The results on gas analysis obtained from MS for a typical experiment (ER = 4.24 and S:F = 0.35) are shown in Figure 3.47a as a function of the time. The data on gas composition have a cyclic dynamic behavior in the vicinity of an average value. However, at first glimpse, it appears that the average is almost constant during the experimental period. Figure 3.47a shows the mole fraction of N2, H2, CO2, CO, CH4, and C2H6 (on a dry basis) along with the average mole fraction and the standard deviation (STDEV) of the data. The data on H2 present the major standard deviation

(3.2) about of the average value of 18.62% whereas the data on CH4, CO2, and C2H6 show a lower standard deviation and the data on CO shows a standard deviation of 1.53. As discussed

earlier, in general for the set of experiments discussed in this paper, the composition value of the gases analyzed fluctuated within ±15% of the average value.

As discussed before, at constant S:F, increasing the ER decreases the O2 supplied with the air at the bottom that implies decreasing Tpeak in the combustion zone. Then, as the temperature is lowered, the reaction C + O2 ^ CO2 is favored. CO2 increases at lower temperatures. More production of CO2 implies consumption of more O2 via CO2, thus, less O2 is consumed via CO and hence less CO is produced (Fig. 3.47b). Also, at constant S:F, increased ER increases the steam-air ratio (S:A), which implies decreased air supplied and hence the combustion of char takes place in a H2O-rich mixture which favors the heterogeneous reaction of char with H2O to produce H2. The rate of H2 and CO produced by the heterogeneous reaction C + H2O ^ CO + H2 becomes important when the reaction occurs at low O2. On the other hand, the concentrations of CH4 and C2H6 were lower (0.43 < CH4 < 1.75 and 0.2 < C2H6 < 0.7) as compared with those of other gases and were almost not affected by the ER.

The effect of the ER and S:F ratio on the concentrations of H2, CO, and CO2 are presented in Figure 3.48a, Figure 3.48b, and Figure 3.48c. At constant ER, higher S:F ratios signify more steam available to react with char to produce CO and H2 (steam char reaction) in the high temperature reducing zone immediately above the combustion zone (i. e. O2 deficient) near the bottom of the bed. The CO produced by the steam reforming reaction reacts with the surplus steam (shift reaction) in the upper zone (reduction) to produce more H2 and CO2; hence, more C atoms contained in the DB result in CO2. It is evident from the graphs of Figure 3.48b that lower ERs

have a lower effect on the CO production compared to higher ERs. Also, the results show that at constant ER, changing the S:F ratio affects the production of H2 more than the production of CO. For instance, at ER = 1.59 changing the S:F from 0.35 to 0.8 increases the production of H2 by 57.5% but decreases the production of CO by only 26.2% (Figs. 3.48a, b). Since the decrease in CO% is less than the increase in H2% then there must be a heterogeneous steam char reaction resulting in production of H2. This is also evident from the lowered Tpeak. Under the operating conditions discussed (1.59 <ER < 6.36 and 0.35 < S:F < 0.80), the CO ranged from ~4.77 to ~ 11.73%, H2 from 13.48 to 25.45%, CO2 from 11 to 25.2%, CH4 from 0.43 to 1.73%, and C2H6 from 0.2 to 0.69%.

3.14.4.2.4 Gas composition results with enriched air and CO2:O2 mixture Figure 3.49 shows the gas composition obtained for enriched air gasification with ER = 2.1. The percentage of carbon dioxide produced increased with increased oxygen percentage due to higher oxygen concentration in the incoming gasification medium. It was accompanied by a decrease in carbon monoxide and an increased production of hydrogen.

Figure 3.50 shows the comparison between the gas composition at 21% O2 obtained for the gasification with air and carbon dioxide at ER = 4.2.

Since carbon dioxide replaced nitrogen in the air the gases produced during gasification had a higher percentage of carbon dioxide, which possibly includes carbon dioxide produced during gasification as well as the carbon dioxide coming in as the gasifying medium. In addition, the heating value of the gases produced using carbon dioxide as the gasifying medium was higher when compared to that of air gasification having nitrogen.

