw computed from air and fuel flow rates. It is seen that yfue is less than ynow. This indicates that the burnt fraction (BF) BF is less than 1.0. Also, note that the ifw requires knowledge of the fuel flow rate. Due to limitations of the feeder, only average flow rates of solids could be measured. Figure 3.22 presents the exhaust equivalence ratio for WYO and WYO:DB blended fuels. Ideally, the data points would follow the ideal line (yflue = ynow). The real data points lie within the experimental uncertainty of each other. This indicates that the values are valid. In all future plots, the у represents the equivalence ratio based on measured air flow rates and the calibrated fuel flow rate.

The superiority of biomass gasification gas as fuel

Biomass gasification gas is a good fuel. The benefits are summarized below: [4]

• Low heating cost:

(i) It has strong adaptability in gas boiler heating load. Within the system, its adjustment is flexible and its gas measurement is simple and accurate, making it easy to adjust the gas supply.

(ii) With less accessory equipment, starting quickly and no fuel preparation system, it can reduce all kinds of consumption of preparation work, and electricity use is less than a coal boiler.

(iii) There are few particulate impurities in the gas, and the boiler will not be exposed to heating corrosion of high or low temperature. There exists no slagging problem. The boiler’s continuous running cycle is long.

• Low equipment maintenance cost:

(i) The equipment of a gas boiler combustion system is simple, so there are less maintenance projects and lower maintenance cost.

(ii) Because there is no slagging and high temperature corrosion due to low operating temper­ature, heating pipes and air preheating element do not need to be replaced so frequently. With the rapid development of the gas industry, gas boilers and the extensive applica­tion of technology, accident-hidden danger are gradually reduced, and various protective measures are increasingly being perfected, which ensures the reliable operation of the gas boiler. Gas fired boilers in China are a new and booming industry, and has shown very broad prospects for development.

THEORY OF GASIFICATION

A fuel gas can be produced from biomass or other feed stocks by partial oxidation at high temper­ature using oxidizing agents such as air, oxygen, steam, carbon dioxide or combination of these. In case of gasification, the temperatures used are typically between 600 and 1000°C. The differ­ent steps occurring in gasification of biomass or other feedstocks are graphically represented in Figure 6.2.

The first step in the thermo-chemical conversion of a feedstock, such as biomass, is the drying of the biomass followed by the pyrolysis, of the cellulose, hemicellulose and lignin to produce char and volatiles, such as permanent gases, light hydrocarbons and tars. Introduction of reducing agents and sometimes also a catalyst further decompose the char and the tars to permanent gases. Constituents such as tars may not be completely converted and also ash from the char will be present in the formed product gas. In view of this description gasification is only one-step of many although they together generally is referred to as only gasification.

The processes graphically represented in Figure 6.3 can generally be specified in the main chemical reactions as described in reactions (6.1)-(6.6) (Engvall, 2011):

Feedstock ^ char + tars + CO2 + H2O + CH4 + CO

+ H2 + (C2 — C5) + impurities (pyrolysis) (6.1)

C + %O2 ^ CO AH°r = -109kJ/mol (partial oxidation) (6.2)

Reaction (6.1) describes the pyrolysis, an endothermic process, which is a very important step for biomasses due to the large fraction of volatiles (70-80% dry basis) in these feedstocks. The reactions (6.2)-(6.6) are the common reactions included in gasification of biomass. Heat for the endothermic reactions can be supplied either by direct partial oxidation directly, as e. g. described in reaction (6.2), or from an external source transferring heat to the gasifier. Other reactions that influence the product gas yield and composition are the cracking of the tars due to thermal conversion at high temperatures (reaction 6.7), or catalytic tar reforming with steam (reaction 6.8) or dry (reaction 6.9) in the presence of, for instance, a catalytically active bed material used in fluidized bed gasification:

The thermal conversion in reaction (6.7) is a simplification. The decomposition is generally much more complex and many different paths as proposed by Devi et al. (2005).

The decomposition of the feedstock is a complex process and depends on parameters, such as biomass feedstock composition, gasifying agent and the gasification process (Dayton, 2002). The gasification process produces a raw gas, generally called producer gas that consists of the permanent gases CO, CO2, H2O, H2, CH4, other gaseous hydrocarbons (C2-C5), char, tars, inorganic constituents and ash. The impurities in form of ash and char particulates, tar, and inorganic impurities, such as H2S, CS2, COS, AsH3, PH3, HCl, NH3, HCN, and alkali salts, have to be removed before utilizing the gas, depending on the application of interest. Of significant importance is the tar produced, present in different amounts, depending on the gasifier technique, feedstock used and process conditions. Tar is often a confusing term because different definitions of tar are used depending on the gasifier type and the gasification operating conditions. Tar usually consists of condensable highly aromatic hydrocarbon organic compounds, ranging from molecular weight above 78 (benzene) and can generally be divided into so-called water-soluble (phenolic) and non-water-soluble (aromatic) compounds (Moersch, 2000).

