F. Bassetti & A. Carrea, Magaldi Power SpA, Italy
As an alternative option to the traditional wet bottom ash system (WBAS), the MAC system is a well proven technology for the dry extraction of bottom ash from pulverized coal (PC) boilers of any size and burning any type of coal. The MAC system, which was developed by Magaldi in the early 1980s, is now one of the most referenced systems on the market, with more than 100 units in operation.
One of the most important effects related to the use of the MAC dry bottom ash technology is its impact on boiler efficiency. When a MAC system is used instead of a WBAS, the overall efficiency of the boiler has been demonstrated to improve, primarily because of the use of air as the ash cooling medium rather than water.
To evaluate the overall impact of the MAC system on the boiler efficiency, Magaldi decided to use a calculation-based method that strictly adhered to ASME PTC – 4 1998 Fired Steam Generators, which is the widely accepted performance test code for determining boiler efficiency.
This article briefly illustrates the basis of the calculation method used and presents the main results, using formula and values specified in ASME PTC 4 – 1998 to compare all relevant heat losses and credits obtained when a boiler is equipped with a MAC system rather than a WBAS.
Although differences in all heat losses and credits are evaluated, particular attention was paid to three process parameters that have the greatest influence on efficiency, namely the impact on heat losses in the lower part of the boiler, the impact of cooling air on air heater performance and the impact of the MAC system on unburned carbon in the bottom ash.
Ash cooling process
In a typical arrangement of a WBAS, submerged chain conveyor (SCC) type, located below the boiler, a hydraulic seal prevents any air entering the boiler throat. There is no difference in terms of boiler efficiency between a WBAS SCC type or an impounded water hopper (IWH) type. Figure 1 focuses on the lower part of the boiler, where bottom ash crosses the boiler throat at quite a high temperature and falls into the water. This bottom ash then leaves the SCC wet and at a low temperature.
Figure 1: In a WBAS, submerged chain conveyor (SCC) type, the ash leaves the SCC wet and at a low temperature
Some water flow in and out from the SCC is necessary to keep the water in the pit at a suitable temperature (approximately 50 °C). A small amount of water evaporates and infiltrates the furnace, and this water evaporation rate can be estimated at around 7.5 per cent of bottom ash rate (BA).
The situation is different in a MAC system, where the bottom ash is cooled by MAC cooling air (MCA) entering the MAC extractor due to the furnace negative pressure through properly sized air intake valves (Figure 2). The system is designed to maximize the counter-current bottom ash cooling. Following the air/ash heat exchange the cooling air enters the furnace through the boiler throat at quite a high temperature.
Figure 2: The MAC system, where the bottom ash is cooled by air, has been designed to maximize counter-current cooling
It has been demonstrated by tests performed in units retrofitted with a MAC system that the cooling air contributes to the combustion process in exactly the same way as the other combustion air provided that the MCA is restricted within certain limits. The ash discharge temperature from the system varies on a case by case basis as a function of the bottom ash rate, the cooling air rate, the MAC system geometry, ash grain size distribution and other secondary factors. Bottom ash cooling can be completed in the downstream part of the MAC system (post-cooler, contact cooler, etc) as required in each case. The counter-current heat recovery continues in this downstream part of the MAC system.
Parameters influencing boiler efficiency
As outlined above the MCA mainly impacts on three boiler parameters, namely unburned carbon (UBC) in the BA, heat recovery in the lower part of the boiler and flue gas temperature. These in turn influence overall boiler efficiency.
The MCA enters the boiler throat at a quite high temperature (normally between 400-500 °C) because of the heat recovered by the bottom ash cooling. This high temperature air creates a hot and oxidizing atmosphere in the lower part of the boiler which produces a strong reduction in UBC (normally in the order of 80-90 per cent). Such UBC reduction has been confirmed by specific site tests by measuring UBC content from bottom ash samples before and after the retrofit of a WBAS with a MAC system. This is clearly a positive contribution to the boiler efficiency increase, since the heat loss associated with UBC in the bottom ash is significantly reduced.
In a WBAS, the energy crossing the boiler throat (both as radiation flux and as ash heat content) is dumped in the water, resulting in a significant heat loss. In contrast, with a MAC system the MCA returns a large part of the energy crossing the boiler throat to the boiler because of the ash/air heat exchange reducing the heat loss from the lower part of the boiler. Again this makes a positive contribution to increasing overall boiler efficiency.
Finally, as mentioned previously the MCA makes up part of the combustion air so consequently the combustion air to the burners is correspondingly reduced. At the same time, this reduces the air crossing the air heater, and because of this the air heater sees the same volume of flue gas (valid in first approximation) but with a reduced air rate, which produces an increase in the flue gas exit temperature. Contrary to the two process parameters above, this will make a negative contribution to the increase in boiler efficiency.
Having said that, if you look at the aggregate effect of MCA on these three process parameters, the impact of a MAC system on overall boiler efficiency is always positive compared to a WBAS.