This represents the heat released during the complete combustion of a sample, determined by burning it in a controlled environment; it is expressed as the energy content per mass unit (kJ/kg, MJ/kg) and can be referred to the wet basis, the dry basis or the dry and ash-free, basis. It can be divided into: higher heating value (HHV) and lower heating value (LHV), depending on whether
water obtained as a product of combustion is considered in liquid phase or vapor. The difference between HHV and LHV is therefore the latent heat of condensation of the steam present in the combustion products. It has to be noted however that the water content to be considered in exhaust gases, when calculating LHV from HHV is only the one derived by the oxidation of hydrogen present in the sample; steam coming from moisture in the sample is not considered in the LHV therefore HHV should be always corrected on a dry sample.

The relations that describe these quantities are:

Higher Heating Value (HHV):

( ACwb MCwb

HHVwb = HHVdaJ 1 — [kJ/kgwb]

( — Cdb

HHVdb = HHVdaJ 1 — [kJ/kgwb]

Lower Heating Value (LHV):

( — Cwb MCwb

LHVdb = LHVdaJ 1 — [kJ/kgwb] (5.7)

/ — Cdb

LHVdb = LHVdaJ 1 — [kJ/kgwb] (5.8)

The HHV is measured in laboratory using a calorimeter. The LHV is obtained, knowing the hydrogen content of the sample (determined through elemental analysis with a CHN analyzer) through the following relation:

LHVdb = HHVdb — 206.0Hdb [kJ/kgwb] (5.9)

in which Hdb is the content of hydrogen expressed in mass and referred to the dry basis. The heating value is a fundamental parameter that gives indications on the energetic potential of the biofuel. In fact the higher the heating value the higher is the energy yield per unit of mass obtained by the conversion process.

At operative level it is better to consider the heating value on dry basis, because it represents the real energy yield of the fuel. Moisture content lowers the energy content of biomass because the evaporation consumes energy that could be used for the thermo-chemical conversion processes.

The heating value can be calculated also through formulas derived from correlations, for exam­ple the Channiwala Parikh formula that correlates HHV with the elemental analysis (Demirbas,

2004) :

HHV = 349.1 C + 1178.3 H + 100.5S — 103.4O — 15.1N — 21.1 Ash [kJ/kg] (5.10)

Other attempts have been made to derive a heating value from proximate analysis (Parikh etal.,

2005) .

5.1.3.2 Carbon, hydrogen and nitrogen content (EN15104, 2011)

The concentration of the three elements in the biomass sample is measured through the ultimate analysis of the sample, that is the combustion of the same in controlled atmosphere and the successive analysis of flue gases. The three concentrations obtained are expressed in % dafb. The carbon/nitrogen ratio can be used as an indicator to identify the most suitable conversion technique for the biomass. When the C/N ratio is higher than 30, the thermochemical conversion process could be adopted, while when C/N is lower than 30, the most suitable conversion processes are biochemical processes.

The entrained flow reactor (EFR) is well-known in coal combustion processes whilst the expe­rience with biomass is limited. In entrained flow gasifiers, no inert material is present but a finely ground feedstock is required. The fuel particles are fed co-currently with the oxidant agent and subsequently are the particles entrained with the gas stream. The EFR gasifiers operate at temperatures of 1200-1500°C, depending on whether air or oxygen is employed. Examples of entrained bed gasifiers are seen in Figure 6.10.

The temperatures in the EFR gasifiers are much higher than those encountered in fluidized bed gasifiers; hence, the product gas does contain low concentrations of tars and condensable gases. The high temperatures allow for the thermal conversion of tar and also some of the methane, thus the composition of the product gas will be close to those indicated by the chemical equilibrium. This even though the residence times are very short only around 1 second.

The conversion in the entrained beds configurations effectively approaches 100% and exhibit, in theory, the highest capacity of all gasifiers. However, the high-temperature operation creates problems of construction materials selection and ash melting.

Figure 6.10.

After the fuel pre-treatment the feedstock enters the gasifier as a pulverized solid by a pneumatic feeding. The mixing between the fuel and the oxygen should be as good as possible in order to optimize the gasification. Depending on the type of fuel, soot may also be formed. To avoid the soot formation, addition of steam in a proportion of 0.1 kg steam per kg oxygen is often necessary. When coal is used as a fuel it is crushed into a fine powder (~50 ^m). This touches upon the main disadvantage with respect to biomass application, as fine milling of biomass is very costly. This drawback can be partially handled by pre-treating the biomass through e. g. torrefaction.

