Design and application of a novel coal-fired drum boiler using saline water in heavy oil recovery

Abstract

In this paper, the design and operation of a novel coal-fired circulating fluidized bed (CFB) drum boiler that can generate superheated steam using saline water were introduced. The natural circulation water dynamics with a drum was adopted instead of the traditional once-through steam generator (OTSG) design, so that superheated steam can be generated for the better performance of the steam assisted gravity drainage (SAGD) technology in heavy oil recovery. The optimized staged evaporation method was proposed to further decrease the salinity of water in the clean water section of the boiler. The evaporating pipes of the salted water section were rearranged in the back pass of the boiler, where the heat load is low, to further improve the heat transfer safety. A CFB combustion technology was used for coal firing to achieve a uniform heat transfer condition with low heat flux. Pollutant control technologies were adopted to reduce pollutant emissions. Based on the field test, the recommended water standard for the coal-fired CFB drum boilers was determined. With the present technology, the treated recovery wastewater can be reused in steam-injection boilers to generate superheated steam. The engineering applications show that the boiler efficiency is higher than 90%, the blowdown rate is limited within 5.5%, and the superheat of steam can reach up to 30 K. Besides, the heavy oil recovery efficiency is significantly improved. Moreover, the pollutant emissions of SO₂, NO_x and dust are controlled within the ranges of 20–90 mg/(N·m³), 30–90 mg/(N·m³) and 2–10 mg/(N·m³) respectively.

Keywords

drum steam injection boiler / natural circulation / recovery wastewater / staged evaporation / circulating fluidized bed (CFB)

Introduction

With the increasing demand for oil, heavy oil becomes an important alternative to light crude oil to keep the stable development of national economy in China. Great efforts have been made to develop advanced technologies for heavy oil recovery in recent years [¹,²], such as the steam-assisted gravity drainage technology (SAGD), which is believed to be successful on both technical and economical standpoints [³]. In such a technique, the steam with high temperature and high pressure is injected into the reservoir to reduce heavy oil viscosity and enhance heavy oil recovery. The mixture of oil and steam condensed water is recovered during this process. Correspondingly, a huge amount of heavy oil wastewater with high salinity and high temperature (usually higher than 90°C) is generated after oil-water separation [⁴].

The steam-injection boiler is used to provide the steam for heavy oil recovery. In water shortage regions, such as Xinjiang Uygur Autonomous Region of China, the treated recovery wastewater is required to be recycled in steam-injection boilers to save freshwater and to effectively utilize the remaining heat in the wastewater. Generally, distillation and reverse osmosis technologies are employed to reduce the salt content in the wastewater [⁵–⁸]. However, due to the high costs, these technologies for deep desalination of wastewater were not widely used in heavy oil recovery. According to the quality of the recovery wastewater which was roughly softened (as listed in Table 1 in Section 2), the properties of the treated wastewater have a low degree of total hardness, a low suspended impurity content, but a high salinity, especially a high content of chloride of sodium and potassium. Consequently, the quality of the treated wastewater causes a great challenge for the design and safe operation of steam-injection boilers.

The traditional steam-injection boiler is once-through type using natural gas or oil as fuel [⁹]. For once-through boilers, the steam dryness is high at the outlet of the furnace. To ensure the safe operation and avoid scaling or corrosion problems when saline water is used as working fluid, the steam dryness at the outlet of the once-through steam-injection boilers has to be limited to 75%–80%, which cannot meet the requirement of steam dryness for SAGD (higher than 90%) [¹⁰,¹¹]. Thus, an external steam-water separator is required to improve the steam dryness, increasing the cost and complexity of the system. Regarding the reuse of heavy oil recovery wastewater in steam-injection boiler, main efforts were made to improve the wastewater quality [¹²,¹³], or to study the deposition characteristics of working fluid in the heated pipes [¹⁴,¹⁵]. Compared with once-through steam-injection boilers, natural circulation boilers have more advantages when it comes to reuse the saline water [¹⁶–¹⁸]. For natural circulation boilers, the steam content in the water walls is low, so that the salt precipitated on the heated surface can be dissolved by the liquid. Moreover, due to the existences of drum and internal steam-water separation, the dryness of steam generated by the natural circulation boiler is increased to be higher than 95%. In addition, considering the high prices of natural gas and oil, it is more economic for Xinjiang Uygur Autonomous Region, where the coal resource is abundant, to use coal as fuel. Therefore, there is an urgent need to develop a coal-fired steam-injection boiler with natural circulation using the treated wastewater as working fluid, so that high dryness/superheated steam can be generated without an additional steam-water separation system.

