Article pubs.acs.org/IECR
Mechanism Research on the Development of Ash Deposits on the Heating Surface of Biomass Furnaces Xuebin Wang,† Yuanyi Liu,† Houzhang Tan,*,† Lin Ma,‡ and Tongmo Xu† †
MOE Key Laboratory of Thermo-Fluid Science and Engineering, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China ‡ Energy and Resources Research Institute, School of Process, Environmental and Materials Engineering/Centre for Computational Fluid Dynamics, University of Leeds, LS2 9JT U.K. ABSTRACT: Sulfate and chlorate are the key species contributing to the formation of ash deposits in a biomass furnace. In this paper, biomass combustion has been tested in a drop-tube furnace, and the ash deposits on a heating surface have been sampled. The physical and chemical characteristics of the deposits were analyzed using transmission electron microscopy in order to investigate the development of the ash deposits. The laboratory results have been compared with the ash deposit samples obtained from a biomass-fired power plant furnace. The results from both the laboratory and the plant samples indicate that most of the alkali chloride has been homogeneously converted into alkali sulfate in flue gas before deposition on the heating surface and that sulfate aerosols play a dominant role in the formation of biomass ash deposits. Finally, major pathways of alkali sulfation and the role of sulfate aerosols formed in the biomass-ash deposition are proposed.
1. INTRODUCTION Biomass is a CO2-neutral fuel because it consumes the same amount of CO2 from the atmosphere during its growth as it releases during its combustion. With the aggravation of the global greenhouse effects, biomass is used more and more in power plants throughout the world in order reduce CO2 emissions from power generation. Currently biomass supply is accounting for approximately 13% of the global energy consumption.1−3 However, alkali and chlorine contents in biomass fuels are often high, compared with coal, which lead to severe problems of slagging, fouling, and corrosion of heat exchangers in biomass-fired power plants.4−7 At high temperatures, sulfur (S) together with potassium (K), chlorine (Cl), and sodium (Na) are released from the biomass as vapor into the flue gas. These species then go through a series of processes of sulfation, condensation, nucleation, and agglomeration leading to the formation of sulfate aerosols and deposit on the heating surfaces of the boiler.7−9 There is enough evidence that the aerosols of sulfate and chloride play a key role in the ash build up on the heat exchangers during biomass combustion, especially those alkali sulfates that have a low melting point.7,9−19 The release of potassium from biomass at high temperatures shows that it can be released in different forms such as (1) K2SO4 for the biomass with a low content of both K and Cl; (2) KCl for the biomass with a high content of both K and Cl; (3) KOH for the biomass with a high content of K but a low content of Cl; and (4) KCN for the biomass with a high content of both K and N.10,20,21 The majority of biomass has a relatively high content of both K and Cl; therefore, potassium species is more likely released in the form of KCl and later forms K2SO4, both of which can condense and deposit on the heat transfer surfaces.11 However, there is still some confusion on the roles of the sulfates in the biomass ash deposition in 100% biomass-fired boilers. © 2012 American Chemical Society
Earlier works on potassium species condensation and deposition have indicated that the gaseous KCl can condense and deposit on heat transfer surfaces when the flue gas temperature decreases, and then it partially converted to sulfate through the heterogeneous reaction with sulfur dioxide (SO2) and trioxide (SO3).7,9,22 This is in agreement with Johansson’s23 report that KCl first condensed onto ash and then converted to K2SO4 with the presence of sulfur species. Due to the kinetic limitation of the sulfation reactions between the solid phase and gaseous phase, only part of the chlorides are converted, even if the potassium should theoretically exist as sulfate under the thermal equilibrium condition at the heat exchanger surface below 1000 °C.8 Experiments by Lind14 in a fluidized bed reactor have confirmed the presence of a high content of KCl in the fine particles from the biomass combustion. The test on the cofiring of biomass and coal in Grena Power Plant has also indicated KCl as the primary component of the deposits formed.24 These investigations, together with that performed on Sandia’s Multifuel Combustor on the deposition of K/Cl/S during straw combustion,25 and the research by Sheth26 demonstrated the significance of the condensation of KCl on solid surfaces and its subsequent transformation into sulfates in the formation of ash deposition. On the other hand, previous research has also indicated that most of the chlorides can be converted to sulfate in the gaseous phase because the sulfate reaction is occurs more easily in the gaseous phase and few to no chlorides were detected in the fine particles and ash deposits in their investigations.18,27 Further, Jimenez and Ballester4,13,15−17,28 conducted some pioneering research on the fine ash particle generation during biomass Received: Revised: Accepted: Published: 12984
July 27, 2012 September 10, 2012 September 10, 2012 September 10, 2012 dx.doi.org/10.1021/ie302009m | Ind. Eng. Chem. Res. 2012, 51, 12984−12992
Industrial & Engineering Chemistry Research
Article
Figure 1. Fuels used in the 25 MW biomass-fired power plant.