The formation of tar is one of the major obstacles in the commercialization of biomass gasifi­cation technologies (Yung, 2009), causing problems in downstream process equipment, such as blocking of pipes and filters, as well as coking on catalysts in gas upgrading processes (Dayton, 2002), even at very low concentrations in the gas. Beside the tars, other impurities important to remove before utilizing the gas are generally particulates, alkali salts and sulfur-containing compounds.

In the case where black liquor is used as a fuel in the gasification, also reactions (6.10) and (6.11) take place due to the presence of high contents of alkali salts in the feedstock (Grace, 1994):

C + 1/2Na2SO4 ^ CO2 + 1/2Na2S (6.10)

C + 1/4Na2SO4 ^ CO + 1/4Na2S (6.11)

As the black liquor droplet enters the recovery unit it is exposed to hot gases and will undergo drying, pyrolysis and char conversion. The amount of salts is very high in the black liquor. This
causes some problems with respect to corrosion, but it also is acting as a very strong catalyst, and thus black liquor can be gasified at 100-200°C lower temperature than the same amount of “normal” biomass.

BIOGAS COMBUSTION AND EMISSIONS

Biogas or landfill gas (LFG) is typically produced from anaerobic decomposition of organic matter in an oxygen-free environment (Saho et al., 2011). It can also be produced through pyrol­ysis and gasification processes. Primary sources include biomass, green waste, plant material, manure, sewage, municipal waste and energy crops. While its composition can vary significantly depending on the source and production process, the main constituents include CH4 (50-75% by volume), CO2 (25-40%), N2 (0-10%), and small traces of H2O, O2, H2, and hydrogen sulfide. It may also contain small amounts of contaminants such as volatile organic compounds, sulfur compounds, siloxanes, halogenated hydrocarbons, ammonia, etc. To account for this variation in composition, previous studies have examined the biogas combustion and emission behavior for some specific compositions. Table 2.4 lists two such representative biogas mixtures based on the two common biomass sources, namely agricultural waste and household waste (Bika et al., 2011). Like natural gas and syngas, biogas can be used as a transportation fuel in IC engines, and for power generation in gas turbines and boilers. It can also be used as compressed natural gas, and in solid oxide fuel cells to generate electricity. Moreover, it can be reformed to produce syngas and then used in the above applications.

There is a large body of literature on methane combustion, including ignition, extinction, flammability limits, flame speeds, cellular instabilities, and emissions. Consequently, detailed thermo-transport and kinetic models have been developed to simulate and analyze methane flames in a variety of configurations. Considerable research has also been reported on fire suppression, which has examined the extinction and blowout of methane-air flames using various diluents, such as CO2, N2, H2O, and chemicals (Gunaseelan, 1997; Quesito, 2011). Most of these studies and the associated models can be readily used for analyzing the combustion and emission characteristics of biogas, whose main constituents are CH4 and CO2 with small traces of H2O, and N2. This section provides a brief overview of the fundamental combustion properties of biogas, and its application in IC engines. For more detailed discussion, the reader is referred to the extensive literature available on methane combustion and emissions.

Biogas has lower energy content compared to natural gas. For example, the volumetric heating values of natural gas (94% CH4) and biogas (60%CH4/40%CO2) are 38.6 and 25 MJ/m3, respec­tively. This has consequences for using biogas in natural gas-fired combustion devices, since the lower heating value implies higher feeding rates and lower flame temperatures. Figure 2.17 compares the predicted adiabatic flame temperatures for methane-air and two biogas-air mixtures, shown in Table 2.4.

 Equivalence ratio Figure 2.17. Comparison of adiabatic flame temperatures of methane-air and two biogas-air mixtures. The biogas compositions are given in Table 2.4. Pressure is 1 atm, and initial temperature 500 K.