In this paper, a novel coal-fired natural-circulation steam-injection boiler, which reuses the treated recovery wastewater as feed water, was introduced. To guarantee the safety of the hydrodynamics, circulating fluidized bed (CFB) technology was applied so that a uniform and moderate heat flux can be obtained inside the furnace with a high combustion efficiency, low pollutant emission, and fuel flexibility [¹⁹]. Besides, the key issues in the design of the CFB steam-injection boiler, including the optimization of the staged evaporation technology, the blowdown process, heating surface arrangement as well as the pollution control techniques, were presented. Moreover, based on fundamental studies and operating experiences, the standards of feed water and boiler water qualities for natural-circulation steam-injection boilers were determined. Furthermore, the engineering applications for the technology of coal-fired CFB steam-injection boiler with natural circulation were demonstrated, and the operation results were discussed.

Coal-fired natural-circulation steam-injection boiler design

The properties of the heavy oil recovery wastewater and treated wastewater in Xinjiang Uygur Autonomous Region, China were listed in Table 1. When the natural-circulation steam-water system was used in the boiler, the blowdown rate of the drum should be extremely large to maintain the salt content in the drum in an acceptable range if no special treatment was made in the boiler deign. To solve this problem, an optimized staged evaporation method was invented and the corresponding heating surface arrangement was introduced in this section. Based on both the experimental and operational results, the standard of water quality for natural-circulation steam-injection boiler was proposed. In addition, the pollutant control methods in the project was presented.

Staged evaporation technology

For a natural circulation boiler, feed water is directly fed into the drum and is then distributed to the membrane wall of the furnace through down comers. The water is heated in the membrane system and a steam-water two-phase flow is generated before it comes back to the drum. After steam-water separation in the drum, the steam leaves the drum while the separated water with salt will participate in the recirculation, resulting in an increasing salinity of the water in the drum (boiler water). Blowdown is a general method to control the quality of the recycled water inside the boiler. According to the salt balance, as shown in Fig. 1, the salinity of boiler water can be calculated by

(1)\begin{array}{l}{S}_{\mathrm{q}}\times \mathrm{1}+S_{\mathrm{g}}\times \mathrm{1}\times {\displaystyle \frac{p}{\mathrm{100}}}=S_{\mathrm{0}}\times (\mathrm{1}+\mathrm{1}\times {\displaystyle \frac{p}{\mathrm{100}}})\\ \Rightarrow S_{\mathrm{g}}={\displaystyle \frac{\mathrm{100}+p}{p}}{S}_{\mathrm{0}}-\mathrm{100}{\displaystyle \frac{S_{\mathrm{q}}}{p}},\end{array}

where p is the blowdown rate of drum; S_g is the mass fraction of salt in the boiler water (salinity of boiler water), %; S₀ is the salinity of feed water, %; and S_q is the salinity of steam, %. The subscripts g, 0, and q represent boiler water, feed water, and steam respectively. S_g decreases with the increase in p. Since the value of S_q is much smaller than those of S_g and S₀, the second term of the right hand side in Eq. (1) can be neglected.

S_{\mathrm{g}}={\displaystyle \frac{\mathrm{100}+p}{p}}{S}_{\mathrm{0}}.

To decrease the salinity of boiler water, the staged evaporation technology was proposed [¹⁷]. As shown in Fig. 2, a drum is divided into the salted water section and the clean water section. Traditionally, the feed water is sent into the clean water section of the drum. After evaporation and steam-water separation, steam leaves and water is returned to the clean water section. Then, the water in the clean water section flows into the salted water section as the feed water of the salted part. Some of the steam is generated in the salted water section as well. The blowdown process is conducted in the salted water part. The salinities of the water in the clean and the salted water sections can be calculated by using Eqs. (3) and (4) respectively.

