|The 300 MW Lodi Energy Center in California is a cutting-edge power plant in the US
Scientists and engineers working to advance thermal fossil-fuelled power generation technologies are faced with a diverse range of challenges as they attempt to create ever more efficient and economic sources of power generation.
However, one of the principal challenges is to produce energy as cleanly as possible and this is a driver which is being increasingly pushed by legislative and regulatory controls.
Burning hydrocarbons in air produces a whole slew of undesirable compounds – carbon dioxide is excluded here although it is an area of significant concern with manufacturers striving to produce energy as efficiently as possible, and therefore with the lowest possible carbon intensity per MWh.
Alongside particulates, of particular concern in exhaust gases are compounds of nitrogen and sulphur with oxygen – referred to as NOx and SOx – as well as carbon monoxide and heavy metals such as mercury and selenium, all of which have deleterious environmental and health effects.
It is no surprise that equipment suppliers have been introducing technologies to address these various compounds and elements for decades with, for example, electrostatic precipitators (ESP) and Selective Catalytic Reduction (SCR) among the most well-known. What is perhaps less well known is that this process is ongoing as technology firms respond to new challenges and ever more stringent controls.
The flexibility problem
One of the most dramatic changes to have taken place within the energy sector in recent years has been the rise of variable-output renewables such as wind and solar. As the installed capacity of these types of generation technologies has grown, so too has the requirement for existing thermal generation installations to ramp output up and down in response to fluctuating demand.
Inevitably this presents challenges as so-called transient events typically require equipment to operate at below optimum efficiency and with less than ideal combustion characteristics. In the case of combined-cycle gas turbines one approach to address this has been to introduce more accommodating heat recovery steam generators (HRSGs) which allow gas turbines unrestricted ramping rates – see ‘Taking the heat’ in the September edition of PEi – which effectively minimizes the period in which emissions spike. There are other approaches though. For example, Siemens has recently introduced its so-called Flex-plant and Clean-ramp system to address emissions during transient events.
Jordan Haywood, principal engineer at Siemens Energy, explains the situation in the US: “Most environmental permits in the United States regulate on a time-averaged requirement, for example over a one-hour rolling average. When engines changed load a few times a day, these short increases in emissions during ramping had little impact, but if an engine changes load frequently, there is no time to average out the peak and there is a possibility that the engine will go out of emissions compliance. In this situation, the plant would have to stop changing load until it had run at stable load for long enough to average out the peak. The expectation is that plants will be changing load more frequently to support renewables.”
Haywood also notes the increasingly stringent nature of environmental regulation, pointing out that within the last decade startup emissions, which had previously been exempt from the regulatory profile, have now become part of the regulatory consideration of a plant’s overall environmental performance.
Similarly, explains Haywood, transient emissions were exempt from being regulated: “Now starting out in California, the transient emissions are becoming a permanent requirement, you cannot be out of permit compliance at all.”
He continues: “Several plants we’ve seen run at a stable load early in the day, and then in the middle of the day they’ll be bouncing around quite a lotà¢€¦ you could have a situation where you would not be able to meet that duty because it would be out of compliance.”
Haywood also notes that while federal regulations are not as strict as those of particular states, such rules are becoming more widespread and are now seen in states such as New York, Massachusetts, Pennsylvania and Texas. Furthermore, he says, “we are starting to see a spread to other regions, including Europe and the Americas”.
Considering the implications of exceeding emissions permits, Haywood says violating a permit limit is a matter of public record in the US and therefore could impact the perception of corporate social responsibility – but, perhaps more significantly, exceeding monthly or annual limits may restrict the operating hours that a plant is able to run, with obvious economic consequences.
Modern gas turbines which feature dry low-NOx combustion systems generally produce more CO and NOx when loads are changing. This is because the extremely lean burn approach to minimize emissions can result in combustor dynamics which can overheat and excite engine components and risk component integrity during rapid, uncontrolled changes in output.
To avoid this undesirable situation, gas turbines are typically supplied with additional fuel to the pilot during load changes to improve stability. The increased pilot fuel results in higher NOx emissions, while excess fuel can also result in incomplete combustion and therefore an increase in carbon monoxide concentrations in the exhaust.
Controlling NOx through an SCR system requires the injection of ammonia and precise control of the ammonia-to-NOx ratio is necessary to effectively limit NOx emissions without introducing excess ammonia. Injecting too much ammonia results in unreacted ammonia passing into the exhaust stream, known as ammonia slip. This is evidently undesirable and is typically regulated in the same way that other compounds are monitored and controlled.
|Relationship between NOx and CO in gas turbines as related to combustion temperature
Credit: Ansaldo Energia
Precisely controlling the volumes of ammonia required in the SCR depends on knowing the instantaneous concentrations of NOx in the exhaust stream. With the engine at stable load this can be accurately measured, but some minutes are required to stabilize the volume of ammonia injected and the NOx in the exhaust.
