Those of you who have read my blogs in recent years will have noticed frequent comment and admiration for the innovative work that has been undertaken by Professor Feng Weizhong, now an independent consultant engineer, with his team from the Shanghai Shenergy Power Technology Co. Ltd.
Together they have achieved significant advances in coal power plant operation performance as they have taken forward several design concepts to improve energy and environmental performance. As you might expect, this trend is continuing and I am privileged to update their story.
Their first major achievement was to uprate the 2×1000 MWe units of the Waigaoqiao No. 3 ultrasupercritical (USC) coal power plant in Shanghai. Operations began in 2008 when annual average unit net efficiency was some 42.7% net LHV basis, with plant capacity factors of 75%. Professor Feng and his team adopted a comprehensive system optimisation approach. This comprised an assessment of the technical and economic feasibility of countering losses in efficiency due to equipment and operational limitations followed by the upgrade of many individual components and systems. By 2011, this efficiency had been increased to some 44.5% (net, LHV basis), with plant capacity factors of 81%. Equally importantly, the operators of the power plant ensured that conventional emissions (SO2, NOX and particulates) are well below the strict standards applicable for coal-fired units in China.
Both Waigaoqiao No. 3 units are of a single reheat design. After the steam has passed through the boiler and the steam turbine it passes through an additional boiler and back through the turbine. This reheat loop increases the overall efficiency by several percentage points and reduces coal consumption although the additional equipment required raises the capital costs. There has been interest in pushing this concept further by introducing a double reheat design, which offers the prospect of a further one to two percentage points increase in efficiency compared to a single reheat system although this too requires additional boiler, turbine and steam piping components. This offers the challenge to boost the efficiency of the most modern single reheat units without the need and associated costs of a double reheat design.
The opportunity arose last year when the China Resources Power Caofeidian 2×1000 MWe USC units were put into operation in April 2019. Professor Feng and his team again applied their skills and expertise to improve the design. This Caofeidian plant became the first one in China to incorporate 3 LP casings for the steam turbine, thereby achieving a 3.3 kPa ultra-low back pressure, to take advantage of its low temperature cooling seawater as the plant is located at the coast. This plant now has a design net efficiency of 46.7% under rated condition, which is even higher than that of the current double reheat units in China. Moreover, after one year’s operation the operational data indicate that the annual average net efficiency can reach 45%, which is some 0.5 percentage points higher than that of Waigaoqiao No. 3 units. Besides the higher efficiency, the Caofeidian plant also represents an excellent return on investment since the capital cost is about 85% of that for the 1000 MWe double reheat plants.
The Shenergy team is applying the lessons learned from that innovative work to develop and demonstrate the next generation advanced USC technology, which is being built at the Pingshan Phase 2 site in Anhui Province.
Indicative layout plan of the Pingshan Phase 2 double reheat cross compound unit (Mao and Feng, 2017)
The aim is to determine and implement overall design changes for a new power plant, which incorporates all the design and optimisation improvements that were made on the Waigaoqiao No. 3 1000 MWe units, together with additional innovative components. It comprises a 1350 MWe double reheat USC coal fired unit with an adapted steam turbine layout. In this arrangement the turbine is split into two trains. The front train comprises the high-pressure turbine (HP) and intermediate pressure turbine (IP1) coaxial with one generator. This is mounted on a high positioned platform near the outlets of the tower type boiler steam headers, which is around 80-85 m above ground level. The rear train, which consists of the two IP2 turbines and the three LP turbines coaxial with another generator as the rear unit, remains in the conventional position, some 17 m above ground level. This approach minimises the lengths of the main steam pipe, cold reheat steam pipe, hot reheat pipes and the second cold reheat steam pipe. The shorter pipework represents a significant cost saving and reduces the pressure drop and temperature loss of steam from the boiler. Its steam conditions are 32.5 MPa/610/630/623°C and the calculated efficiency is 49.6%. There is close cooperation with Siemens, GE, SEC and the East China Electric Power Design Institute. The Pingshan Phase 2 plant has been designated as a national demonstration project, which is expected to be the world’s most efficient coal-fired plant when it enters service in 2020.
Professor Feng and his team are also looking at the refinement of this concept to limit piping on a smaller (660MWe) unit that will include five casings (1HP + 1IP1 + 1IP2 + 2LP), which can be accommodated on a single (elevated) shaft. Design work is currently underway on the first planned plant to use an ‘all-casings-elevated’ configuration, combined with the latest versions of Feng’s advanced energy technologies. Work to date suggests that net efficiency of this new design is very promising with an estimated performance that should surpass that of Pingshan II.
