|Opportunities in Industrial Boiler Efficiency|
|Written by John Alleman|
|Tuesday, 08 June 2010 10:04|
Opportunities in Industrial Boiler Efficiency by John Alleman
For many food processors, the boiler is the most energy-intensive piece of equipment in the plant. Even with the current recession driving natural gas costs down to about $0.35 per therm from last year’s high at about $1.10 per therm, there are many viable and cost-effective opportunities to increase boiler efficiency within an acceptable payback period.
A 2005 paper published by the Oak Ridge National Lab (ORNL 2005) found that boilers in the food processing industry are:
Accordingly, this article will focus on energy efficiency methods for small (300 hp or less) firetube boilers. For the purpose of savings analysis, let’s assume that this typical 300 hp boiler operates at an average 50% maximum load, 5 days per week, two 8 hour shifts per day, 50 weeks per year and operates at 1.5% low flame for the remainder of the year. Thus, natural gas use is about 208,000 therms per year.
First it’s important to discuss where the majority of energy flows in and out of boiler systems. Figure 1 below illustrates where energy flows in a typical 300 hp firetube boiler.
Figure 1: Energy Balance of a Typical Food Processing Boiler
Economizers & Air Pre-Heaters
The energy lost in the flue represents the largest opportunity for improvement, although the associated capital costs are comparatively high. Economizers are essentially cross flow heat exchangers mounted in line with the flue that use flue gas to pre-heat feed water prior to pumping it into the boiler. A typical economizer installation includes a high-pressure economizer, followed by a low-pressure economizer, which can also be followed by an air pre-heater which uses input combustion air as the cooling medium.
A good rule of thumb is that a 40°F reduction in flue gas temperature will result in a 1% improvement in efficiency. Typical flue gas outlet temperature is about 360°F for 300 hp boiler, at 125 psig steam pressure, and 50% capacity. Accordingly, a typical high-pressure economizer can reduce flue gas outlet temperatures by 100 to 120°F, thus improving efficiency by about 3%. With an additional low-pressure economizer plus an air pre-heater, it’s possible to gain about 2% additional efficiency.
Most older boilers have a single dampener positioning motor with linkages to control air and fuel amounts. As a result, excess oxygen has to be kept at 4-7% to be efficient over a wide range of operating conditions. A good rule of thumb is that every 2% of excess oxygen reduces boiler efficiency by 1%.
Parallel positioning uses separate motor actuators for air and fuel and can typically keep excess oxygen within 2-5%, thus achieving a 2% decrease in excess oxygen.
If the boiler fluctuates over a wide range of operational steam loads, it may be feasible to monitor excess oxygen in the flue gas, and with a feedback loop, control the air damper position to maintain a steady excess oxygen ratio just above that needed for complete combustion. Excess oxygen control can improve efficiency by 1-2%. Combining both parallel positioning and excess oxygen control can yield a total efficiency improvement of 3-4%.
Nearly all boilers require periodic bottom and surface blowdowns to control total dissolved solids (TDS) and prevent carryover and water hammer in steam piping. For most plants, blowdowns are directed to a quench tank and then periodically dumped to the waste water system. It is possible to use the blowdown piping as a heat source to pre-heat feed water at the de-aerating feed tank (DFT) or the makeup water storage tank. About 85% of the 3% efficiency loss can be recovered with a heat exchanger installed in either tank– thus, gaining about 2% overall fuel-to-steam efficiency. It should also be noted that both methods (DFT or heated makeup tank) help remove dissolved CO2 from feed/make-up water which also reduces boiler chemical use.
Insulating Pipes and Repairing Steam Leaks
Any un-insulated condensate and feed piping represents both a personnel burn hazard and a significant source of lost energy. Over a 100-foot run, schedule 40 4-inch diameter un-insulated steel pipe with 120°F (0 psig) water will lose about 85% of its energy. For the small 300 hp example, this equates to 1-2% of boiler efficiency. Likewise steam and condensate leaks, stuck steam traps, etc. can cause significant efficiency losses.
Low Flame and Refractory Losses
The 2005 ORNL paper estimates that food processing plants use (on average) 31% of their boiler’s actual capacity. Recall that, in the Figure 1 example, it was assumed that the boiler was at 50% of average load. During these low- or no-demand periods, the boiler will continue to use fuel at a “low flame” rate (1-2% of the fully-loaded rate) to keep the refractory warm and support blowdowns. Additionally, for plants which have very large cycles in steam demand, there will be further heat losses because of the relatively large size of the firetube boiler and the required extra fuel burn to meet the rapidly changing steam demand.
There are different boiler designs specifically engineered to rapidly meet changing steam demand. A good example is the Miura package vertical floating water tube boiler. These boilers will startup from cold iron to normal operating pressure in about 5 minutes, which eliminates nearly all low-flame losses and efficiently respond to high cyclic steam demand. If considering a replacement boiler for a high cyclic steam demand, it would be wise to evaluate the potential cost savings of these small watertube boilers.
Summary of Savings
Table 1 is a summary of the savings that can be generated from implementing these improvements. Approximate project costs have been estimated using data from Willems (2005) and prices have been corrected for inflation (2009 dollars).
Project Costs Source: Advanced System Controls & Energy Savings For Industrial Boilers, 2005, D. Willems, Cleaver-Brooks, Inc.
The above savings are computed assuming a 300 hp boiler, operating at 50% load, 16 hours per day, 5 days per week, 50 weeks per year, and a natural gas price of $0.60 per therm. Natural gas currently (as of 4/29/2009) trades about $0.35 per therm due to the recession, but is reasonably expected to rise back towards 2008 levels as the economy improves.
Also note that payback period has been computed with and without the Business Energy Tax Credit (BETC). In Oregon, the BETC guidelines stipulate that, for efficiency projects which have over a 10% annual energy savings, a 35% tax credit will be applied to the applicable project costs. Thus, for the sum of all of the improvements, the BETC lowers payback time significantly.
IPC can assist in developing an appropriate financial model for boiler efficiency improvement project planning. For further information, call John Alleman at 503.327.2214.
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