(3)S_{\mathrm{j}}=S_{\mathrm{j1}}={\displaystyle \frac{\mathrm{100}+p}{n+p}}{S}_{\mathrm{0}},

(4)S_{\mathrm{y}}=S_{\mathrm{y1}}={\displaystyle \frac{\mathrm{100}+p}{p}}{S}_{\mathrm{0}}=S_{\mathrm{pw}},

where S_j and S_y are the salinities of the water in the clean and the salted water sections respectively, S_j1 and S_y1 are the salinities of the water in the heated pipes of the clean and the salted water sections respectively, S_pw is the salinity of boiler blowdown water, and n is the ratio of the steam provided by the salted water section to the total evaporation. The subscripts j and y present the clean water section and the salted water section respectively. The subscripts j1 and y1 are the heated pipes of the clean and salted water sections respectively, and the subscript pw represents the boiler blowdown water.

According to Eqs. (3) and (4), when p is set to be 10% and n is 15%, the salinity of the water in the clean water section (S_j and S_j1) is only four times higher than that of feed water (S₀), and the salinity in the salted water section (S_y and S_y1) is 11 times greater than S₀. Since most of the steam (85%) is generated by the water walls of the clean water section, the quality of the recycled water is improved and the safety of the heating surface is guaranteed.

To further decrease the salinities of the water in clean and salted water sections, the traditional staged evaporation technology was optimized. As shown in Fig. 2, the feed water position was changed from the drum to the dowcomer of clean water section. Besides, the outlet of the steam-water separator in the salted water section (12 in Fig. 2(b)) is connected with both the salted water section and the blowdown pipe (10 in Fig. 2(b)), and the concentrated water in salted water section was discharged from the steam-water separator in the present design. Then, the salinities of the water in the clean and the salted water sections can be calculated by using [²⁰]

(5)S_{\mathrm{j}}={\displaystyle \frac{\mathrm{100}+p}{n+p}}{S}_{\mathrm{0}},

(6)S_{\mathrm{j1}}={\displaystyle \frac{\mathrm{100}+p}{n+p}}{\displaystyle \frac{n_{\mathrm{j}}-\mathrm{1}}{n_{\mathrm{j}}}}{S}_{\mathrm{0}},

(7)S_{\mathrm{y1}}=S_{\mathrm{y}}={\displaystyle \frac{\mathrm{100}+p}{p}}{\displaystyle \frac{n_{\mathrm{y}}-\mathrm{1}}{n_{\mathrm{y}}}}{S}_{\mathrm{0}},

(8)S_{\mathrm{pw}}={\displaystyle \frac{\mathrm{100}+p}{p}}{S}_{\mathrm{0}}.

where n_j and n_y are the circulating ratios in the clean and the salted water sections respectively. Comparing Eqs. (5)–(8) with Eqs. (3)–(4), it can be found that the salinities in the clean and the salted water sections are further decreased.

Design of heating surface

The salinity of the water in the salted water section is higher, thus the heating surface should be rearranged to ensure its operating safety. In engineering applications, the middle of the drum was built to be the clean water section and its two sides were set to be salted water sections. Due to the higher salinity of water in the salted water sections, it is more suitable to arrange the evaporating pipes of salted water sections into the second pass of the boiler, where flue gas temperature is low, so that the heat transfer security of the heating surface can be improved. In addition, it is more economic for heating surface maintenance. As shown in Fig. 3, the steam-water system consists of a drum, downcomers, furnace water walls, water cooling panels and evaporating pipes for the salted water section. The water in the clean water section flows through downcomers into the furnace water walls and four water cooling panels which are installed on the front wall of the furnace. The water in the two salted water sections is heated by the evaporating pipes in the second pass.

The detailed structure of the evaporating pipes for salted water sections is presented in Fig. 4. The main part of the evaporating pipes was vertical, while the inlet section was arranged with an up dip of 7°. The operation results indicate that a dip angle of 7° can guarantee the hydrodynamic security of the boiler. Since the up dip section was used for the water inlet of evaporating pipes for salted water sections, the steam fraction in this section is low. The calculation results show that the dryness should be less than 0.08 in the up dip pipe section, so that the heating surface can be cooled successfully by the saline water and scaling can be avoided. Furthermore, superheaters were installed in the second pass to further increase the steam quality and provide superheated steam.

Standard of water quality

Appropriate water quality is of great importance for boiler safe operation. In this project, since the treated recovery wastewater with a high alkalinity and a high salinity is used as working fluid of the boiler, it is easier for the water to cause some operation problems. Meanwhile, considering the distinct operating conditions of steam-injection boilers, that the operation pressure is generally higher than 5 MPa and the steam temperature is lower than 370°C, it is necessary to comprehensively study the limitations of feed water and boiler water qualities for the novel steam-injection boilers.