As indicated, the biogas flame temperature is about 100-200 K lower than that of methane. Lower temperatures imply lower flame speeds and thermal NO for biogas flames compared to those for methane flames. The comparison of laminar burning rates for freely propagating methane and biogas flames is shown in Figure 2.18, which plots the flame speed as a function of equivalence ratio and pressure. The flames were computed using the PREMIX algorithm in CHEMKIN software along with the GRI-3.0 kinetic mechanism. As expected, results indicate lower flame speeds for biogas-air mixtures compared to those for methane-air mixtures. The effect of pressure on flame speed is qualitatively similar for all three cases shown, with the flame speed first decreasing sharply and then relatively slowly as the pressure is increased.

Since biogas is potentially a cleaner and more sustainable alternative to natural gas, it is relevant to analyze methane and biogas flames over different combustion regimes. Figure 2.19 from Aggarwal (2009) depicts the computed structures of methane-air and biogas-air partially premixed flames in terms of temperature, velocity, and species mole fraction profiles.

The counter flow flames were established at Ф = 1.4, pressure = 1 atm, and strain rate = 200 s-1, using the OPPDIF algorithm and GRI-3.0 mechanism, as stated earlier. For all three cases, the flames exhibit a double flame structure with a rich premixed reaction zone (RPZ) located on the fuel side and a non-premixed reaction zone (NPZ) on the oxidizer side near the stagnation plane, which is located by the zero value of the axial velocity. The fuel is completely consumed in the RPZ, producing CO, H2, and intermediate hydrocarbons, which are transported to and consumed in the NPZ. The two reaction zones can also be located by the two local peaks in axial velocity profiles (Fig. 2.19), and by the peaks of CO and CO2 mole fractions, respectively. For example, the RPZs for the three flames are located at 0.85, 0.9, and 0.91 cm, respectively, from the fuel nozzle, based on the peak CO locations, while the NPZ are located at 0.975, 0.963, and 0.95, respectively, based on the peak CO2 locations. The NO profiles indicate a significantly lower level of NO formation in biogas PPFs compared to that in methane PPFs. This may be attributed to the less thermal NO and prompt NO formed, indicated by lower temperatures and C2H2 peaks, in biogas flames compared to those in methane flames.

Like syngas, there are relatively few studies on the performance and emission behavior of biogas-fed combustion devices. Henham and Makkar (1998) and Yoon and Lee (2011) reported experimental investigation on the combustion and emission characteristics of dual-fuel CI engines using diesel and biogas. These studies were able to point out the viability of dual-fuel engines for using fuels with low energy content like biogas. Bedoya et al. (2011) performed an experimental

 Equivalence ratio

 Pressure (atm) Figure 2.18. Comparison of laminar flame speeds of methane-air and two biogas-air mixtures. Flame speed is plotted versus equivalence ratio (top) and pressure.

study of biogas combustion in an HCCI engine for high efficiency and ultra-lowNOx emissions. Kohn etal. (2011) performed experiments on a SI engine operating onLFG and syngas. The syngas addition was found to improve the engine efficiency and reduce emissions of CO, UHC, and NOX.

Temperature profiles for air gasification

Temperature profiles are measured every 60 seconds along the gasifier axis. A typical gas analysis is presented in Figure 3.44a for an experiment at ER = 3.18 and S:F = 0.8. It is apparent that the temperature profile achieves almost state steady in the last ten minutes; therefore, it is appropriate to assume steady state conditions during the last 10 minutes of each gas analysis. The temperature profiles discussed in this paper correspond to the average measured during the last ten minutes. As discussed before, in a fixed bed gasifier, the oxidation of char (heterogeneous oxidations) occurs near to the bottom of the bed where mostly char reacts with the oxygen and steam to produce CO, CO2, H2, and the heat required for driving the gasification process is released. Because under gasification conditions char oxidation of large particles is almost diffusion controlled, the char oxidation rate is dependent upon the availability of O2in the gas stream. The temperature in the combustion zone (Tpeak) depends upon the concentrations of O2, H2O, and CO2. Above the combustion zone, the temperature decreases since oxygen concentration is negligible and most of the reactions occurring there are endothermic. Below the combustion zone, the temperature is lower because it corresponds to ash temperature. It is apparent from Figure 3.44a that the peak temperature occurred at ~5 cm above of the grate indicating no ash accumulation.

