Most of the fuel-bound nitrogen is normally converted to mainly ammonia and smaller quantities of other gaseous organic nitrogen containing compounds (50-80%) during gasification of biomass. These compounds are all in the vapor phase and will therefore pass through all particulate removal devices and they will cause potential emissions problems by forming NOx, during subsequent combustion.

There are in principle four options of approaching the problem of nitrogen oxide emissions. These may be used singly or in combination:

• Reduction of the formation of NOx, by limiting fuel-bound nitrogen in the feedstock through careful selection of biomass types and/or blending.

• Wet scrubbing, which will remove ammonia and other soluble nitrogen compounds, but results in loss of sensible heat and thus hamper efficiencies.

• Use of selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR) down­stream the subsequent combustion. Both these methods involve a reaction between ammonia and NO, to form nitrogen and water. These are well-established technologies and they are often used for NOx-reduction. However, there is a cost and an efficiency penalty.

• Use of sophisticated gasification techniques for minimizing the conversion of fuel-bound nitro­gen to ammonia and subsequent use of alternative sophisticated combustion techniques for enhancing the in-situ SNCR reactions.

6.4.2.3.2 Sulfur

The sulfur content of biomass is relatively low, 100-200 ppm. Thus the product gas will contain about the same amount of sulfur, primarily as hydrogen sulfide. These levels are not problematic from an environmental viewpoint, but for processes involving subsequent catalytic upgrading must they be reduced by up to 90%. This because sulfur readily forms metal sulfides, which makes it an often encountered catalyst poison, even at (very) low sulfur concentrations. This property also makes metals or metal oxides suitable for sulfur purification or sulfur adsorption. However, a suitable candidate for sulfur capture should preferably be relatively easy to regenerate, a property that often is difficult to combine with that of good sulfur adsorption properties. The most often employed sulfur purification methods are the “wet” methods, which call for relatively low temperatures. In flue gas, cleaning is this normally not problematic as an efficient combustion process call for relatively low flue gas temperatures. However, during gasification, with subse­quent catalytic upgrading, is it preferable not to lower the temperature before the upgrading for reasons of energy efficiency. Thus are high temperature dry desulfurization methods to prefer, but these are yet to be fully developed.

6.4.2.3.3 Chlorine

Chlorine is another potential contaminant, which may originate from pesticides and herbicides as well as in waste materials containing, for example PVC. A level of 1 ppm is often quoted, but this is a function of the temperature, chlorine species and co-contaminants, etc. Chlorine tends to form low melting salts with nickel and calcium (dolomite) with may hamper the subsequent catalytic tar cracking or catalytic reformation and upgrading of the product gas. Chlorine can be removed by absorption in active material either in the gasifier, in a secondary reactor or through wet processes.

TXL was used as the base case fuel. TXL and WYO were fired as blends with two DB fuels. Each coal was blended with each DB fuel in 100-0, 95-5, 90-10, and 80-20 blends on a mass basis. Note that on a heat basis, the percent of heat attributed to each fuel type was much less compared to percent mass basis. For example: for the 80:20 WYO:HA-PC-DB-SoilS fuel, 80% of the mass was WYO, but more than 94% of the heat came from WYO. All fuel and air flow rates were calculated from a program developed by Goughnour (2006). For each blended fuel, the equivalence ratio was varied from 0.8 to 1.2 in 0.1 increments. Combustion any leaner than

0. 8 created a heavy strain on the compressor and was also useless for industrial applications. The 80-20 blends were too rich in DB to be used in industrial applications, but were used in order to get more data points for the study. In the rich regime (equivalence ratio > 1.0) the HA-PC-DB-SoilS (Fig. 3.3b) quickly clogged the sampling port due to high ash content and may not be suitable for co-firing with coal; however it could be used as fuel for producing low heat value gases using gasifiers (see the section on gasification). The coal: biomass blends needed slightly more fuel flow rates compared to pure coal in order to compensate for the lower energy content of biomass for same thermal output.