Table 2 lists the feed water quality standards for traditional OTSG in different countries. Comparing Table 1 with Table 2, it can be observed that the quality of the treated wastewater can basically meet the traditional OTSG water quality standard. However, the natural circulation and the staged evaporation technologies were used in this project. Since the hydrodynamic characteristics of natural-circulation steam-injection boilers are significantly different from those of once-through steam-injection boilers, the water quality standard, including the hardness of water, dissolved oxygen, salinity and other impurity contents [²¹] should be further investigated for the coal-fired steam-injection boilers with natural circulation.

Previous study declared that the high contents of Ca/Mg ions are the main cause of scaling on the heating surface [²²], and the existences of {\mathrm{HCO}}_{\mathrm{3}}^{-}, {\mathrm{CO}}_{\mathrm{3}}^{\mathrm{2}-}, {\mathrm{SO}}_{\mathrm{4}}^{\mathrm{2}-}, and {\mathrm{SiO}}_{\mathrm{3}}^{\mathrm{2}-}have positive effects on scaling formation. Consequently, it is easier for corrosion to occur on the heating surface where scaling happens [²³]. With continuous evaporation, the salinity of water increases, the limitation for the water hardness becomes much stricter. The concentrations of Ca/Mg ions and other negative ions should be strictly limited to prevent heating surface from scaling.

Under a high pH level condition, {\mathrm{HCO}}_{\mathrm{3}}^{-} shall be decomposed into OH^– and CO_2, leading to a sharp increase of alkalinity degree. When the pH value is higher than 13, caustic embrittlement would occur, which is a great threat to the safe operation of heated pipes. The concentration of {\mathrm{HCO}}_{\mathrm{3}}^{-} in feed water should be limited within 100 mg/L by adding oxalic acid or HCl into the feed water, so that the pH value of boiler water is lower than 12. In addition, the existence of {\mathrm{Cl}}_{}^{-} in the feed water could cause pitting corrosion on the heated surface [²⁴,²⁵]. The analyses of corrosion products and the mechanism of corrosion occurrence show that dissolved oxygen and H⁺ are the major factors for {\mathrm{Cl}}_{}^{-}-related corrosion. By decreasing the dissolved oxygen and controlling the pH value in the feed water, {\mathrm{Cl}}_{}^{-}-related corrosion can be decreased. To test the corrosion behaviors of different boiler steels under the operating condition of low dissolved oxygen and high salt contents, corrosion experiments were conducted in a high-pressure reactor at the pressure of 12–15 MPa and the temperature of 325°C–345°C. The influence of water scour on the corrosion behavior of the water wall in the boiler was considered by using a rotation device to drive the tested material to spin in the working fluid. Figure 5 exhibits some of the corrosion test results in a low dissolved oxygen, high pH value and high salt content condition. The experimental results suggest that 304 L and 316 L stainless steel have a greater corrosion resistance than the other three materials. Under such a circumstance, the corrosion rate for 304 L and 316 L is lower than 0.04 mm/a which is acceptable in engineering applications. In all material cases, the corrosion rate decreases apparently as the test time is over 200 h. The major reason for this is that a protective oxide skin is generated, which prevents the occurrence of further oxidation reaction. Thus, for actual operation, corrosion can be controlled as long as the dissolved oxygen and the pH value are strictly maintained so that the life span of oxide skin can be prolonged. The dissolved oxygen in the feed water should be lower than 0.007 mg/L and the pH value should be 8.0 –10.5.

The operation results demonstrate that the existences of Na/K ions have little impact on heated pipes even when the steam fraction is high [¹⁸]. Therefore, the limitation for mineralization is not very strict. For natural circulation boilers, the drum water level shall be monitored carefully to ensure that enough water is fed to avoid the damage of waterwalls due to the lack of water. However, the high contents of silica, oil, grease and suspended impurities in the recycled water may lead to priming which have to be avoided [²⁶].

Ultimately, based on the corrosion test and operation experiences, the quality standards for feed water and boiler water of the coal-fired CFB steam-injection boilers with natural circulation are summarized in Table 3.