However, this is at present a relatively unproven technology. In addition, in order to reach these high temperatures more product gas has to be combusted, which will hamper overall efficiency and thus costs. Another possibility is to pyrolyze the biomass to pyrolysis oil to be able to feed the oil into the reactor. This is possible if the pyrolysis oil is directly introduced in the gasifier and not stored since the oxygen-rich pyrolysis most likely will polymerize when stored. The polymerized pyrolysis oil forms two phases where the most energy-rich phase is very viscous and not possible to pump.

Entrained flow gasifiers may be found as slagging and non-slagging. In the slagging entrained flow gasifier configuration, melted slag products, originating from the fuel, are condensed, usually on the reactor wall, as it is the coldest part of the gasifier. The melt will accumulate on the wall, forming a slag layer that protects the wall from the high temperature corrosive atmosphere of the gas. The liquid slag is removed from the bottom of the gasifier. In order to get a liquid slag with the right viscosity at the given temperature, so-called fluxing material must usually be added. In coal-based power plants, limestone or other Ca-rich materials are often added into the bed.

The non-slagging entrained flow gasifiers do not produce any slag due to small amounts of minerals/ashes in the fuel. A small amount of soot is often deliberately generated in order to get condensation surfaces in the gas bulk by nucleation to preventing fouling of the walls by the slag.

Entrained flow gasifiers may also be used for gasifying black liquor. An example is the operation at SCA Ortviken in Sweden with recovery of liquors from a Ca-sulfite plant during 1970 and 1986. The reactor had injection of black liquor together with oil in the upper center and air was introduced around it. The residence time was very low, just a few seconds, and the temperature around 725°C. The gas was cooled in a waste heat boiler and solids separated in a Venturi scrubber. Since black liquor consists of large amount of alkalis sever materials problem is to be expected at high temperature. AT SCA Ortviken, the material problems were handled in the reactor and proved that at least 725°C was practical for long-term operations using Hoganas bricks as insulation inside the reactor.

While the ultimate and proximate analyses provide information on the basic elements of fuel and quality of fuels (heat value, volatile matter, emission potential (based on N and S contents)), the temperature of the start and end pyrolysis, and rate of release of VM are provided by Thermo gravimetric Analyses (TGA). In addition, the modeling of pyrolysis, gasification, various energy conversion processes (see Chapter 2 of this book by Agarwal), co-firing, and reburn behavior and also understand the ignition behavior and NOX reduction characteristics of biomass and coal fuel, fundamental pyrolysis and ignition experiments must be conducted to generate data on fuel properties. Experiments were conducted in a Thermo Gravimetric Analyzer/Differential Thermal Analyzer (TGA/DTA) in both N2 and air environment.

Comparing with coal, burning biomass results in lower levels of SOX. Some straw has no sul­fur content, which could decrease the SOX emission with co-combustion coal and biomass.

NOx emissions may increase, decrease, or remain the same, depending on the fuel, combustion condition and operating conditions (Baxter, 2005).

1.2.1.1.1 The influence of solid biomass fuel

During co-combustion of coal and solid biomass fuel, biomass went through four stages, namely dehydration, biomass pyrolytic and volatile burning, the combustion and solid carbon of volatile surface burning coexisting, and solid carbon burning at the surface. Many products of these stages have a reducibility onNOx.

Based on a small drop-tube furnace, it is shown that 50-70% of NO emissions could be reduced using wood chips, orange peel, and rice husk as the co-combustion fuels (Li et al.,

2004) . An experimental study on the biomass co-combustion in a multi-function test-bed was carried out by Cheng et al. (2007). The influence of the fuel properties, the operation parameters (fuel particle size, and fuel ratio) and operation parameters (co-combustion zone temperature, excess air coefficient, residence time, initial NO concentration) on biomass co-combustion were researched with wheat straw and corn stalk, peanut shell and sawdust as biomass material. The effect of biomass co-combustion on the reduction of NO emissions was similar as pulverized coal burning.