1.2.3.1 The feasibility of biomass gasification gas as fuel

As China’s energy situation becomes worse, the policy of national environmental protection has become stricter, and it is necessary to adapt to the actual situation of power generation tech­nology to reduce operation and maintenance costs (Dong et al., 2007). The safe operation of power generation enterprises has become more and more urgent. Using gas fuel especially some flammable industrial waste as a supplemental fuel cannot only help the power generation enter­prises to reduce the production costs but also reduce the NOx emissions of pollutants effectively.

Biomass is one of the earliest sources of energy, especially for the rural areas where biomass is the only accessible and affordable one. The waste from agriculture and industry can be used as raw material in biomass gasification for electric power generation. Gasification efficiency and system efficiency increase and tar content in fuel gas decreases when the gasification technology is put into operation. From the chemical point of view, the composition of biomass is a C-H compound. It exists in the conventional mineral sources of energy such as oil, coal and other similar sources (coal and oil are all biomass). Biomass generates organic compounds through photosynthesis by plants, including agriculture and forest waste (such as straw, straw, branches, etc.), firewood, crop residues from sugar industry, municipal organic waste, energy crops and animal waste. Their characteristics and utilization patterns have been very similar to those of fossils. Some biomass gas main compositions and low calorific values have been listed in Table 4.8.

China is rich in biomass resources, and except for firewood and livestock feed, most of them are burnt directly and with low utilization rate. Therefore, it is important to change the situation and to increase the energy utilization efficiency, which would promote the national economic development and environmental protection. Biomass energy utilization started in China late, but the development of boiler capacity has been very fast. More and more medium and large cities have formulated a corresponding demand. Regulations and restrictions on the use of fuel gas boilers, such as in Beijing, Shanghai, Xi’an, where construction of new coal-fired boiler room plant is no longer approved. After 10 years of development, from the pure laboratory research to pilot scale, biomass gasification has been successfully used in production practice. The Thermal Engineering Department of Harbin Institute of Technology has developed fluidized bed combus­tion technology for biomass energy utilization and has resulted in manufacturing of domestic combustion boilers, such as a 12.5 t/h bagasse fluidized bed boiler, a 4t/h rice husk fluidized bed boiler, and a 10 t/h wood and sawdust fluidized bed boiler. The efficiencies are very high, up to 99%.

Klas Engvall, Truls Liliedahl & Erik Dahlquist

6.1 INTRODUCTION

Modern society is profoundly dependent on fossil feed stocks to produce multiple products, such as transportation fuels, fine chemicals, pharmaceuticals, detergents, synthetic fibers, plastics, fertilizers, lubricants, solvents, waxes, etc., as well as heat and power (Demirbas, 2006). The fossil resources are not endless. Their price is increasing continuously due to increasing scarcity, and not regarded as sustainable from an environmental point of view (Kamm, 2006). A versatile resource, especially in terms of producing carbon-based products, to replace fossil feedstocks is biomass (Vlachos, 2010) or other sources originating form biomass, such as black liquor (BL). Conversion of biomass to other products can be performed either by biochemical or thermo­chemical processes. In the case of large-scale production of, for example, carbon-based products, thermo-chemical conversion is considered more efficient compared to biochemical processes (Zhang, 2010). Techniques for thermo-chemical conversion can be divided into pyrolysis, gasifi­cation, combustion and liquefaction. Among these techniques, gasification is a versatile platform for production of multiple products, as illustrated in Figure 6.1.

Gasification has a long history starting with Thomas Shirley who experimented with “carbu­reted hydrogen”, today called methane, in 1659 (Basu, 2010). In the beginning of the 17th century and onwards to the early 19th, gas from gasification of coal was mainly used for lightening of homes and streets and for heating. New inventions in other fields expanded the utilization of the gas in diverse applications, such as fuel for steam engines, feedstock in chemical production

of chemicals and motor fuels. During this time the major commercial gasification technologies, Winkler’s fluidized bed gasifier in 1926, Lurgis pressurized moving bed gasifier in 1931 and Koppers-Totzek’s entrained flow gasifier were developed before the Second World War. After the Second World War the availability of abundant oil eliminated the need for gasification as a basis for production of chemicals and motor fuels. Today, the driving force for the renewed interest is the concerns about global warming and about the accessibility to fossil resources.