Pollutant control methods

The circulating fluidized bed technology is used in the present project. Due to the uniform temperature distribution and more intensive heat and mass transfer processes in the furnace, it is easy to desulfurize and denitrate in the furnace [²⁷–³⁰]. To satisfy the pollutant emission rules, the limestone injection method (in situ desulfurization), low NO_x combustion technology, selective non-catalytic reduction (SNCR) technique, and bag filter were used in the present coal-fired CFB steam-injection boiler.

For CFB boilers, the limestone which is used as sorbent can be added into the furnace together with coal to achieve satisfied desulfurization in the dense-phase zone [²⁹]. The limestone size distribution significantly affects the desulfurization efficiency [³¹]. Generally, a smaller-sized one which has a bigger specific surface area can improve the desulfurization ability. However, fine particles tend to escape from cyclones as fly ash and it is easy for coarse particles to be discharged as bottom ash. In this study, a limestone size of 0–500 μm with a 50% cut size of 140 μm is suggested [³²]. Previous studies show that the highest desulfurization efficiency can be achieved at a bed temperature of 800°C–850°C [³³]. A low bed temperature (about 850°C) of CFB boiler is appropriate for the desulfurization in the furnace. In addition, to optimize the combustion, the proportion of primary and secondary air should be properly adjusted. The ratio of the primary to secondary air is suggested to be 6:4 and should be adjusted based on the actual operation.

As for NO_x, the fuel NO_x constitutes the primary source for the combustion in a CFB boiler due to the low bed temperature. Previous research [³⁴,³⁵] indicates that many factors, such as coal types, bed temperature and excess air coefficient, are of great importance for the control of NO_x. NO_x emission increases with the increases of bed temperature, oxygen content and the secondary air. The operating condition should be controlled carefully. Besides, the metallic compounds existing in the circulating material can catalyze the reduction reaction of NO_x. The original generation of NO_x can be controlled within 200 mg/(N·m³). The SNCR method is used to further remove NO_x, in which NO_x is reduced to N₂ and H₂O by ammonia without catalyst.

Results and discussion

The coal-fired CFB steam-injection boiler with natural circulation technology has been developed and used in Fengcheng Heavy Oil Field (TG-130/9.6-M) and a steam supply station of Yucheng Thermo Ltd. (TG-130/14-M1) in Xinjiang Uygur Autonomous Region. By now, these steam-injection boilers have been in stable and economical operation. The operation results are shown in the following section.

The feed water for coal-fired CFB steam-injection boiler in the test station in Fengcheng Heavy Oil Field is a mixture of 60% treated wastewater and 40% softened freshwater, that for the coal-fired CFB steam-injection boiler in the steam supply station of Yucheng Thermo Ltd. is the high temperature reverse osmosis water. Table 4 presents the feed water qualities of the two places. By improving the feed water quality (decreasing the salt contents in feed water), the blowdown rate of the steam-injection boiler in the steam supply station of Yucheng Thermo Ltd. is controlled within 5.5%, but that of the steam-injection boiler in the test station in Fengcheng Heavy Oil Field is controlled to 8.4%, to maintain the boiler water quality within the water quality standard listed in Table 3 (according to Eq. (7)) due to its higher salinity in feed water.

The heated wall surfaces, including the surfaces of the boiler drum, water walls, and economizer, were checked after a long-time operation. By controlling the dissolved oxygen content within 0.007 mg/L in the feed water, the Cl⁻-induced corrosion is diminished. As shown in Fig. 6, the phenomena including corrosion, deposition and wall thinning did occur on the heated wall surfaces. The operation results show that the water quality standard listed in Table 3 can guarantee the safe operation of coal-fired natural-circulation CFB steam-injection boilers.

To provide superheated steam, superheaters are installed in the boilers both test stations Fengcheng Heavy Oil Field and Yucheng Thermo Ltd.. Table 5 shows theirs steam quality and operating data. Compared with once-through steam-injection boilers, the natural-circulation coal-fired CFB steam-injection boiler can provide superheated steam with a superheat of 25–30 K. The boiler efficiency is higher than 90.5%, can even reach 92.79% for the boiler in the test station in Fengcheng Heavy Oil Field, according to the China Special Equipment Inspection and Research Institute and Xi’an Thermal Power Research Institute Co., Ltd. by using the counter-balance test method.

