This work examined the emission for SO 2, NO x, and CO 2 gases from the combustion of pulverized coals, raw and torrefied biomasses, and 50- 50 wt% blends thereof. The fu el samples chosen for this work are a high- sulfur bitum inous coal (Illinois #6), a low -sulfur sub -bituminous coal (Powder River Basin), a herbaceous biomass (corn straw) and a crop -related biomass (rice husk). All fuel samples were burned in a laboratory -scale electrically -heated drop -tube furnace (DTF), operated at 1400 K, and combustion emissions were measured in the furnace effluent. Coal and biomass particles were in the range s of 75- 90 µm and 90- 150 µm , respectively. Results showed beneficial synergism s for almost all blend samples that can reduce the SO 2 and N Ox emissions to values below those predicted by linear interpolation of the SO 2 and NO x emissi ons of the involved neat fuels. Key Words: Coal -biomass blends, Raw and Torrefied Biomass , Emissions 1-Introduction The concerns about environmental issues from using fossil fuels in electricity generation have promoted the use of alternative renewable source s of energy , such as solar, wind and biomass. Based on the scenario projected in EIA ’s world energy outlook 2016 [ 1], nearly 60% of all new power generation capacity to 2040 could come from renewables [ 1]. In 2012, bioenergy accounted for 10% of the total global energy consumption which was greater than any other renewable sources of energy [2]. About 50% of the total bioenergy came from traditional use of biomass in wood stoves in developing countries, and 12% used in biomass -based electricity generation systems [ 2]. Co-firing is one of the most economic technologies in power generation in many countrie s, and t he overall cost of co -firing depends on the location and biomass feedstock and these costs are still significantly lower than the cost of 100% biomass dedicated power plant [ 3]. Greenhouse Gases Emissions . Biomass may be considered nearly CO 2-neutral as there are s till emissions related to biomass harvesting, tr ansportation, pre -treatment etc . Co-firing biomass residues reduce greenhouse gases by avoiding the CH 4 release from the landfilled biomass, as CH 4 is 21 times more potent than CO 2 in terms of global warming impact. As storing biomass wastes cause anaerobic release of CH 4, NH 3, H2S, volatile organic acids, and other chemicals, combusting the biomass helps reducing these emissions, which has been reported by other researchers [4-15]. SO2 and NO x Emissions . Most biomass fuels have little sulfur , therefore their co -firing with coal should result in lower SO 2 emissio ns than neat coal firing . Additionally, most of the fuel nitrogen in biomass is converted to NH radicals (mainly ammonia, NH 3) during combustion, where ammonia can reduce NO to molecular nitrogen [14]. Research on co- firing coal and a c rop-related biomass (rice husk) [ 16] has reported that while increasing the mass ratio of biomass decreases the emissions of NO and N 2O, it increases the amount of SO 2 emissions slightly . Co-firing a bituminous coal with a herbaceous biomass (straw) , with the biomass increasing fraction of 10 - 20% thermal basis in a full -scale power plant analysis showed a net decrease in the amount SO 2 1 Sub Topic: Coal and Biomass Combustion and Gasification and NO x emissions . In a study of co- firing coal with Bagasse, wood chips (Julia flora), sugarcane trash and coconut shell biomasses in India [ 5] researcher reported that co -firing has a benefit on SO 2, NO x and suspended particulate matter (SPM ) reductions . Co-firing sub-bituminous coal with different types of biomass with variable nitrogen contents, such as wood chips, sugarcane bagasse, cotton stalk , and shea meal , showed that the addition of those type s of biomass reduced NO x emissions of the neat coal [17]. Overall it can be concluded that co-firing has been shown to have beneficial synergism in reduction SO 2, NO x, and CO 2 emissions [5, 9, 18-21], as well as higher boiler efficiency and fuel cost saving s [5]. Whereas co -firing of coal and raw biomass has been studied extensively in the past and there have been reports on the gaseous emissions therefrom, little has been reported on the emissions from c o-firing coal with torrefied biomass . This work compares the gaseous emission of carbon, sulfur and nitrogen oxides from co -firing coal with both raw and torrefied biomass . The properties of all fuel samples are given in T able 1. 