Steam boilers are commonly used throughout industry. Operation of these boilers is energy intensive and can therefore represent a significant proportion of an organisation’s energy costs. According to the Carbon Trust, in the UK, boilers account for some 60% of carbon emissions from industrial operations and buildings. Being so energy intensive, steam boilers therefore offer many energy savings opportunities to businesses and these actions can achieve significant cost savings whilst reducing a company’s carbon footprint. Consideration of a switch from fossil fuel to biomass steam raising is one change that is gathering momentum and biomass boilers are now firmly in the options mix. Biomass is any solid nonfossil-based organic fuel and includes wood (either grown specifically as a fuel or as waste material), straw, types of grass and many other organic by-products.
The lower steam raising flexibility offered by biomass burners compared with that of oil or gas fired alternatives is however a major concern for operators of plant and investment decision makers. The main obstacle to achieving acceptable biomass boiler performance is the inherent slow response to step changes in demand, especially for batch production processes using steam as the heating medium. There are other potential draw-backs effecting energy efficiency and ramping time such as the degree of wetness and variable quality of the fuel, and the consequences are a substantial delay in meeting the demand, loss of pressure and temperature to satisfy process requirements, potential loss of production and risk to product quality. All of these problems may be solved by the inclusion of a steam accumulator in the steam supply system.
So what is ‘steam accumulation’?
It is the storage (in a pressure vessel) of surplus steam produced at times of low demand for subsequent release to supplement the output of the boiler at times of high demand. Any industrial manufacturing process having a variable demand for steam and where an effective differential exists between boiler and process pressures can benefit from this energy efficient technology.
What is the prime benefit?
The steam held in reserve is available immediately to meet a step change or rapidly rising demand during the time needed for the biomass boiler to ramp up to the required level of output and conversely the accumulator provides a means to absorb and store the excess steam produced from residual heat in the furnace during ramp down. Where the demand is of a highly fluctuating nature, a steam accumulator provides the means to convert this into a steady load on the boiler.
What else can ‘steam accumulation’ achieve?
Imagine a boiler able to reach a ‘peak’ demand ten times higher than its rated output. Consider that two out of a bank of three boilers might be eliminated and still allow the remaining boiler to reach the same ‘peak’. Contemplate maintaining a low pressure steam supply whilst diverting boiler output to critical high pressure consumers. These are just three examples of the many applications possibilities for ‘steam accumulation’.The potential benefits are always significantly lower energy costs through increased operating efficiency and can additionally provide immediate response to the steam demand, a secure steam supply at constant pressure, temperature and dryness, elimination of boiler low water lock-outs and priming due to wide load swings, assure product quality, a reduction in emissions and where new steam or power plant is required, lower capital cost.
How do accumulators work?
Water is used as the heat storage medium. The reason becomes evident when the greater heat storage capacity of water is compared with that of the same volume of steam vapour at any given state of temperature and pressure. Most accumulators work on the ‘pressuredrop’ principle whereby steam from the boiler is charged into the water causing a rise in temperature (and pressure) and steam for process is discharged from the water (as ‘flash’) at low pressure to meet the demand. The size of the storage vessel depends on the difference in pressure raised in the boiler and that of the pressure required by the process, and the amount of storage required. Storage vessels can range in size from 10m3 to 75m3 in volume (and more), or be a multiple of vessels to overcome logistical problems.
How is the water heated?
Charging of an accumulator takes place when ‘surplus’ or ‘excess’ steam from the boiler is condensed in the water space of the accumulator. This is achieved by directly injecting the steam into the water by means of special charging nozzles and results in a continuous rise in the temperature of the water which continues until the corresponding pressure in the vessel has reached that of the charging steam.
What happens when steam is released?
Discharging takes place when the steam in the head space above the water is released which causes a momentary reduction of pressure leading to an imbalance of that lower pressure with the saturation temperature of the storage water, whereupon the surface water starts to boil and more steam is released as ‘flash evaporation’ to replace that just released. This process may continue for as long as steam is required to be discharged or until the minimum required by the process has been reached.
How can an accumulator benefit boiler sizing?
The dangers in oversizing or undersizing a biomass steam boiler are more pronounced than those for fossil fuel fired applications. All boilers reach highest efficiency when operated near rated output and under steady load conditions. However, lower combustion flexibility compared with that of oil or gas fired burners makes the biomass boiler less suitable for meeting a variable demand, seasonal output requirements and low loads. A steam accumulator can add the steaming flexibility that the biomass boiler alone lacks and can reduce the periods during which it operates less efficiently. Variations in fuel quality can be compensated for, repeated shut-downs and start-ups are reduced, load factor is increased, boiler reliability is improved, longevity is increased and emissions are minimised.
Which industries can benefit?
Steam is an essential commodity for many industrial processes. The technology can be successfully applied in biomass boiler applications to any manufacturing process where steam is used for batch processing or at highly fluctuating rates. These conditions can be found in food production, canning, distilling and brewing, chemical manufacture, textiles, rubber, paper and board, plastics, pharmaceuticals, steel making, laundries, ammunitions, brick and concrete curing, turbine testing, tyre manufacture, combined heat and power.
Accumulator control
The latest accumulator control methods are designed to optimise boiler efficiency in conjunction with modern burner technology. Matching the amount of steam stored to burner performance, connecting the storage to the steam supply system in the most effective way, and customising the microprocessor accumulator controls are the keys to a successful installation.
The energy saving economics
Other than the basic cost of fuel, the most important energy cost savings to be gained from steam accumulation derive from a reduction in the boiler output in combination with load stabilisation. In all cases, there are prospects for energy savings emanating from: the elimination of excess steam generating capacity and part-loading that causes a low load factor, the removal of a rapidly fluctuating demand and idle live capacity that increases standing heat and power losses. Therefore the possibility of a steam accumulator meeting a fluctuating steam demand with a smaller boiler operated at a higher load factor and at constant output with higher efficiency is real.
By David Oakland – Steam Power Technology Limited UK