To put all this into context, during the last two decades China has seen a massive surge in GDP as it has increased its industrial output on the back of a rapidly growing coal-based power generation fleet. This was achieved through very focused policy relevant initiatives and tight regulations that saw China’s coal fleet transformed with rising operational efficiencies through the introduction of larger scale units with higher steam temperatures and pressures, together with the introduction of state-of-the-art control systems for very low conventional pollutant emissions. The introduction of higher steam temperatures and pressures led first to operation at supercritical (SC) and then subsequently with USC conditions. The SC units could boost efficiencies from 39% to about 41%, while USC units have achieved efficiencies that have steadily increased from just above 42% to about 44% for single reheat and about 45% to 46% for conventionally designed double reheat (net, LHV basis). The current approach is to install plants that typically comprise 660 and more usually 1000 MWe USC units with state-of-the-art conventional pollutant control systems. These are commonly known as High Efficiency Low Emissions (HELE) plants.
The overall coal power capacity is close to 1100 GWe and now comprises a mix of 300-600 MWe subcritical units, 600 MWe SC and 660-1000 MWe USC units. A 300 MWe subcritical unit can at very best achieve close to 38% efficiency while a state of the art 1000 MWe USC unit can have an efficiency close to 46.7% (net, LHV basis). The Chinese Government has continued its strict approach to increasing efficiency. Depending on the technology mix within each power company fleet, this is likely to require an increase in the proportion of zero carbon emissions sources such as renewables and nuclear together with steps to improve the average emissions profile of the coal plants. The introduction of drivers to encourage closure of the older of the 300 MWe subcritical units has been matched with moves to ensure all new plants are USC HELE units. These include the setting of regulations that require each power company to achieve a tough efficiency standard for its overall power fleet so limiting CO2 emissions per unit of power generated.
In addition, the Chinese Government requires that by 2020 all the coal-fired power units should achieve an annual average unit net efficiency (UNE) greater than 39.6% (LHV). This is designed to push the lower efficiency units off the grid. While the SC and USC units can readily achieve this target, for most subcritical units this will not be possible. The latter represent a significant part of the overall coal power fleet since their operational capacity is about 350 GWe, accounting for close to 32% of the total installed coal-fired power capacity. These units have typical steam parameters of 16.7MPa/538/538oC in the size range 300 – 600MWe. The heat rate of a typical subcritical turbine is greater than 8000 kJ/kWh at THA (turbine heat acceptance) condition, which equates to an annual average UNE below 37% (LHV). The ages of such subcritical power plants vary, with the newer units being less than fifteen years old, having been built just before the policy driven switch to SC and the USC options. There is an incentive to upgrade these units, to achieve cost effective higher efficiencies, since their expected operational lifetime could be some 40 years.
In order that this incentive can benefit China, Professor Feng has turned his attention to providing a cost-effective high temperature retrofit for those existing 300/600 MWe subcritical coal power units, which will result in a significant efficiency improvement that can be achieved at an acceptable investment level. His approach has been to increase the maximum temperature of the main and hot reheat steam from 538/538°C to 600/600°C, while keeping the steam pressure unchanged. Together with the temperature improvement modification, other energy-saving and improvement technologies have also been included so that the unit efficiency can be further improved. The expectation is that the upgrade will enhance the unit’s power output efficiency to 42.9% for the 300 MWe units, and even higher for the 600 MWe ones, reduce its emissions by more than 10% and extend its overhaul interval from six to 12 years. Coal consumption will be lowered by more than 10% to under 290 g/kWh, which is close to the USC level. In overall terms, such a modification will increase the generation revenue of the plant, help reduce performance degradation so lowering maintenance costs significantly while improving the unit’s flexibility and availability.
To turn these innovative technological ideas into engineering reality, Shanghai Shenergy Power Technology Co. Ltd. signed a contract with China Resources Power for the demonstration of an upgrade of a 320 MWe subcritical unit at Xuzhou coal power plant, Jiangsu Province, China. This had typical steam conditions of 16.7 MPa/537°C/537°C and began commercial operation in June 2004.
The basic design of the modified plant was completed by the end of September 2018, while construction drawings were essentially complete by the end of November 2018. The construction of the modified components was started during December 2018 and completed in June 2019. While the retrofit is comprehensive, involving the boiler, turbine, piping and other related systems, the overall principle was to change as little as possible to minimise costs while ensuring the operational target will be achieved. For example, the boiler steel structure changed little and the height of the boiler was unchanged. The total costs were about 1/3 of that of a new build unit.
Siemens signed an agreement with Shanghai Shenergy Power Technology to implement a high-temperature subcritical upgrade for the 320 MWe steam turbine unit at the Xuzhou Power Plant. This included adapting the control stage, incorporating advanced blade designs, plus ensuring additional steam extraction for the high-pressure pre-heater. The boiler modification included some innovative items so that the 600/600°C condition could be achieved with low construction cost. The required modifications to unit 3 were completed in August 2019, after which a 168-hour trial operation was undertaken.