Increase of ER, at fixed S:F ratio implies a decrease in the oxygen supplied; thus, heat generation due to char oxidation decreases resulting in lower Tpeak and hence results in a lower temperature

profile (Fig. 3.44b). Due to the presence of oxygen at the bottom of the bed, the peak temperature occurs near the bottom. The temperature of the particle under the assumption of negligible char — steam reaction and diffusion-controlled combustion can be derived as (Annamalai and Puri,

2007) :

cp(Tp — Toq )

-^-p——- — = B (3.42)

he

where hc = hcI for CO, hc = hcII for CO2 produced, Tp = particle temperature, B = {YO—/vO2}, Vq2 = 1.33 for CO, 2.33 for CO2 produced, YO2— = Oxygen mass fraction, and cp specific heat of the gases. In particular, for ER = 1.59 and S:F = 0.68 the peak temperature measured is about 950°C (Fig. 3.44b); however, this value is lower compared to (1191 °C) obtained with the equa­tion 3.42 (cp of air = 1.15kJ/kgK, cp of the steam = 2.3kJ/kgK, cp of mixture = 1.28kJ/kgK, YO2— = 0.203, and hcI = 9204kJ/kg). The lower experimental temperature compared to that of the model indicates that (i) the char may react with both O2 and steam at the bottom of the bed to produce CO and H2 and (ii) combustion may not be diffusion controlled. On the other hand, if the steam carbon reaction was included in the model and if diffusion limited hetero­geneous reactions was assumed, the estimated Tp would be lower than the estimated using equation 3.42.

Figure 3.44c shows the effect of change in ERs and S:F ratio on the peak temperature (combus­tion temperature zone). Also are presented two Tpeak (1098 and 998°C) obtained for gasification with only air at ER = 2.12 and ER = 3.18. At lower ERs, the effect of the S:F ratio is higher. For instance, at ER = 1.59 the peak temperature difference between the curves of S:F = 0.35 and

0. 80 is 185°C while at ER = 6.36 the difference between the same curves is 91°C only since oxygen availability is limited. The curves from Figure 3.44c suggest that at constant S:F, the peak temperature is affected almost linearly by changes on the ER. Increased S:F causes the Tpeak to decrease. This can occur due to (i) decreased amount of air, (ii) change in the cp of the mix­ture, (iii) regimes of combustion: kinetics vs. diffusion controlled, and (iv) steam-char reaction. At ER = 2.12, the peak temperature for gasification with air only is 147°C (15.45%) higher as compared to that of gasification with air-steam at ER = 2.12 and S:F = 0.35 while at ER = 3.18, the difference in peak temperature between gasification with air and gasification with air-steam is ~132°C (15.24%). In general, for the range of operating conditions (ER and S:F) investigated the Tpeak ranged between 519 (ER = 6.36, almost pure pyrolysis) and 1015°C (ER = 1.59).

3.13.1.2.1 Temperature profiles for enriched air gasification and CO2:O2 gasification For the gasification experiments with higher oxygen percentages, at ER = 2.1 and S:F = 0, the temperature profiles obtained are plotted in Figure 3.45. The peak temperatures obtained can be compared to that of the theoretical values obtained using the B number.

From Figure 3.45, the peak temperature obtained when using enriched air mixtures is observed to increase with increased oxygen concentration. The numbers obtained experimentally were almost same as the values calculated theoretically using B number calculations.

Enriched air results in the presence of nitrogen in syngas, which lowers the heat value of gases. Also, carbon dioxide (CO2) can be separated easily from products compared to nitrogen (N2) in the event CO2 sequestration is necessary to enhance the heat values. Hence, experiments were performed using carbon dioxide-oxygen mixture as the gasification medium instead2 of air. In this case, carbon dioxide is substituted for nitrogen in the air mixture. Also the carbon dioxide produced as a result of gasification can be separated and circulated again into the reactor at high temperatures (e. g. as cooling medium for the gasifier) in order to increase the efficiency of the reactor and also to sustain the reaction within the gasifier. This will also increase the upper limit on ER. This in turn helps to reduce the amount of carbon dioxide released into the environment. CO2 has a slightly higher specific heat (cp) than N2 at higher temperatures. The cp of the mixture of CO2 and O2 is higher than cp of the mixture of N2 and O2. Hence, Tpeak, using CO2 instead of N2, is expected to be low. The difference in peak temperatures can be observed in the temperature profiles (Fig. 3.46) obtained using carbon dioxide in the gasifying medium instead of nitrogen (Thanapal et al., 2012).

 Figure 3.45. Steady state temperature profile, ER = 2.1, S:F = 0 (adopted from Thanapal, 2010).

 Figure 3.46. Temperature profile, 21% oxygen, ER = 4.2, S:F = 0 (adopted from Thanapal, 2010).