216 J. Jpn. Inst. Energy, Vol. 97, No. 8, 2018

Fly Ash Transformation and Fouling Tendency during Co-firing Biomass with Coal

Dedy Eka PRIYANTO※1※ 2†, Chikako WADA※1, Naoki SATO※1, Shunichiro UENO※1, and Kazuhiro MAE※ 2

(Received January 15, 2018)

The co-firing of coal with biomass is a promising method for reducing net CO2 emissions from existing coal-fired power plants. This present study examined the effect of the co-firing of coal with biomass on the produced fly ash and the fouling tendency under condition representative of reheater region of pulverized-coal (PC) boiler. The fouling tests were conducted in a drop-tube furnace by inserting a water-air-cooled deposition probe to the point where the inner furnace temperature was 800-900 °C and probe’s metal temperature was kept at 500 °C. Bituminous coal was mixed with up to 50% (energy basis) of three types of biomass, namely; Palm Kernel Shell (PKS), Japanese cedar (without bark) and bark of cedar respectively. Fouling tendency was evaluated by determining the ash deposition ratio during co-firing tests. The properties of fly ashes and ash deposits for each sample were analyzed in detail by XRF and CCSEM analysis. Compared to coal firing, PKS or bark co-firing significantly increases the fouling tendency, whereas cedar co-firing does not affect the fouling tendency. The increase of minerals with low melting point, particularly Ca-Fe-Al-Si and K/Na-Al-Si, in fly ash accelerated the fouling tendency during co-firing.

Key WordsBiomass, Co-firing, Fly ash, Fouling

※1 Chemical Engineering Department, IHI Corporation Yokohama, Japan※ 2 Department of Chemical Engineering, Kyoto University Kyoto, Japan†Corresponding author: dedy_priyanto@ihi.co.jp

Journal of the Japan Institute of Energy, 97, 216-221（2018）

Special articles: Asian Conference on Biomass Science Ⅰ特集：アジアバイオマス科学会議Ⅰ

Original Paper

1.　IntroductionJapan has the ambitious target of reducing its total

greenhouse gas emission to 26% of the 2013 level and increasing the ratio of electricity generated from renewable energy to the total electricity generation to 22%-24% by 2030 1) 2). Owing to the introduction of the feed-in tariff (FIT) scheme for renewable energy in 2012, there has been a significant increase in the electric power produced from renewable energy sources in the country, such as biomass to replace or mix with coal. Co-firing technology, which involves the addition of some biomass to the coal used in coal-fired power plants, is a promising means of reducing greenhouse emissions from coal-fired power plant, especially considering its lower additional investment requirement compared with CO2 capture technology or oxyfuel technology 3). It has been implemented in several

coal-fired power plants in Japan, although the biomass ratio in the country is still low, being

217J. Jpn. Inst. Energy, Vol. 97, No. 8, 2018

focused on co-firing of coal with different biomasses at relatively low or moderate ratio (below 20%). Some of the biomass with relatively low deposition rate, such as sawdust, were observed to be relatively safe, since co-firing coal with these biomass would not significantly influence or even decrease the ash deposition rate. However, there have been limited studies on co-firing using high biomass ratios, and on the effect of such on ash deposition, particularly fouling, and fly ash transformation in PC boiler applications.

In this paper, effects of high ratio co-firing of biomass on fly ash transformation and fouling tendency in a lab-scale pulverized combustor were investigated in detail. A bituminous coal was mixed with up to 50% (energy basis) of three types of biomass. The chemical and physical characteristics of ash deposits and fly ash are analyzed using X-ray fluorescence spectrometry and computer-controlled scanning electron microscopy (CCSEM) analysis.

2.　Experimental　2.1　Fuel

Pulverized bituminous coal (

218 J. Jpn. Inst. Energy, Vol. 97, No. 8, 2018

The fouling tendency was evaluated by calculating the ash deposition ratio (ADR) using the following equations:

(1)

where Mdep is the mass of the ash (g) deposited over a certain time (min); Mfuel and Cash are respectively the flow rate of fuel (g/min) and ash content (wt%) of the fuel; and Stube and Sfurnace are respectively the cross-sectional areas of the probe and furnace (m2).

Fly ashes were also collected by thimble filter (35 mmφ×120 mm, ADVANTEC) equipped in exhaust gas line during combustion.