Table 1. Proximate and Ultimate Analysis of the Fuels proximate analysis (wt %)
element analysis (wt %)
fuel
Qnetd
Mad
Ad
Vdaf
FCad
Cd
Hd
Nd
Od
Sd
Cld
wheat straw corn straw wood coal plant fuel
18.09 17.69 19.31 23.03 16.41
10.09 11.46 11.43 7.88 9.92
6.5 8.49 0.7 20.65 2.57
79.33 80.21 82.63 38.13 71.1
17.38 16.03 15.51 45.23 25.47
44.70 51.80 50.65 66.06 46.11
3.43 3.19 4.46 3.12 5.9
0.81 1.31 0.26 0.69 0.32
44.22 34.98 43.8 8.98 35.01
0.3 0.24 0.14 0.51 0.18
0.218 0.327 0.054 0.013 0.141
gaseous phase then the main depositing species will be potassium sulfate together with inorganic ash. The homogeneous transformation of alkali and chlorine species and the subsequent formation of sulfate aerosols will play a central role in the ash deposition formation on the heating surfaces of the boilers. In the present work, biomass combustion has been tested using three different types of biomass in a drop-tube furnace (DTF) and in a 25 MW biomass-fired power plant furnace in order to further investigate the role of sulfate on the formation of biomass ash deposition on heating surfaces. The physical and chemical characteristics of the ash were analyzed using TEMEDS-ED (transmission electron microscopy−energy dispersive spectroscopy−electron diffraction) to reveal the development of the ash deposition. The results from the DTF have been compared with the sulfate profiles of the ash sample from the 25 MW furnace. Major pathways of alkali sulfation and the role of sulfate aerosols formed in the biomass-ash deposition are proposed.
combustion, and it was clearly shown that K2SO4 nucleated and condensed from 900 °C, but condensation of KCl remarkably did not occur until the temperature was lower than 360 °C and most of the chloride was converted into sulfate before its nucleation and condensation. A test by Hansen29 on biomass cofiring in a 150 MW power plant showed that the content of chlorine in the ash is lower than 0.5%. Further, the results from Iisa30 obtained in an entrained flow reactor indicated that the conversion rate of KCl to sulfate was up to 100% in the gaseous phase and only 0.5−2% in solid phase. It has been experimentally and theoretically shown that the conversion of KCl to sulfates in the gaseous phase mainly occurs at approximately 800 °C, which is much higher than the dew point of KCl.7,9 Assuming the sulfate conversion of KCl and KOH mainly took place in the gas phase, Glarborg and Marshall18 presented a detailed mechanism to predict this process, and the results obtained agreed well with the experimental results by Iisa.30 Clearly, the degree of alkali sulfation and the deposition of chloride species are dependent on fuel properties, furnace design, and operating conditions. For the typical biomass-fired grate furnace that is investigated in this paper, if most of KCl is converted to sulfate in the 12985
dx.doi.org/10.1021/ie302009m | Ind. Eng. Chem. Res. 2012, 51, 12984−12992
Industrial & Engineering Chemistry Research
Article
shown in Figure 1. The proximate and ultimate analysis of the fuels used in the laboratory and the power plant tests are shown in Table 1. The chlorine contents in the biomass tested are in the region of 0.054−0.327% by mass, which is much higher than that in the tested Huating coal. Sulfur contents in the biomass are also high, approximately 30−60% of that in the coal. 2.2. DTF System and Sampling Probe. A schematic diagram of the DTF system used is shown in Figure 2. The furnace is heated by a silicon molybdenum with a maximum temperature 1650 °C. There are four temperature zones that are individually controlled to an accuracy of ±1 °C. The inner corundum tube is 100 mm in diameter and 2600 mm in length. Two draft blowers are employed to transport the fuels and adjust the pressure in the furnace to reduce air leakage. A microscale spiral feeder is used to make a stable feeding of the fuels at a rate in the region of 8−20 g min−1. The main combustion temperature is maintained at 1000 °C, and the air ratio is kept constant at 1.25. Gasmet DX-4000 was used to record the gas species concentration online at the flue gas outlet. The ash deposit sampling probe is located in zone TC-4 located in the lower part of the furnace, as shown in Figure 3. Two thermocouples are welded on the surface of the sampling probe to obtain average surface temperatures, and water or air cooling is used to control the sampling surface temperature, fluctuating in the range of ±10 °C. Each sampling test is continued for 1 h. The microstructure of the ash deposits are detected using a transmission electron microscope (JEM-3010, made by JEOL, Japan). Ethanol is selected as the dispersing agent to avoid the dissolving of KCl. EDS and ED are used to give the elemental composition and crystal structure of the deposition samples. 2.3. The 25 MW Biomass Power Plant and Deposition Samples. Ash deposits from the 25 MW biomass-fired grate furnace in a power plant were sampled after one year’s continuous operation when severe deposition and slagging occurred on the second stage superheaters and where the operation flue gas temperature is around 750 °C and the surface temperature is in the range of 500−750 °C. The
Figure 2. Schematic description of the DTF facility.
2. EXPERIMENTAL SECTION 2.1. Fuels. Three kinds of biomass (wheat straw, corn straw, and wood) from Shaanxi Province in China have been used in the DTF experiments. The Huating bituminous coal is also tested to compare with biomass. The fuels used in the power plant are crop wastes mixed with a small quantity of wood as
Figure 3. Schematic description of ash deposit sampling. 12986
dx.doi.org/10.1021/ie302009m | Ind. Eng. Chem. Res. 2012, 51, 12984−12992
Industrial & Engineering Chemistry Research
Article
Figure 4. Ash deposits sampled from the 25 MW biomass-fired gate furnace.
Figure 5. TEM images for the ash depositions at different wall temperatures from straw combustion.
The microstructure and the chemical element profile along
deposits were analyzed and compared with the laboratory DTF studies. A photo showing the ash depositions inside the furnace, together with the treated ash deposits, is shown in Figure 4. The patterns of the ash deposition can be identified as three typical layers by color, i.e. from the bottom to the top: (1) the dark-red layer attached to the tube, with a thickness of
