Formation of Acid Gases from Co-firing of Coal with Raw and Torrefied Biomasses

: 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 fuel samples chosen for this work are a high-sulfur bituminous 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 ranges of 75-90 µm and 90-150 µm, respectively. Results showed beneficial synergisms for almost all blend samples that can reduce the SO 2 and NO x emissions to values below those predicted by linear interpolation of the SO 2 and NO x emissions of the involved neat fuels.

and NOx 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 SO2, NOx 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 types of biomass reduced NOx emissions of the neat coal [17]. Overall it can be concluded that co-firing has been shown to have beneficial synergism in reduction SO2, NOx, and CO2 emissions [5,9,[18][19][20][21], as well as higher boiler efficiency and fuel cost savings [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 co-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 Table 1.

2-Experimental Procedure and Fuel Properties
All of the solid fuels were burned in an 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 found 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 standard 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]. Pulverized corn straw and rice husk were obtained from Harbin Institute of Technology in China. The ultimate and proximate analyses 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 the 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].

3-1-Equivalence Ratios
In this work the combustion 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, , as given by Eq. 1 based on the input of fuel and air, are shown in Fig. 2.
The lower for corn straw raw sample can be partly explained based on its lower bulk density [23]. The torrefied biomass samples experienced higher in general, as their bulk densities are higher than those of the raw samples. The neat coal samples experienced the highest due to higher bulk densities comparing with biomass samples and the equivalence ratios 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-CO2 Emissions in the Combustion Effluents
The measured volume fractions CO2 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 torrefied samples the mole fractions of CO2 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 CO2 emissions because of their higher carbon contents. The neat coals emitted about 9% of CO2 in their effluents, as also reported in previous works [22,24]. These differences can be explained based on the different bulk equivalence ratio ϕ among the fuel samples and differences in their carbon content. Mass fractions were also calculated in order to normalize such differences in the CO2 mole fractions, i.e. (g or mg)/(g of dry fuel). Therefore, the differences between the raw and torrefied biomass and their 50-50 wt% blends were reduced and became comparable when CO2 mass emissions were also plotted in Fig. 3. Therein, it can be seen that the mass emissions of neat 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 coals respectively. Mass-based CO2 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.  In addition, for all blend cases there is a beneficial synergism for reducing the SO2 emissions to values even lower than those expected based on the interpolation 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 SO2 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][25][26]. By considering the aforesaid SO2 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 SO2 emissions in the effluent gases. Therefore, during the process of torrefaction some amount of sulfur in the raw biomasses 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 devolatilization at lower temperatures, such as the torrefaction temperatures used in this work. The NOx 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 work, as shown in Table 1, all fuel samples have rather similar nitrogen contents in their structures, in the range of 0.94 -1.5 wt%, with all biomass samples herein having higher nitrogen content than the coals. The mole fractions (ppm) shown in Fig. 5, represent lower NOx emissions for the biomass cases, perhaps due to lower CO2 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 content in fuel samples as shown in Table 1, therefore 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 than the weighted emission averages from the neat coal and neat raw and torrefied biomass, which suggests that beneficial reduction synergisms were present. Moreover, in the case of coal-corn straw (raw and torrefied) blends, the mass emissions 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 N2. Therefore, blending coal and biomass would help reducing the NOx emissions, as biomass may act as reburning fuels for NOx reduction.

Conclusions
• Torrefied biomass samples have higher carbon content than the raw ones, which typically generates more CO2 emissions. In addition, CO2 mass emissions from both raw and torrefied biomasses are lower than coals. • Blending coal-biomass (raw and torrefied) showed beneficial synergism for reducing SO2 and NOx emissions from the combustion of coal. • Biomass has typically lower sulfur content and higher total alkali content than coal with rare exceptions. Therefore, SO2 emissions therefrom were lower than those from coal. • Torrefaction reduced sulfur content of biomass and hence, it also reduced the SO2 emissions in the combustion effluents.