During transients the measurement system is typically not sufficiently accurate to determine exactly how much ammonia should be injected. As a solution, Siemens developed its Clean-Ramp technology which uses patent-pending features to change how the gas turbine is controlled so the system can accurately predict what is going to happen when a load change is requested.
Haywood says: “It works with the control algorithm of the gas turbine so it modifies how the gas turbine is controlled while it changes load, and it integrates that with the ammonia system for the SCR. It’s an integration algorithm that makes both systems talk to each other and maintains the stack emissions and the ammonia slip while the engine is changing load.”
Currently the company is introducing this technology on new plant designs – there are some limitations on possible retrofitting to some of the existing plant designs – and there are a number of projects currently operating which feature this technology. Among those is NRG Energy’s 550 MW El Segundo Energy Center in California, which was commissioned in September 2013.
More recently, Siemens commissioned its first Flex-Plant in Texas with the 758 MW Panda Temple Power project claiming a large operating window from low plant turn-down to high plant output with CO emissions less than 10 parts per million (ppm), and NOx emissions of less than 2.0 ppm.
A complex balancing act
Within a combustion system there is typically an inverse relationship between the various exhaust gas components. Higher temperatures imply a higher thermal efficiency but also tend to increase the production of NOx when burning gas. Conversely, lower temperatures suggest lower efficiency and lower NOx, but also may result in incomplete combustion and therefore higher emissions of undesirable CO. Achieving an effective balance between these competing factors is the central challenge many OEMs are attempting to address.
|Siemens commissioned its first Flex-Plant in Texas with the 758 MW Panda Temple Power project
Credit: Panda Power Generation
In its approach, Ansaldo Energia has launched its ProMeteo product to control carbon monoxide emissions at low loads.The patented operating management programme analyzes the conditions of the major plant components such as the gas and steam turbines, boiler and condenser as well as an intake air conditioner fitted with an anti-icing heater and cooling systems.
A processing unit is built into the power plant’s automation system and coupled to CO and NOx monitoring equipment, providing feedback which regulates the degree of heating or cooling of the inlet air.
At low loads, when combustion is typically over-cooled, producing carbon monoxide, the heater is used to reach the desired combustion conditions. Ansaldo says its ProMeteo method reduces the fuel consumption required to achieve emission-compliant combustion and extends the minimum required load to operate the gas turbine within emissions limits.
Used primarily at low loads, the method favourably changes the conditions in the combustion chamber, the company says, by modifying the flow rate and temperature of the surged air.
It argues that its method is particularly effective at reducing emissions, especially when gas turbines operate near the minimum technical load.
Similarly, reciprocating engine manufacturer Wärtsilä is also focusing on addressing the relationship between thermal efficiency and NOx.
Wärtsilä Power Plants company spokesman Jussi Laitinen explains: “If you want to cut NOx emissions on a reciprocating engine, efficiency goes down and unburned hydrocarbons increase due to colder combustion conditions. This means the CO2 footprint also grows. Which is better for the environment? You should look at the big picture and not just a single type of emissions. A golden middle way has to be found.”
Wärtsilä believes it has made progress in this regard: “For us this has been a key area of R&D for quite some time. We have actually been able to push down the seesaw on both ends: we have been able to improve efficiency and keep the emissions at low level at the same time.”
Wärtsilä is also considering emissions within an overall energy market context. Laitinen says: “The efficiency of our gas engines has been improved by about 4-5 per cent in 10 years and is now about 48 per cent.”
He adds: “If you want more than 15 per cent of wind and solar of all the electricity, you need fast backup.” Internal combustion engine (ICE) plants are challenging gas turbines in this application. ICEs are faster, can start and stop many times per day, and are better at part-load efficiency. Wärtsilä already has wind-balancing power plants in the range of 200 MW in Texas and Colorado, for example. The company is also working to improve combustion characteristics further, recently launching a new research programme with MAN Diesel & Turbo.
The HERCULES-2 research project is aimed at minimizing emissions in large engines, and although principally directed at marine applications the findings effectively apply to all reciprocating engines. It follows the decade-long initial HERCULES programme, which was initiated in 2004 as a joint venture by the two major European engine manufacturing groups.
Pending approval under the Horizon 2020 EU Framework Programme for Research and Innovation, the HERCULES-2 project is aimed at developing fuel-flexible engines focused on four areas of integrated R&D including the work programme WPG IV – a near-zero emissions engine.
The new project aims to achieve significant reductions in fuel consumption and exhaust emissions and includes several full-scale prototypes. The research consortium is made up of 32 partners, of which 30 per cent are industrial and 70 per cent are universities and research institutes.