However, because of the difficulty in predicting the increase in the flue gas temperature, coupled with healthy scepticism, some clients have expressed concern relating to this statement. Thus, Magaldi decided to respond to this concern by using a rigorous method in compliance with ASME PTC 4 – 1998 Fired Steam generators performance test code to demonstrate that the MAC system did indeed improve overall boiler efficiency.
Heat losses and heat credits
The use of ASME PTC 4 – 1998 as a method to compare two different configurations, i.e. WBAS and MAC, required a completely new formulation of the problem by calculating all boiler parameters affected by the change from a WBAS to MAC technology, as indicated in the Chapter 5.14 of the code.
In order to calculate the boiler efficiency, all pertinent heat losses and heat credits had to be considered as per the ASME code. The two areas most affected by the introduction of the MAC technology are the lower part of the boiler and the air heater. The physics of the dry technology had to be duly introduced into the ASME code framework.
The calculation was further complicated by the need for iterative calculations because of two reasons.
Firstly, the prediction of the air and flue gas temperature at the air heater outlet. This prediction had to be performed by running the air heater code in two different situations. After a number of checks with the results of the air heater code, a correlation was found to forecast the air heater exit temperatures, which corresponded with the output of the air heater code.
Secondly identifying the basis of the right comparison, which was found to be the same ‘gross power output’: assuming there is an efficiency improvement with the MAC system the coal rate to the boiler will be lower. Although the effect is small it has important downstream implications, i.e. less coal means less combustion air and flue gas, which in turn reduces the pressure drop in the main ducts and pressure differential between the air heater, and consequently reduces the air heater leakage. The latter contributes to a further reduction in the air to the air heater, and so on, which partially reduces the negative impact of the increase in the flue gas temperature.
As regards heat recovery in the lower part of the boiler, it should be noted that while ASME PTC 4 – 1998 provides specific information and rules to evaluate the ‘ash pit loss’ for a WBAS, there is no guidance on how to perform the same balance in the lower part of the boiler fitted with MAC technology. However, by following the spirit of ASME PTC 4 – 1998, the ash pit loss in a MAC installation can be achieved by outlining a boiler boundary as illustrated in Figure 3 and performing a heat balance calculation as follows: ash pit lost = ash sensible heat + heat dispersion – air sensible heat.
Figure 3: Parameters required for evaluating the ash pit loss in a MAC installation
It should be noted that in an energy balance comparison it is not only boiler efficiency that is affected by a switch from a WBAS to a MAC system. Boiler auxiliary power consumption is also affected. In fact because of the change in the coal rate, the combustion air rate and the flue gas rate and temperature the power consumption of all the running equipment in these lines is affected.
Confirming boiler efficiency increase
Conducting the above mentioned calculations is a long and time consuming process so Magaldi devised a spreadsheet containing relevant instructions and illustrations of the formula utilized. Such a tool is important not only because makes it easier to evaluate what the efficiency improvement and the auxiliary power saving can be, but it is possible now to simulate different operating conditions (variations in bottom ash rates, UBC content, cooling air rates, etc) and find what is the MCA value that maximizes the improvement, while also optimizing the ash cooling process.
By rigorously following the ASME PTC 4-1998 code when calculating boiler efficiency improvement through the substitution of a WBAS with a MAC system Magaldi has been able to confirm that its MAC dry technology produces an increase in overall boiler efficiency.
In Magaldi’s experience, the boiler efficiency improvement seen by switching from a WBAS to a MAC system normally falls within the 0.1-0.5 per cent range, depending mainly on the coal quality. Normally, poor coals give better boiler efficiency improvements because of the higher ash quantities generated and relevant higher heat losses associated to them when using a WBAS.
The results show that the improvement of boiler efficiency can be expected to range between 0.10.2 per cent for normal bituminous coals, while with high ash content coals an improvement in boiler efficiency of up to or even higher than 0.5 per cent is achieved.
As a sample calculation, two typical units (one burning typical Indian coal [Plant A] and one burning typical imported coal [Plant B], have been investigated and the results are shown due to space constraints only the most significant values are shown. In the table of results, the positive or negative effect on energy saving is shown for each heat loss and credit. The overall efficiency improvement is indicated in the final row.
The availability of a spreadsheet that easily performs all the calculation, including iterative computation due to backwards effect in some recalculated boiler parameters, has shown efficiency improvement better than expected and discloses auxiliary power saving too.
Further, by using this calculation tool it is now possible to quickly indentify the MCA value that maximizes the boiler efficiency improvement. In this way, the boiler and the MAC system can be considered integrated, creating a single system, whose overall performance can be optimized in the design phase contributing to an important efficiency recovery.
Relevant savings in terms of coal rate reductions, auxiliary power reduction, as well as other environmentally important parameters like carbon dioxide emissions and cooling water savings can be easily calculated to quantitatively appreciate, within the framework of ASME PTC4-1998, the major benefits of the MAC technology.