The industrial operation data for heavy oil recovery, such as oil yield, oil-gas ratio and production- injection ratio, is collected over a long-time industrial operation, as listed in Tables 6 and 7; the corresponding data uncertainty is about 5%. Due to the increase in steam dryness and superheat, the heavy oil recovery efficiency is significantly increased. In Fengcheng Heavy Oil Field, for instance, the preheating time of SAGD in No. 18 well area is decreased by 36 days by using the steam generated by the coal-fired CFB boiler compared with that by using the gas-fired once-through boiler. During the preheating period, 1800 tons of steam is saved per well group, the total steam consumption is decreased by 191500 tons, and the costs are decreased by USD 2.7 million. The oil yield in No. 18 well area is increased by 1.9 t/d per well group, the oil gas ratio is increased by 0.026, and the water content is decreased by 2.3%, as listed in Table 6. The comparison of oil production between coal-fired and gas-fired boilers in No. 18 well area of Fengcheng Heavy Oil Field is depicted in Fig. 7. The oil production by using the coal-fired boiler is much higher than that by using the traditional gas-fired boiler.

As for vertical horizontal steam drive technology (VHSD), the oil yield is increased by 25 t/d in the test well area (No. 32 well area) by using the coal-fired CFB boiler, while the steam injection quantity is decreased by 100 t/d. The oil-gas ratio and the production-injection ratio are increased by 0.09 and 0.36 respectively (as listed in Table 7).

The design and operation pollutant emissions are listed in Table 8. The pollutant emissions for NO_x, SO₂ and dust are determined according to the methods in Refs. [³⁶–³⁸] respectively, and the measurement uncertainties are within the requirements in the standards. For the test station in Fengcheng Heavy Oil Field, the ratio of the primary to secondary air was set to be 6:4. During operation, it is found that this ratio has resulted in the fluctuation of SO₂ emission and a poor combustion stability. Based on this experience, the proportion of the secondary air is increased and the ratio is adjusted to 4.5:5.5–4:6 in Yucheng Thermo Ltd. As a result, the SO₂ emission is decreased to be lower than 50 mg/(N·m³) and the furnace desulfurization efficiency reaches to 98.5%. Appropriate methods are adopted to control combustion. The original generation of NO_x is controlled within 250 mg/(N·m³) by using the low NO_x combustion technology. The SNCR method is used in Yucheng Thermo Ltd. to further decrease the final emission of NO_x to 30–90 mg/(N·m³). The bag filter is used in the test station in Fengcheng Heavy Oil Field to collect the dust, which controls the dust emission to be within 80–300 mg/(N·m³), while the high-efficiency electrostatic bag filter is used in the test station in Yucheng Thermo Ltd., which decreases the dust emission to 2 –10 mg/(N·m³).

Conclusions

To provide high dryness/superheated steam for heavy oil recovery, to conserve water resources and to decrease costs, a coal-fired CFB steam-injection boiler which reuses recovery treated wastewater as feed water was designed, in which the natural circulation technology instead of the traditional once-through technique is used. The main conclusions obtained from this work can be summarized as follows:

The staged evaporation method was optimized in this project. Compared with the traditional staged evaporation, feed water is sent into the downcomer of the clean water section in this work instead of into the drum, and the blowdown position is changed to the outlet of the steam-water separator of the salted water section. The salinities of the water in the clean and the salted water sections are further decreased by the optimization. To ensure the security of the heating surface of the salted water section where the salinity of water is higher, the evaporating pipes of the salted water section are arranged in the second pass where the temperature is relatively low.

The quality standards of the feed water and the boiler water for coal-fired CFB steam-injection boilers are determined respectively, according to the results of corrosion tests and industrial operation. Compared with the feed water quality standard for the once-through boiler, stricter controls for dissolved oxygen, pH value and total hardness are required in the present study, while the limitation for the mineralization is more relaxed. The operation results show that the water quality standard guarantees the safe operation of coal-fired CFB steam-injection boilers with natural-circulation.

By using suitable pollutant control methods, including the limestone injection method, the low NO_x combustion technology and the SNCR technique, the pollutant emission can be limited within the allowed range.

The operation results show that the boiler efficiency is higher than 90.5%, the blowdown rate is controlled within 5.5%, and the superheat of the steam can reach up to 30 K. By using the high dryness/superheated steam, both the recovery efficiency and economic benefits are significantly increased.

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