Han et al. (2008) studied biomass co-combustion for reduction of NOx and indicated that the performance order of reducing NOx is: wheat straw, peanut shells, and sawdust. The appropriate condition were co-combustion temperature of 950 to 1050°C, co-combustion rate of 15 to 25%, excess air rate of 0.6 to 0.8, and residence time of 1s.

Yang et al. (2009) researched the influence of alkali metal K on nitrogen conversion in co-combustion of coal and straw. They showed that a certain proportion of straw in the de — ashed samples is effective to control NO emissions. The formation rate of NO was changed from one peak to two peaks with the proportion of straw increased in TG. Increasing K from 0 to 3% in the samples mixed with a proportion of straw at 50% and a proportion of KOH at 25%, has the strongest catalytic effect on the reduction reaction of NO. When the K reaches a certain value, the catalytic effect was stable. The lower the O2 content was in the co-combustion condition, the better the reduction reaction of NO.

The simplest technology for biomass combustion is the pile burner which is the engineered version of the primitive bone fire solution. Biomass is piled inside a refractory chamber through a screw conveyor, in an underfeed system, or dropped on top of the pile in overfeed systems (Fig. 5.12) and it is ignited manually or with an oil or gas start-up burner. Primary air is blown directly inside the bed through holes in the refractory lining while secondary air is provided above the bed through chute openings. Combustion is mainly surface driven, therefore adequate radiation from the combustion chamber walls is necessary and vaulted ceilings are usually utilized. The conical pile settles according to the friction angle of the material and in underfeed burners new fuel pushes inside the pile causing a bottom-up vertical gradient of the mass loss and the ashes on top to fall at the sides of the pile where the ash pits are located. In overfeed burners no forced movement of the pile is present and a gravity driven top-bottom gradient of mass loss is present causing the ashes to fall into the ash pit which is positioned below the bed.

Ash is usually removed from the ash pit by manually extracting the container as a drawer, or else by cleaning the grate directly when the ash is cooled. In either case the cyclic operation of the burner results in high maintenance requirements which contribute to the low efficiencies (<70%) and difficulties in controlling the process. Automated ash extraction through augers may be present in bigger scale applications to increase availability. Pile burners are not suitable for load following operation while they are simple, inexpensive and suitable to burn wet and dirty fuels.

A typical overfeed pile burner for small to medium scale application is the so called Dutch oven which is the oldest technology still in use, especially in the forestry products and sugar industry.

With reference to Figure 5.13 the Dutch-oven technology is a two-chamber furnace made of refractory walls with holes (tubes) for combustion air inlet and fuel is dropped on a grate from a

hole on the ceiling. Drying and gasification takes place in the first chamber where radiation from the walls and ceiling is essential to speed up the process. The construction characteristics vary considerably according to the different biomass size and humidity, although the Dutch-oven can handle easily fuel with up to 50% humidity thanks to the heat reservoir of the pile which however does not allow abrupt load changes.

When a single chamber pile combustor is considered, the underfeed solution in a cylindrical combustion chamber is the so-called cyclonic furnace (Fig. 5.14) which is still used in northern Europe. Combustion gases are extracted from the top of the brick lined furnace and are conveyed to the heat exchanger while biomass is pushed by an auger through a hole at the center of a circular grate on top of which the pile is formed. For this reason the cyclonic furnace may also be considered as a grate combustor. During combustion the solid fuels proceeds radially from the center to the periphery where ashes are discarded. Primary air is fed preheated through the grate while secondary air is provided tangentially on the top of the furnace as to provide a cyclonic vortex that optimizes the combustion of volatiles. The cyclonic furnace shows a relatively high efficiency, also on wet fuels, and low dust emission while its disadvantages are mainly related to its high maintenance costs and poor automation possibilities.