Syngas combustion in gas turbine engines (i) using an IGCC facility is quite promising for efficient, low-emission power generation, and for carbon capture and storage. Research in this area has focused on using syngas in natural gas-fired combustors (Monteiro, 2011). Similarly, some studies (Luessen, 1997; Colantoni et al., 2010; Boehman et al., 2008) have demonstrated the viability of using syngas in spark ignition (SI) and compression ignition (CI) engines. Sahoo et al. (2011) examined the effects of using syngas on the performance and emission characteristics in a diesel engine operating in a dual-fuel mode, using a combination of diesel pilot injection and syngas fumigation in the intake air (Boeman et al., 2008). In this mode, the ignition is initiated through the auto ignition of diesel fuel. Results indicated that the engine performance and emissions are strongly influenced by the syngas composition, depending upon the load and other conditions. In general, increasing the H2 fraction in syngas was found to improve engine performance, reduce CO and hydrocarbon emissions, but increase NOX emissions. Thus, further experiments and simulations are needed to optimize the engine performance and emissions for various operating conditions and syngas composition. Research should also focus on examining the use of syngas in new engine designs, such as HCCI (Homogeneous Charge Compression Ignition) and low temperature combustion.

The use of syngas in SI engines also offers advantages, such as better anti-knocking prop­erties and operation with leaner mixtures. Improved knock resistance is due to the presence of CO and CH4, and enables operation at a higher compression ratio, leading to higher thermal efficiency. However, a higher burning rate due to the presence of H2 can result in higher end gas temperature and increased propensity to knocking. The presence of H2 can also increase

NOX emissions, which may be controlled by using leaner mixtures (Boeman et al., 2008). Bika et al. (2011) examined such issues by performing single cylinder experiments for different syngas compositions, compression ratios, and equivalence ratios. For a given Ф and spark timing, the knock limited compression ratio was observed to increase with increasing CO fraction. The burn duration and ignition lag also increased with increasing CO fraction.

3.13.1.1 Fuel properties

Ultimate and proximate analysis (on an as received basis) of the DB (separated solids) used as feedstock in the current gasification experiments are presented in Table 3.3. Using as received analysis (ar), dry and dry ash-free (DAF) values are calculated and reported. Also, empirical chemical formulae are presented in Table 3.3 for gasification of DAF DB. Air gasification of DAF DB at ER > 5.8 (or A:F < 0.87) implies insufficient oxygen for the reaction (C + %O2 ^ CO) and hence, incomplete conversion of char, which means char as byproduct. On the other hand, at ER < 5.8 (or A:F > 0.87) there is more oxygen than that required for the conversion of all FC to CO and the FC could be gasified completely to CO and CO2. However, in gasification processes where the reaction time is not infinity (not ideal), incomplete conversion of char can be possible even with ER < 5.8.

3.13.1.2 Experimental results and discussion

To estimate the uncertainty in gas composition, standard deviation was determined for the data. The uncertainty for each gas was calculated as the ratio between the standard deviation and the average value measured. Additionally, the uncertainty of the temperatures was estimated as the ratio between the uncertainty of the device (±1.5°C) and the measured value. In general, the gas composition values fluctuated within ~15% and the temperature values within ~0.55% of the average value measured.