2-Experimental Procedure and Fuel Properties All of the solid fuels were burned in a n electrically -heated, laminar -flow, alumina drop- tube furnace (DTF). A schematic of the combustion setup is shown in Fig. 1. A detail of the experimental setup can be foun d in previous works done in this laboratory [22, 23]. All experiments were repeated three times and each point shown on each plot represents the mean of the three test and the error bars represent the s tandard error of the mean . A low sulfur, high alkali sub-bituminous coal ( PRB , Wyodak, Wyoming) and a high sulfur, low alkali bituminous coal (Illinois #6) were received from the Pittsburgh Coal Bank, and have been extensively characterized [22, 24]. Pulve rized corn straw and rice husk were obtained from Harbin Institute of Technology in China . The ultimate and proximate analyse s of all fuel samples are shown in Table 1 . The coal particles were in the range of 75-90 µm and the biomass particles (both the raw and the torrefied) were in t he range of 90-150 µm. Torrefaction process for the biomass samples was carried out in laboratory -scale muffle furnace in a nitrogen flow as thoroughly described in Ref. [ 23]. Fig.1. Schematic of the electrically -heated laminar -flow drop tube furnace setup for solid fuel combustion and emissions monitoring. 3-Results and Discussion 3-1- Equivalence Ratios In this work the c ombustion experiments were conducted burning neat coals, raw and torrefied biomass, as well as their 50- 50 wt% blends , in air under fuel-lean conditions. The calculated equivalence ratios, 𝜙𝜙 , as given by Eq. 1 based on the input of fuel and air, are shown in Fig. 2. 𝜙𝜙=�𝑚𝑚𝑓𝑓 𝑚𝑚𝑎𝑎𝑎𝑎𝑎𝑎�𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎𝑎 �𝑚𝑚𝑓𝑓 𝑚𝑚𝑎𝑎𝑎𝑎𝑎𝑎�𝑠𝑠𝑎𝑎𝑠𝑠𝑠𝑠𝑎𝑎ℎ𝑠𝑠𝑠𝑠𝑖𝑖𝑖𝑖𝑎𝑎𝑖𝑖𝑠𝑠𝑎𝑎 (1) 2 Sub Topic: Coal and Biomass Combustion and Gasification The lower 𝜙𝜙 for corn straw raw sample can be partly explained based on its lower bulk density [23]. The tor refied biomass samples experienced higher 𝜙𝜙 in general, as their bulk densities are higher than those of the raw samples. The neat coal samples experienced the highest 𝜙𝜙 due to higher bulk densities compari ng with biomass samples and the equ ivalence rati os for the 50 -50 wt% blends were between those values of their corresponding neat coal and biomass. Table 1. Chemical compositions (wt%) and energy contents (MJ/kg) of the selected raw biomass Figure 2. Equivalence ratios of neat coals, neat biomasses, and the 50 -50 wt% blend thereof. 3-2- CO 2 Emissions in the Combustion Effluents The measured volume fractions CO 2 in the combustion effluents of all samples are shown in Fig. 3 (a: represents bituminous -biomass samples and b: represents subbituminous -biomass samples, same for Fig.4 and 5). For all neat biomasses including raw and torr efied samples the mole fraction s of CO 2 were between 2.2 - 5.5%, where the lowest are those of corn straw and the highest are those of rice husk. In addition, the neat torrefied samples and their blends resulted in higher CO 2 emissions because of their higher carbon contents. The neat coals emitted about 9% of CO 2 in their effluents, as also reported in previous works [22, 24] . These difference s can be explained based on the different bulk equivalence ratio ϕ among the fuel samples and differences in their carbon content. Mass fra ction s were also calculated in order to normalize such differences in the CO 2 mole fractions, i.e. (g or mg)/(g of dry fuel). Therefore, t he difference s between the raw and torrefied biomass and their 50- 50 wt% blends were reduced and became comparable when CO 2 mass emissions were also plotted in Fig. 3. Therein, it can be seen that the mass emissions of neat Rank / Fuel Source Herbaceous biomasses Crop -related biomasses Coals Corn straw (Raw) Corn straw (Torrefied) Rice husk (Raw) Rice husk (Torrefied) Sub-Bituminous (PRB) Bituminous (Illinois #6) Proximate Analysis (dry basis) Volatile Matter (%) 75.88 67.55 65.16 55.62 44.86 40.83 Fixed Carbon (%) 17.15 24.56 15.06 21.58 47.57 45.78 Ash (%) 6.90 7.87 19.69 22.80 7.57 13.39 Ultimate Analysis (dry basis) Carbon (%) 45.84 52.77 44.69 44.16 69.77 66.1 Hydrogen (%) 5.11 5.32 4.71 4.41 5.65 4.6 Oxygen (%) 39.12 32.46 27.54 26.73 15.64 9.9 Nitrogen (%) 1.28 1.50 1.15 1.22 0.94 1.1 Sulfur (%) 0.21 0.07 0.09 0.03 0.43 5.5 Calcium (%) 0.48 0.56 0.36 0.35 1.10 0.53 Sodium (%) 0.01 0.02 0.12 0.02 0.09 0.11 Potassium (%) 0.64 1.19 0.20 0.73 0.02 0.18 Magnesium (%) 0.25 0.29 0.05 0.08 0.19 0.05 Chlorine (%) 0.63 0.18 0.07 0.04 -- 0.14 Heating Value (MJ/kg) 16.8 19.4 14.7 16.1 29.6 28.2 3 Sub Topic: Coal and Biomass Combustion and Gasification raw and torrefied biomass were around 1.5 and 1.6 g /(g dry fuel) respectively, i.e., lower than the 2.11 and 2.43 kg/(kg dry fuel) for bituminous and sub- bituminous coal s respectively . Mass- based CO 2 emissions for the 50- 50 wt% samples were between 1.8 -2.15 g/(g dry fuel), whereas corn straw (raw and torrefied) blends experienced higher emissions comparing to rice husk (raw and torrefied) blends. Figure 3. Carbon dioxide mole fractions (%), mass fractions based on the amount of fuel introduced to the furnace. 