Aiming for ultra-low exhaust emissions, including a 70 per cent reduction of NOx and a 50 per cent reduction in particulates by 2020, the programme targets the development of engines with extreme operational pressure and temperature parameters. It considers the thermo-fluid-dynamic and structural design issues including friction and wear, as well as combustion, air charging and electronics. To achieve the emissions target, combustion and advanced after-treatment methods will be concurrently developed, a statement says.
Cleaning up coal
Compared with relatively clean-burning gas turbines and engines, coal-fired boilers are a considerably more challenging emissions environment and consequently are the subject of considerable industry research and development. Despite the rise of renewables, there is little doubt that coal will continue to deliver a significant proportion of the world’s electricity and, as such, they too are becoming subject to increasingly stringent emissions control regulations.
One trend has been the recent move to convert existing coal-fired assets into biomass-fuelled stations, for example as demonstrated by Drax, the UK’s single largest power station.
Indeed, according to recent figures published by the UK government in the second quarter of 2014, bioenergy accounted for a record 5.6 TWh of the country’s electricity generation, around 7 per cent. This is an increase of 8.8 per cent compared with a year earlier, due mainly to the biomass conversions at Drax and Ironbridge.
Dr Guisu Liu, director of technology at Mobotec, points to the company’s expertise in computational fluid dynamics (CFD) and a number of its products which support optimum combustion both in fossil-fired applications and in biomass conversion.
Over the last two decades, Mobotec has completed 14 biomass conversion and co-firing projects across Europe and conducted four biomass conversion feasibility studies for larger boilers. For instance, in 2005 an early biomass conversion was carried out on a 240 MWt, tangentially-fired boiler in Helsingborg, Sweden.
Its principal product in this application is the Rotating Opposed Fired Air (ROFA) system which is designed to optimize combustion, minimizing emissions of CO and, more importantly, NOx. NOx emission from 100 per cent biomass firing is significantly reduced in comparison with baseline coal-fired operation, predominantly due to low nitrogen biomass fuel along with the combustion staging afforded by the ROFA system. With low nitrogen biomass fuel and ROFA installation, NOx emissions are expected to comply with the EU Industrial Emission Directive (IED) limits of 200 mg/Nm3 which are due to come into effect in Europe from January 2016.
Similarly, another Mobotec product, Rotamix with Chlorout chemical injection, reduces fouling and corrosion risk in biomass conversion applications. These retrofit technologies utilize specially-designed and high-kinetic-energy air jets to improve furnace mixing for combustion optimization while achieving claimed lower NOx levels. The locations of the air nozzles are determined and optimized through advanced CFD modeling, while emissions reduction agents such as urea can also be injected deeply into flue gases through these nozzles to improve chemical utilization.
Babcock and Wilcox’s (B&W) Power Generation Group is also working to optimize coal-fired boiler combustion to minimize emissions, in particular looking at optimizing the interactions between the various emissions control parameters and systems.
Kip Alexander, vice-president of technology at Babcock & Wilcox Power Generation Group, explains, citing as an example the injection of powdered activated carbon (PAC) or brominated PAC as an approach to reducing mercury emissions in power plants. He says: “We are worried about heavy metals and waste water, sometimes they are pushing and pulling in different directions for the most optimum solution, so we’re starting to focus on how we work more efficiently as a system.
“We’re doing some interesting developments on additives that can either reduce the need for PAC and brominated PAC or, in some cases, eliminate it. There are some interesting things to minimize selenium in waste water and minimize the need for bio reactors.”
One principal area of research is in controlling the pH and oxidation reduction potential (ORP) in the scrubber where the partitioning and oxidation states of selenium and mercury are influenced by a combination of these two factors.
“We discovered from our research that the upstream unit operations are greatly affecting the downstream, especially during load changes. We’re trying to dampen that effect by integrating all the systems so that we can maintain a lower ORP which would be ideal for for selenium and mercury speciation in the wastewater, along with other parameters impacting downstream treatment of the waste water,” says Alexander.
The aim is to orchestrate individual environmental control unit operations to limit the use of chemical additives for emissions control to a minimum, thus lowering operational costs. Similarly, B&W is working on improving the efficiency of the electrostatic precipitators with a new three-phase power supply and new electrode geometry.
Looking ahead, Alexander says the company is anticipating the launch of this new software and hardware optimization platform during 2015. Services will be also be provided as needed for each facility’s unique characteristics – depending on the existing emissions control equipment, the fuel used and the operating constraints of each particular plant. Alexander adds: “We have not released the trade name for this product yet. But we are just starting to market it to potential customers.”
For the future, it is clear that regulatory regimes will certainly become more stringent, demanding that power plant operators and the technologies they rely on to control emissions become more effective and more efficient over time. It’s equally clear that changing energy sector dynamics are presenting further challenges to meeting emissions standards. Nonetheless, it seems that in addressing these challenges manufacturers are determined to be at the cutting edge of technology in the drive towards emissions-free power.
David Appleyard is a freelance journalist specializing in the energy and technology sectors
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