Key facts. Combustion, biofuels and zero net carbon. Capturing the economic benefits of carbon neutral fuels

Transcription

1 Key facts Combustion, biofuels and zero net carbon Capturing the economic benefits of carbon neutral fuels Oil pre-heat Testing variations in biofuel characteristics and the importance of filtration Waste to energy Specific fuel dedicated DDCC channels for multi fuel switching Calculating carbon emissions Issue number 3 in the series of Dunphy Key Facts booklets Multifuel burners, CHP, community energy networks, waste to energy applications

2 Capturing the benefits of carbon neutral fuels There are three main approaches to achieving zero net carbon in buildings or factories using regulated energy avoid, reduce, offset. The first two avoid and reduce are important considerations when specifying new or retrofit combustion equipment. For example, a well designed, axial air flow burner will, by nature of its design, significantly reduce CO 2 emissions. Specification of biofuel as the primary or supplementary energy source will contribute to the avoidance and net reduction of CO 2. But-the benefits of carbon neutral fuels are only achieved when 100% of the fuel combusted is of renewable origin. To date, few burner manufacturers have proven technologies for the safe and efficient combustion of multi-fuel systems using biodiesels and recovered waste vegetable oils and can offer only blended bio-diesels which may contain relatively low levels of carbon neutral oils. Optimising energy efficiency from carbon neutral fuels depends on a number of variables including: Capturing the economic benefits of bio-fuels as a cost effective fuel source relies on the design of the burner being fit for purpose. An off-the-shelf single or multi-fuel burner without burner design and controls configured to specific biofuel characteristics will result in higher energy use and unplanned maintenance requirements. Green fuel is not fully carbon neutral if it s only a blend of organic and fossil derived fuels characteristics of fuel used local site conditions design of the combustion chamber heat release rate the design of the burner and how that complements the geometry of the boiler design of fuel pump Dunphy axial air flow burner Equipment design for bio-fuel combustion Energy saving single or multi fuel combustion relies first and foremost on perfect axial air flow to the burner head at all levels of firing including, in particular, low fire operation. 2

3 Characteristics of biofuels The most common biofuels currently used in the UK include virgin rapeseed and recovered vegetable oils. Bio-fuels are derived from a variety of sources. The quality and consistency of waste and pure vegetable oils vary far more than those of manufactured fossil fuels. It is, therefore, essential that potential bio-fuel supplies are analysed for heating value, moisture and ash content as well as purity and contaminant levels. A typical comparison of a bio-diesel compared with diesel is set out below. Characteristic Bio-diesel Diesel oil Rapeseed oil Net calorific value MJ/kg Kinematic O C O C 38@40 O C viscosity cst Acidity ph 2.3 to to 5.5 Ash weight % <0.02 < 0.01 <0.01 Density 0.8kg/litre 0.83kg./litre 0.9kg/litre Water weight % Solid weight % 0.05 V % combined 0.05 V % combined 0.75 V % combined The calorific value of biofuels is about 10% less by weight than that of diesel, but because of the higher density. The volumetric content of the heat value is about the same. Most vegetable oils have a higher kinematic viscosity than diesel and there can be large differences between various types of vegetable sources. On average, the densities of vegetable oils will be around 10% higher than that of diesel fuel. Ash particles from biofuel combustion can be chemically reactive and may result in heavy deposits in the combustion chamber. Preheating bio-fuels Accurate temperature control of biofuel is essential to avoid swings in viscosity which result in combustion variation. The functionality in our burners of oil pre-heating is, therefore, one of the key design features which ensures that Dunphy carbon neutral burners operate with consistent combustion efficiency, high turn-down ratios and low NOx emissions. 3

4 Test biofuel characteristics before specifying the burner The combination of the rush for carbon neutral developments, the enormous variety of bio-fuels available and the move towards blending conventional and bio-fuels is making it increasingly difficult to write specifications for consistently high efficiency, low emissions plant - without first testing the efficiency of bio-fuels and blends in burners. Dunphy s fuel testing programmes (often run in the presence of consultant and end user clients) cover the critical functions of filtration, blending, optimum temperatures, calorific value differences and emissions. The main carbon neutral sources are currently extracted waste vegetable oil and extracted pure vegetable oils (for example, oils derived from rapeseed, soya or palm). The importance of filtration Virgin rapeseed and recovered vegetable oils (including waste oil from chip, crisp and other food processing operations) must be filtered to remove impurities and particles left over from first use. Vegetable oils differ in thickness depending on their plant source and processing also affects the final product properties. Waste oils vary considerably in contamination levels and quality. If the base oil has been reused frequently, it will have become very thick through increased hydrogenation and contamination. Thorough cleaning will be essential. Case study Test analyses run recently for a community heating scheme assessed the outputs of virgin rapeseed and used vegetable oils. The oils were gravity fed via an in-line duplex filter from a fixed holding tank into the burner hydraulic pump. Oil temperature was raised to 60 o C after the pump and before entry to the burner. A range of performance data was collected and measured. Of particular interest to the client was the evidence that the virgin rapeseed oil was blocking the filter (75u mesh) with husks and particles which were not visible in liquid form to the naked eye. Once the filtration process was tailored specifically for this type of rapeseed fuel, then combustion operated smoothly. 4