No.3 unit at Xuzhou Coal Power plant, Jiangsu Province (Shenergy Power Technology 2019)
Steam turbine upgrade underway (Shenergy Power Technology 2019)
The following tables provide a summary of the key formal testing results, with emphasis on efficiency and environmental performance at various operational unit loads.
Formal test results for the High Temperature Retrofit Project undertaken on Unit 3
of Xuzhou coal power plant, as provided by GE and Siemens (Li 2020)
|Highest boiler efficiency||94.32%|
|Lowest net coal consumption||281.8g/kWh|
|Highest net unit efficiency||43.59%|
|Unit load (%)||100||75||50|
|Boiler efficiency (%)||94.32||94.07||93.92||93.95||93.34||93.25|
|Turbine heat rate (kJ/kWh)||7463.3||7456.5||7573.6||7540,3||7886.6||7887.5|
|Total unit power (MW)||320.8||319.6||233.1||243.0||160.7||162.4|
|Auxiliary power (MW)||13.9||13.8||11.5||11.8||9.5||9.4|
|Net unit power (MW)||307.0||305.8||221.6||231.3||151.1||153.1|
|Net coal consumption (g/kWh)||281.8||282.1||288.7||287.0||304.6||304.3|
|Net efficiency (%)||43.59||43.54||42.55||42.79||40.32||40.36|
Formal test results for the High Temperature Retrofit Project covering load turn down ratio undertaken on Unit 3 of Xuzhou coal power plant, as provided by a domestic test institute authorized by the power grid (Li 2020)
|Item||Turn down ratio as required by the power grid||Test results|
|Lowest stable load||20% load (target for 2030 as proposed by the NEA)||19.39% (11 years before the National Energy Administration requirement)|
|Time from 50% load to lowest load (hours)||<1.5||1.28|
|Duration of lowest load (hours)||>4||4.08|
|Time from lowest load to 50% load (hours)||<1||0.67|
Formal test results of emissions at 19% load as provided by a domestic test institute authorized by the power grid (Li 2020)
|Pollutant||National requirement||Level achieved at 19% load|
These results show that the modifications have increased efficiency, improved environmental performance, while achieving a very good load following capability. Thus;
- Before retrofit, the net coal consumption for #3 unit of the Xuzhou Power Plant was 318 g/kWh. The retrofit target was 287 g/kWh, but this level was improved upon to give a formal test result of 282 g/kWh. In line with these changes, the operational cycle efficiency improved from 38.63% to 43.56%, an increase of almost five percentage points. The new level is equivalent to that of an USC unit. If these improvements are applied to the large number of 300-600 MWe coal power units, this will significantly boost the national efficiency of the global coal fleet, resulting in a corresponding reduction in coal use and lowering of carbon emissions.
- After retrofit, the turndown ratio improved from 50%-100% with low efficiency and high emissions to 19%-100% with high efficiency and ultra low emissions. This modified unit successfully completed the National Energy Administration proposed certification test on deep load-regulation and achieved a lowest stable load of 19%. In addition, the unit has a much shorter response time from low load to full operation compared with the 600-1000 MWe SC and USC units, due to the preservation of the previous boiler drum and the adoption of an upgraded turbine control stage. Consequently such upgraded units will provide a very flexible means to balance and stabilise the grid, both in terms of the ability to maintain a low load (<20%) and a fast response to load following demands due to the need to integrate intermittent renewable energy options onto the grid.
- Overall conventional pollutant control is maintained at all conditions, with emissions well below the strict national standards. This includes effective operation of the SCR NOx removal system at a minimum load of 19%, which compares favourably with the fact that the SCR had to be shut down when the unit was operating under 50% load before retrofit.
- The key modifications include replacement and changes to the high temperature parts of the unit, such as the boiler superheaters and reheaters, steam pipes between the boiler and the turbine, and the turbine HP and IP. Consequently the unit can safely extend the operational time prior to its next major overhaul and service.
- This upgrade approach represents an excellent return on investment as the cost is about one third of that for building a new unit, while achieving so many advantages.
In summary, this innovative approach in China has provided the basis for major upgrade of the 300-600 MWe subcritical coal power plant units, which represent about 32% of the coal power fleet capacity in China. These will achieve not just a reduction in carbon emissions but will also ensure better use of fossil fuels with much improved environmental performance, in line with the aims of UN Sustainable Development Goals.
My thanks to both Prof Feng Weizhong and Mr Li Li for their continued cooperation and assistance. I look forward to hearing and writing about their ongoing developments in the coal power sector.