　2.4　Ash characteristicsThe morphological data of ash particles, such as the

average diameter, perimeter, and shape index, were obtained from the backscattered electron (BSE) image, and the constituent elements of each particle were determined by computer-controlled scanning electron microscopy analysis (CCSEM) analysis of the EDX spectra (SEM:JSM-5600, EDX: EDAX Genesis). The details about CCSEM procedure are available in another paper 15). Chemical composition of ash was analyzed using XRF (PW2404, Phillips). The water-soluble compounds (Ca, Na, K, SO42-, Cl-) in the deposits were also analyzed according to JISZ-J302-6.

3.　Results and Discussion　3.1　Fly ash transformation　3.1.1　Morphology

Fig. 1 shows BSE images of ash particles obtained during the combustion process. The bright particles in the figures are the ash particles. The co-firing of cedar using ratios of up to 50% produced no significant morphological changes in the fly ash particles. In contrast, the significant morphological changes occurred during the co-firing of bark or PKS using mixing ratios as low as 15% as evidenced by the existence of large numbers of spherical ash particles. In addition, co-firing also produced coarser fly ash particles due to the interaction between biomass ash and coal ash.

The spherical ash particles can be quantified by using the shape index of each particle obtained by CCSEM analysis 15). Based on the circularity of a circle of 1, the numbers of particles with shape indexes ranging between 1.0 and 1.19 were compared, as shown in Fig. 2. As can be clearly observed, the ratio of spherical ash particles significantly increased with increasing ratio of bark or PKS. Compared to pure coal-firing, the ratio of spherical particles for 30% bark 30% and 30% PKS are more than threefold. In addition, PKS co-firing produced more spherical particles than bark co-firing. The ratio of spherical particles during

co-firing coal with biomass, increased in the following order;　PKS >Bark > Cedar Coal

The sphericity of an ash particle indicates that the entire particle or its surface became molten during the combustion. This strongly suggests that co-firing, especially

ADR =Mdep

Mfuel × t × Cash

StubeSfurnace

Fig. 1　Backscattered electron (BSE) images of fly ash particles

Fig. 2 Comparison of the ratios of spherical particles with shape index of 1.0-1.19, produced by coal firing and co-firing with biomass

219J. Jpn. Inst. Energy, Vol. 97, No. 8, 2018

when using bark or PKS, significantly affects the ash melting behavior.　3.1.2　Chemical composition

Fig. 3 shows the compositions of fly ashes based on CCSEM analysis parameter. Fly ashes of coal predominantly comprised aluminosilicates minerals, particularly kaolinite. Generally, the amount of aluminosilicates starts to decrease, while the amount of AAEM minerals increases as the increasing ratio of biomass. However, the increase level of the AAEM minerals depends on the ash content and composition in biomass. A slight increase in the calcium minerals was observed for co-firing using low-ash biomass (cedar), while the significant increase was detected for co-firing using high-ash biomass, such as bark or PKS.

Composition of calcium minerals in fly ashes also differs among biomass employed in this study. During cedar or bark co-firing, the calcium minerals in fly ashes predominantly comprised a calcium aluminosilicate (Ca-Al-Si) mineral, probably in form of anorthite or gehlenite. In contrast, Ca-Fe-Al-Si mineral was the main calcium mineral in fly ashes for co-firing using PKS which has high Ca and Fe contents. Ca-Fe-Al-Si is probably formed from the reaction between iron and calcium compounds (mainly CaSO4) from PKS ash and aluminosilicates from coal ash. The reaction is as follows:　Fe2O3 + CaSO4 + xAl2O3 ･ ySiO2

→ Fe2O3 ･ CaO ･ xAl2O3 ･ ySiO2 + SO2 + 1/2 O2　(R1)It should be noted that iron-containing calcium

aluminosilicates have lower melting point compared to calcium aluminosilicates 16), and this may increase the ash deposition tendencies of the former.