5 Waste to energy: carbon neutral sources Industrial and municipal waste contains potential for energy recovery. (The biodegradable proportion of the waste can be classed as renewable under the Renewables Obligation Directive.) The production and combustion of waste derived fuels can supply low or zero carbon energy for individual boiler/burner plant or for CHP community energy networks. Dunphy has extensive experience in designing and installing multi fuel burners in CHP, community energy networks, waste and wastewater treatment operations. Case study One of the world s largest wastewater biological treatment plants installed Dunphy biogas /oil burners because of their advanced technology for safely and efficiently burning sewerage gas and light fuel oil with very low carbon and ultra low NOx emissions. Biogas boosters and gas trains were manufactured with cast iron bodies and stainless steel fittings suitable for withstanding the corrosion properties of bio fuels. Due to the biological process involved in producing sludge gas, the calorific value can vary by up to 20%. It was essential, therefore, that the burners were fitted with a quick response fuel trim system. Integrated into each burner is a Ratiotronic digital control system with adaptive fuel trim which can respond within 15 seconds of a change. The Ratiotronic digital system manages operations with O 2 levels of less than 2% and with NOx levels not exceeding 30mg/m 3 corrected to 3% oxygen. The high turndown ratio over the burners enables the flow temperature to be maintained within 1 o C irrespective of load requirements. Burner mounted Ratiotronic direct digital combustion control unit Dunphy s Ratiotronic TM 6000 DDCC system is designed to maximise safe and efficient fuel switching between fossil derived and carbon neutral multi fuels - or a mix of different types of carbon neutral fuels. Because of the differing characteristics of each fuel, the control system includes a dedicated channel of control for each individual fuel. This means that fuel change-over can be carried out automatically or at the flick of a switch as well as enabling the safe, simultaneous firing of fuels. 5

6 Calculating carbon emissions DEFRA and The Carbon Trust have published methodologies for measuring CO 2 emissions from energy projects. The default emissions factors for a range of fuels are set out in the adjoining table. Traditional fuels : CO 2 emission levels Standard conversion rates Fuel type CO 2 emission factors (kgco 2 /kwh) Fuel can be purchased in a variety of units. The most common conversion are as follows: 1 tonne oil equivalent = 11,630 kwh 1 therm = 29.3 kwh 1 giga joule = kwh Coal 0.30 Electricity 0.43 Natural gas LPG Gas/diesel oil 0.25 Heavy oil 0.26 In instances in which fuels are purchased by volume or by weight, the calorific value of the fuel is needed to convert to energy units. The main properties and conversion factors for traditional fuels are shown on the opposite page. Source: The Carbon Trust Examples of CO 2 emissions calculations Example A : Electricity emissions Annual CO 2 emissions (tonnes) = energy consumption (kwh) x emission factor (kgco 2 /kwh) x x number of operating days per annum Eg: A heating system using 500kWh of electricity per day operating at 70% over 350 days p.a. will generate annual CO 2 emissions of 53.9 tonnes p.a. Example B (based on Part L2A factors): Natural gas emissions Assuming gas calorific value of 38.7MJ/m 3 (or 10.75kWh/m 3 ) and output of 1,000kW, gas consumption will be113.44m 3 /hr. {1,000/(0.82 x 10.75)} m 3 /hr x 10.75kWh/hr. = 1, kwh. of gas consumption which will generate 236 kg/hr of CO 2 (1, x factor from table above). Across a 24/7, 365 day operational year, this generates CO 2 emissions of 2, tonnes p.a. Note: These emissions levels are based on a specific point in the turndown range and do not take into account the additional efficiency gains and fuel savings through the improvements in turndown obtained with Dunphy axial air flow burner designs. 6

7 Key data for multi fuel burner specification Properties of common gases Properties of common oils Comparative viscosities Viscosity has a number of different measurements. The most familiar are: Kinematic viscosity (centistokes) based on the amount of force required to beat the internal friction of fluid and Dynamic or absolute viscosity (centipoise) used for expressing the internal friction of oils at low temperatures Kinematic viscosities The chart on the right shows the comparative kinematic viscosities of rapeseed, corn, soybean and diesel oils. 7

8 Dunphy low carbon products and services 300kW to 13MW multi fuel burners designed specifically for use with 100% carbon neutral fuels in centralised or distributed heat and steam, CHP, community district heating and waste-to-energy applications. Expert burner design skills for all biofuels, tallow oils, recoverable solvents, sewerage gases and fossil derived fuels. Quick, simple retrofitting of Dunphy fossil fuel burners to enable effective combustion of carbon neutral fuels. Ratiotronic direct, digital combustion control with dedicated fuel management channels for safe and simple flick of a switch multi fuel changeover. Touchscreen options - interfacing with all BMS and SCADA systems. Single and duplex gas boosters For further information about energy efficient, carbon neutral combustion or to book a bio-fuels lab. test or site energy audit, contact sharon.kuligowski@dunphy.co.uk Dunphy Combustion Ltd, Queensway, Rochdale, OL11 2SL