3-3- SO 2 and NO x Emissions in the Combustion Effluents Figs. 4 shows the results of SO 2 emission in the forms of mole fractions and mass emissions from the combustion of all fuel samples . The sulfur content s of the coals are higher than those of the biomass fuels used in this work. The SO 2 emissions from sub- bituminous coal are about 188 ppm or 7.2 mg/g of dry coal, which are much low er than emission s from the bituminous coal, which are 3180 ppm or 110 mg/g of dry coal when burned under similar conditions . The SO 2 emissions for the raw and torrefied rice husk are about 10 and 5 ppm, respectively, with corresponding mass emissions of 0.5 and 0.2 mg/g o f dry fuel. For the case of corn straw raw and torrefied the emissions are 17 and 9 ppm, respectively, with the corresponding mass emissions of 1.7 and 0.7 mg/g of dry fuel . For the all cases of the 50 -50 wt% blends the SO 2 mole fractions and mass emissions, lay between the emission values of neat coals and neat biomasses. In addition, for all blend cases there is a beneficial synergism for reducing the SO 2 emissions to values even lower than those expected based on the inte rpolation between the two values of the neat coal and biomass. This can be explained first due to lower sulfur content of biomasses as well as the high amount of alkali earth metals in their structure that can react with the released SO 2 gases and capture them in the ash at the bottom of the furnace. Capturing sulfur in ash with the alkali metals available in the fuel structures was extensively studied in previous research in this laboratory [ 22, 24-26]. By considering the aforesaid SO 2 emission values , it is clear that the emissions in the case of torrefied biomasses are lower than those from raw biomasses. In addition, by comparing the crop related (rice husk) and herbaceous (corn straw) biomass samples here, the crop related samples have lower sulfur content and their mass emissions are lower , as expected. Based on the results herein, it can be concluded that torrefaction can help reduce the amount of SO 2 emissions in the effluent gases. Therefore, during the process of torrefaction some amount of sulfur in the raw biom asses was released, and this resulted in decrease in sulfur content of torrefied samples. Sulfur can be found in both organic and inorganic compounds in biomass; the organic sulfur can be released during the devolatiliz ation at lower temperatures, such as the torrefaction t emperatures used in this work. The NO x emission in the forms of mole fractions, and mass emissions from the combustion of all fuel samples are shown in Fig. 5. Generally speaking, the nitrogen content in coals structures vary less than those in biomasses structures. In this w ork, as shown in T able 1, al l fuel samples have rather similar nitrogen content s in their s tructures, in the range of 0.94 -1.5 wt%, with all biomass samples herein having higher nitrog en content than the coals. The mole fractions (ppm) shown in 4 Sub Topic: Coal and Biomass Combustion and Gasification Fig. 5, represent lower NO x emissions for the biomass cases, perhaps due to lower CO 2 emissions, therefore, direct comparison would not be an accurate method herein. Consequently, using mass emissions would be a better way to compare the emission results. Torrefaction process increased the amount nitrogen conte nt in fuel samples as shown in T able 1, theref ore the emissions from the torrefied biomass samples are higher than those from raw samples. The mole fraction (ppm) of all 50-50 wt% samples were lower t han the weighted emission averages from the neat coal and neat raw and torrefied biomass, which suggests that beneficial reduction synergism s were present . Moreove r, in the case of coal -corn stra w (raw and torrefied) blends, the mass emiss ions were even lower than the values of the corresponding neat coals and biomasses. These biomasses have high volatile nitrogen content that may convert to NH radicals, which may reduce NO to N 2. Therefore, blending coal and biomass would help reducing the NO x emissions, as biomass may act as re - burning fuels for NO x reduction. Figure 4. Mole fractions of sulfur dioxide in the combustion effluents (ppm) and mass fractions based on the amount of fuel introduced to the furnace ( mg/g). Figure 5. Mole fractions of nitrogen oxide in the combustion effluents (ppm) and mass fractions based on the amount of fuel i ntroduced to the furnace ( mg/g). Conclusion s • Torrefied biomass samples have higher carbon content than the raw ones, which typically generates more CO 2 emissions. In addition, CO 2 mass emissions from both raw and torrefied biomasses are lower than coals. • Blending coal -biomass (raw and torrefied) showed beneficial synergism for reducing SO 2 and NO x emissions from the combustion of coal. • Biomass has typica lly lower sulfur content and higher total alkali content than coal with rare exceptions. Therefore, SO 2 emissions therefrom were lower than those from coal. • Torrefaction reduce d sulfur content of biomass and hence, it also reduced the SO 2 emissions in the combustion effluents. 5 Sub Topic: Coal and Biomass Combustion and Gasification