Erik Dahlquist

The major thermal biomass conversion techniques are combustion, gasification, pyrolysis, and tor — refaction. Combustion means 100% oxidation of all organic contents of the fuel using air/oxygen, while gasification means partial combustion where some 15-30% of the oxygen is added in rela­tion to what would be needed for 100% oxidation. In pyrolysis we only heat but without adding air and thereby gaseous components of the organic material are evaporated and later condensed as liquid hydrocarbons. Torrefaction is when you do partial pyrolysis but only to remove some of the gaseous components, where the purpose not is to produce liquid hydrocarbons but make a compact residue that can replace coal in coal fired power plants. Only combustion is really used on a large scale commercially today for biomass, although significant work has been done on development of the other techniques as well. The hurdle has been the cost as the fossil alternatives with natural gas, oil and coal have been “too cheap”. As different type of penalties are introduced on fossil fuels to compensate for the costs caused by environmental impact like greenhouse effects and acidification, the relative competitiveness will change. As the new technologies are improved they will also be cheaper, and with new system designs we can foresee a commercial expansion within the next coming 10 years also with respect to all other technologies apart from combustion. Several countries like the USA, Denmark, Finland, Germany, the Netherlands and Sweden all have strategies for research and demonstration of biomass for multiple uses such as for production of plastics, textile fibers, and many different chemicals (Andersson, 2012).

Already today, significant amounts of biomass are converted mostly into heat. The estimate is that approximately 13% of all global primary energy utilized is biomass. Still, most of this is converted with very low efficiency technologies like burning in an open fire. Then the efficiency from fuel to useful heat for e. g. cooking food is just around 10%. By introducing simple ovens the efficiency may then be increased several times, and by introducing very efficient cogeneration technologies the sum of heat and electric power in relation to the heating value of the biomass fuel may even be 117% in e. g. Sweden. This is actually quite common in the large-scale CHP (combined heat and power) plants in Sweden, where we have a heat demand at least most time of the year. In hot climates, the alternative is CCP, combined cooling and electric power production. The efficiency then can be quite high, although not as high as in Scandinavia, where also condensate heat can be utilized from the exhaust gas to reach the 117%.

In China, approximately 15% of the coal is gasified today and coal is used in the production of 50% of all chemicals. In 2005, China produced 232,820,000 tonnes of coke, 8,950,000 tonnes of calcium carbide, 25,000,000 tonnes of chemical fertilizer and about 3,500,000 tonnes of methyl alcohol from coal. Shenhua Baotou coal to olefin program has a production of 1.80 million tonnes/year; coal-to-carbinol is 600,000 tonnes/year. There are more than 10,000 coal gasification stoves in operation in China. Fixed-bed gasifiers are the most common. In ammonia-fertilizer industries, the number of water-coal gasifiers exceeds 4000 units; there are also more than 5000 two-phase gasifiers where e. g. Lurgi gasifiers are used for producing industrial fuel gas (Yasuyuki, 2007). Here the potential for gasification of biomass should be very high, as there are major resources of straw just being wasted today. As gasification is already common, it should be easier to get acceptance also for biomass gasification. Still, there is a demand for the right incentives like price or regulatory directives. Torrefaction also has a major potential, as the product is compact and easy to transport long distances in an economic way. The heating value may be up to 25 MJ/kg dry substance (DS), which is in the same range as coal. Another advantage is that the

2 E. Dahlquist pellets or briquettes produced from torrefied biomass can be used in normal coal mills without having to modify the grinding equipment normally used for the coal. This makes it easy to start using biomass as a complement to coal on a large scale.

Gasification can be used to produce different type of chemicals. Either methane can be produced and separated directly, or the syngas with CO + H2 is converted through catalytic processes to different chemicals using e. g. the Fischer-Tropsch process. An alternative can be to heat biomass without introducing air, and we then get pyrolysis instead of gasification. Then a more complex mixture of gaseous and liquid components is produced. This can be refined in a way similar to how crude oil is refined in refineries. This technology is now being developed both “on its own” and as part of gasification systems. For example, CORTUS has a process where biomass is first pyrolysed and the pyrolysis gas is then combusted to heat the char, which is gasified using steam (http://www. cortus. se/, 2012). Chalmers in Gothenburg is also working with a similar technology together with Metso Power. Andritz is working with the Carbona process and several companies in among others Japan are working on processes with gasification combined with combustion in two separate fluidized beds. Here the char is combusted to produce heat for the gasification and to get rid of the residual char coal. A pilot torrefaction plant in Ornskoldsvik also is using pyrolysis gas for heating and driving the torrefaction, although using lower temperatures than are normally used in pyrolysis.