It represents the quantity of water inside the sample, expressed as a percentage of the weight of the material. The moisture content (MCwb) referred to wet basis is expressed as the ratio between water content and the weight of biomass (obtained as the sum of the components: water, ashes, dry and ash free matter):

Moisture on dry basis (MCdb) is expressed as the ratio between the weight of water contained in biomass and the weight of ashes plus the weight of dry matter without ashes:

Moisture on dry and ash-free basis (MCdafb) is expressed as the ratio between the weight of the water contained in biomass and the weight of dry matter without ashes:

mH2O i™mH2O,, ,,,

MCdafb = 100————— 2———- = 100—— (5.3)

mb mH2O mash mdafb

Moisture on wet basis is calculated as a variation in mass between the sample as received and the oven dried sample at a temperature equal to 105 ± 2°C till the weight variation becomes negligible.

The bubbling fluidized beds operate at relatively low gas velocities, typically below 1 m/s. In the BFB gasifier most of the conversion of the feedstock to product gas takes place in the dense bed region in the bottom of the gasifier, even though some conversion continues in the freeboard section owing to reactions associated with entrained (small) particles. Because of the relatively low gas velocities in the BFB gasifier, freeboard gasifier elutriation is minimal and the addition of new bed material limited. A schematic of a BFB gasifier is shown in Figure 6.7a. The circulating fluidized beds, illustrated in Figure 6.7b, operate at much higher gas velocities, 3-10 m/s, and are significantly different in their hydrodynamics, compared to a BFB reactor. The solids are dispersed all over the tall riser, allowing for a long residence time for both the gas and the fine particles. In the CFBG, the particles are separated from the gas in a cyclone and recycled back to the bottom of the reactor. For fly ash/ dust removal are in both configurations a particle filter employed.

The inert bed material will enhance the heat exchange between the fuel particles, and therefore the fluidized beds will operate under almost isothermal conditions. For both configurations, the maximum operating temperature is limited by the ash-induced melting point of the bed material that typically will lie between 800 and 900°C. At these relatively low temperatures, coupled with the prevailing relatively short gas residence times, will the (slow), especially heterogeneous, gasification reactions normally not reach chemical equilibrium. This is especially true for the faster CFB gasifier. Thus, for example, methane concentrations tend to be much higher than those suggested by the chemical equilibrium. Additionally, tar levels are normally between those of the downdraft and the updraft fixed bed gasifiers.

The conversion of the feedstock is high in a fluidized bed gasifier, almost 100%. This only, however, if the carryover of fines is limited, which predominantly may occur in top-feeding configurations.

Due to the geometry and the excellent mixing properties, fluidized beds may be scaled up with confidence. However, fuel distribution may become problematic in large beds, although multiple feeding can solve the problem partly. The energy throughput per unit of reactor cross­sectional area is higher for a CFB gasifier than for the BFB gasifier. Both configurations can be operated under pressurized conditions, which will further increase the energy throughput. Pressurized conditions are also beneficial if the eventual subsequent downstream upgrading calls

for pressurized conditions, as for instance the Fischer-Tropsch synthesis. The intense mixing allows the reactor to perform well over a broader fuel particle size distribution, starting already with relatively fine particles. Furthermore, in contrast to other reactor systems, the fluidized bed gasification allows for the possibility to use additives, e. g., for in-situ removal of pollutants (like sulfur) or primary measures to increase tar conversion via employment of catalytically active bed materials. Alternatively, a subsequent catalytic or thermal reactor can be added.

Loss of fluidization in fluidized beds due to bed sintering is one commonly encountered prob­lem, depending on the thermal characteristics of the ash. Alkali compounds from the biomass ash form low-melting eutectics with the silica sand, which is the usual bed material. This may result in agglomeration and bed sintering, resulting in eventual defluidization and subsequent shutdown of the gasifier. The presence of chlorine will amplify this problematic effect, as often alkali and chlorine go together. By applying proper counter measures, such as adding additives with alkali — attracting properties may part of this problem handled though. With biomass of high ash/alkali content, it may be advisable to use alternative bed materials such as alumina or magnesite. The main drawback with these more sophisticated bed materials is that of cost.

The CFB can run with most kinds of fuels, from coal to waste, including biomass. For larger CFB gasifiers, it is often preferable to employ a few smaller cyclones in parallel in contrast to only one large cyclone.