Beside calcium minerals, the presences of sodium aluminosilicate (Na-Al-Si) and potassium-aluminosilicate (K-Al-Si) were also observed particularly during PKS co-firing and bark co-firing respectively. Compared to aluminosilicate or Ca-Al-Si, alkali aluminosilicate has much lower melting point, and this may also increase the ash deposition tendency. Alkali aluminosilicates were formed through the following reactions 17).　Na2SO4 + xAl2O3 ･ ySiO2

→ Na2O ･ xAl2O3 ･ ySiO2 + SO2 + 1/2 O2　(R2)　K2SO4 + xAl2O3 ･ ySiO2

→ K2O ･ xAl2O3 ･ ySiO2 + SO3 + 1/2 O2　(R3)　2KCl + xAl2O3 ･ ySiO2 + H2O

→ K2O ･ xAl2O3 ･ ySiO2 + 2HCl　(R4)

　3.2　Fouling TendencyThe profiles of the ash deposition ratio for co-firing

and coal or biomass firing are shown in Fig. 4. The ash deposition ratio is an indication of the fouling tendency of the fuel. The higher the ash deposition ratio, the higher the fouling tendency. The test results showed that co-firing using cedar did not increase the ash deposition ratio at least up to 50%. In contrast, the ash deposition ratio increased with the increasing biomass ratio for co-firing using bark or PKS. Co-firing using PKS produced the highest ash deposition ratio among all the considered biomasses. Generally, fouling tendency during the co-firing increased in the following order;　PKS >Bark > Cedar ~~ Coal

The results of the fouling propensity agree with the transformation tendency of fly ash properties during co-

Fig. 3　Mineralogical compositions of fly ashes produced by pure coal firing and co-firing with biomass

220 J. Jpn. Inst. Energy, Vol. 97, No. 8, 2018

firing; specifically, co-firing using bark or PKS significantly affects the properties of the ash, whereas the use of cedar does not. As noted earlier, co-firing increases the concentration of calcium and alkali minerals in fly ashes, with bark and PKS producing more significant changes. Although the fouling tests were conducted in the region with low temperature (800-900 °C), the combustion temperature reached more than 1300 °C. It caused particles, which contain calcium or alkali minerals, still partly molten when impacted on the probe surface. As is well known, the ash particles tend to adhere to the probe tube with increasing amount of molten ash particles. In addition, the existence of iron in calcium mineral (Ca-Al-Si) could further lower the melting point (i.e. Ca-Fe-Al-Si), resulting in the highest fouling tendency for PKS co-firing.

From the results of water-soluble analysis of the deposits, co-firing, particularly bark or PKS co-firing, also produces higher concentration of sulfates (i.e. CaSO4 and K2SO4) than pure coal firing. The sulfates originated from the reaction between alkali or alkaline earth species from biomass with SO2 gas from coal combustion through the following reactions. 　2 KCl + SO2 + 1/2 O2 + H2O → K2SO4 + 2HCl (R5)　CaO + SO2 + 1/2 O2 → CaSO4 (R6)

The condensation of sulfates, particularly alkali sulfates, on the probe surface also can act as binder between ash particles 18), accelerating ash deposition during co-firing. However, because the ratio of water-soluble compounds was below 10%, the condensation of sulfates is probably less dominant than inertial impaction of low-melting point compounds as the main factor of fouling under the conditions of our experiments.

The fact that no increase was observed in the fouling

tendency for co-firing using the low-ash biomass (cedar) is also noteworthy. This finding is consistent with our previous studies that co-firing using cedar did not increase the slagging tendency and corrosion rate of superheater tubes 15) 19). Thus, from the viewpoint of preventing severe slagging and fouling problems in a PC boiler during co-firing using a high ratio of biomass, a low-ash biomass is preferable. Installing soot blower in reheater region can be also an alternative way to mitigate fouling problem when using PKS or bark as biomass fuels.

4.　ConclusionCo-firing of coal with high ratios of three different

types of biomasses in a drop tube furnace was investigated in this study. The properties of fly ashes and ash deposits were examined in detail through CCSEM analysis and XRF analysis. Compared to pure coal firing, co-firing produces no significant changes in the morphology and chemical composition of the ash products when using a low-ash biomass such as cedar up to 50% (energy basis). In contrast, the use of even a small ratio (15%) of high-ash and high-calcium/iron biomass such as bark or PKS produces a large amount of molten ash particles with a higher calcium and alkali minerals content. An increase in fouling propensity was only observed for co-firing using bark and PKS. This was because of the significant change in fly ashes properties and the formation of low melting compounds, such as Ca-Fe-Al-Si and Na/K-Al-Si.

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