Aside of these thermal conversion processes we have microbial processes as well as mechanical compaction in different ways. Concerning microbial processes, in China these can be from small batch fermenters in single households to produce gas for cooking food to large scale plants like Tianguan’s biorefinery inNanyang (Henan Tianguan Enterprise, 2012), where 150 x 106m3 gas will be produced annually.

The processes are of batch type as well as continuous and the temperature can be from room temperature over mesofilic around 35°C to thermofilic around 55°C. In all these processes the basic principles are still the same. We use different types of microorganisms to convert biomass through biochemical process routes into something that is more valuable for us as humans than only CO2.

In combined systems, we can see that it would make sense to use easily decomposable sub­stances like house hold waste for biogas production, while dry, solid waste is better to convert in the thermal conversion processes. An advantage with the microbial processes is both that all nutrients like P, N and K can be recirculated to farmland after the processing, and also there will be an organic residue that has the properties to keep moisture in the soil when distributed on farmland. As the organic content has a tendency to decrease rapidly today with a lot of cereal production and less animals, this is of high importance in many countries and should be taken into account in many more in the future, to create a sustainable agriculture. To make it possible to recycle the organic material on the other hand we need to be careful with what we put into the reactors. This is especially true in wastewater treatment plants, where many different chemical substances may come to the plant like pharmaceuticals, tensides, oil, etc. Thus, we can foresee a major demand on separation of waste and avoidance of disposing toxic chemicals into waste and wastewater in the future. The complete material handling system will be integrated with the energy system.

In reality, we will need to recycle also the inorganics from the thermal conversion plants to sustain the productivity in forestry and other areas long term. Here we have just started, and have a very long way to go until we reach sustainability.

Concerning the mechanical conversion techniques the major focus is on robustness, so that the equipment and tools will last and not need replacement too frequently. For that reason e. g. briquetting may be easier than pelletizing, as the friction surface is smaller.

For pyrolysis the major difficulty is that we get a process that gives a different composition depending on what we put in. The chemical composition is affecting the liquid phase composition a lot, and if we want to produce a very homogeneous product, we have a problem. Still, by measuring the chemical composition of the biomass we put in we can to some extent control the process in such a way that we can get more homogeneous results. This is also relevant for the

optimization of combustion, gasification, and biogas production processes. In this book, we cover these aspects especially looking at NIR (near infrared spectroscopy) and RF (radio frequency) sensor systems, which are introduced for on-line applications to determine moisture content and chemical composition of biomass. Several installations are done by e. g. Bestwood for this in Sweden.

This chapter only has the aim to give a very broad overview of the different technologies and you will read more about everything in the rest of the book. Thus only a few references have been included, as more comes in later chapters instead.

REFERENCES

Andersson, K.: Report on bioenergy based economy Bioenerginytt 2, 2012. http://www. cortus. se/ (accessed March 2012).

Henan Tianguan Enterprise Group Co., Ltd: http://www. tianguan. com. cn/english/about. asp (accessed March 2012).

Yasuyuki, A.: Report on applying coal gasification technology in China’s coal based chemical industry. UNESCO report, 2007, http://www. unescobeijing. org (accessed March 2012).

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Recall that O2 in the exhaust is an indicator of ф used in experimentation. Thien (2002) derived an expression for the burnt fraction of a solid fuel approximated as:

BF « * 1 — —

ф XO2,A

where BF is the burnt fraction, ф is the measured equivalence ratio from flow rates, XO2 is the mole fraction of oxygen in the exhaust gases (dry basis), and XO2A is the mole fraction of oxygen in the ambient air (dry basis). This equation can be used for rich or lean mixtures. When BF is very high, near unity, it implies that all of the fuel was combusted. Note that BF is larger than 1 for some of the extremely lean experiments. These values demonstrate the limitations of equation

(3.2) as well as experimental uncertainties including fuel compositions. As is to be expected, BF decreased with increasing equivalence ratio. In rich combustion, insufficient air was provided to completely oxidize all fuel carbon to CO2, leaving unburned fuel in the ash. This caused the BF to be less than 1.

Figure 3.27 and Figure 3.28 present the BF for TXL and TXL:DB blended fuels and WYO and WYO:DB blended fuels, respectively. Even in the very rich combustion (ф = 1.2), approximately 83% of the fuel was burnt.

According to the agricultural biomass energy industry development planning for 2007 to 2015 of China’s Agriculture Ministry, 1000 straw gasification gas supply stations, with an annual capacity of 365 million cubic meters of gas from straw had been built by 2010, to solve the basic energy demands and changing the way China’s countryside is used; 2000 straw gasification gas supply stations will be built by 2015, with an annual capacity of 730 million cubic meters of gas from straw.

In China, the institute of energy in Shandong province is the representative, whose rural straw gas centralized supply system gained major popularization and application. About 300 gas supply projects were built and a total investment of one hundred million Yuan was carried out. China has dozens of companies engaged in producing and marketing of rural straw gas centralized supply devices. Various types of straw were supplied as the main raw material in the straw gasification technology. Thermal energy was supplied for cooking gas or drying food by central supply gas system. Farmers, especially the richest in well-off localities, eager to use clean energy, have changed the healthy and clean appearance of their villages. Because of straw gasification, farmers burn gas instead of wood, which met the demand of the farmers to improve the quality of life, and gained farmers’ welcome and love.

There exist some problems in the straw gasification application for gas supply process. First of all, the gas centralized supply system of rural straw applied in China is the production of low calorific value biomass fuel gas with air medium. The flammable gas ingredient is CO, whose content is more than the regulations of the state civil gas standards. Farmer’s culture, science and technology quality is low, especially in China, they have a big hidden danger of security with cooking gas. Secondly, the waste gas, produced by burning low value gas, would pollute the environment. In addition, the biggest problem of the Chinese straw gasification units in operation is the removal and processing of the tar. Therefore, the pipeline is easily blocked during the process because of tar precipitation, which is too little to recycle and be reused. If it is not removed, the environment will be polluted. Tar cracking is an effective method to solve the problem of pollution by tar, as showed in China’s new energy nets. The problem of waste water containing tar could be solved if the quantity of tar is decreased greatly. It is hard to make the tar become cracked completely with the present technology, and water washing is needed to some degree. So wastewater treatment and recycling is necessary.

The important properties of biomass of significance for gasification can roughly be divided as

follows:

• Physical and thermodynamic properties: The physical properties of biomass depend markedly on the type of feedstock. For instance, density may vary from 100kg/m3, for balsa, bagasse and straw, to 1200 kg/m3, for lignum vitae (Di Blasi, 1997). Permeability to gas flow and thermal conductivity not only vary with the biomass species, but also along, across and tan­gentially to the wood fiber-chains. Furthermore, the effective physical properties of char probably reflect those of the virgin biomass and thus show significant variation with the feedstock. All cereal straws resist compaction, which make them difficult to compress for economical transport and storage. Processed biofuels do generally have a higher calorific value than the raw material itself, caused by the disposal of the moisture content and air in the structures of the biomass during processing. Since gasification is a thermochemi­cal conversion process the thermodynamic properties of a biomass plays a significant role in the gasification process. Biomass, as such, is highly anisotropic with different thermal conductivity along the fiber compared to across the fiber bundles. The conductivity also depends on factors such as moisture content, porosity and the present temperature. An exam­ple of the thermal conductivity dependency on the dry density along and across the grain is shown in Figure 6.4. Other important properties are the specific heat, depending on the moisture content and the actual temperature, and the ignition temperature. The ignition tem­perature is the temperature where the thermochemical process becomes self-sustainable and is generally lower for biomass fuels with a higher volatile matter content. However, it also depends on the surrounding conditions such as oxygen partial pressure, particle size and heating rate.

• Chemical composition and energy content: All organic compounds in lignocelluloses contain oxygen, but the degree of oxidation varies, so that the polysaccharides contain more oxygen than lignin and the extractives. The quantities of cellulose, hemicellulose and lignin vary between different types of biomass plants. Each of these components has their own pyrolysis chemistry in thermo-chemical conversion, and therefore the composition of the volatilized intermediary compounds can vary substantially depending on biomass used and (Mohan, 2006) and hence also influence the composition after gasification. The energy content varies between different biomass fuels as exemplified by straw, refused derived fuel (RDF) and municipal solid waste (MSW), algae and non-organic residue with low heating values (LHV) of 18.0, 22.9, 23.1 and 33.0MJ/kg, respectively (Phyllis, 2012).