Here is a message I sent to REPP Stove Discussion Group (Stoves@listserv.repp.org) regarding the Excess Air. It was my first message on this discussion group.
Date:
Sat, 3 Jan 2004 20:50:19 +0545 Hello everybody My name is Kanchan Rai. I am a research assistant at Research, Development and Consultancy Unit in Kathmandu University in Nepal (I have Completed Mechanical Engineering in 2002 from same university). I am working on Development of an Improved Cooking Stoves in the Mountain Areas of Nepal. At the same time I am also working on Waste management project in the university. I am working on paper briquettes and thermal insulating bricks (composed of paper, plastics, glass powder and some additional materials) from waste materials. I am a regular receiver of emails of stove discussion. This is my first time writing on discussion lists. There have been lots of discussion regarding EXCESS AIR last week. I like to add my view on that though I am not specialist on combustion. I am just a begineer. What is stoichiometric air? Is this sufficient? This is the theoritical amount of air required to burn the fuel completely. For e.g. 1 kg of pinewood ( C 44.45%, O 33.96 %, H 5.93%, H2O 15 %, Ash 0.3% w.b.),(Relative humidity 40% ), needs 5.71 kg of air to burn completely. Theoritically at this amount of air fuel produced maximum of its energy but it practice it is not so. The sad thing is all air entering the combustion chamber donot participates in reaction. This greatly depends upon the stove design and fuel loading pattern. Hence we need some more air to complete combustion and the air is called EXCESS AIR. In small scale biomass combustion applications, it is necessary to have excess air ratio well above one, usually 1.5, to ensure a sufficient mixing of inlet air and fuel gas. DOES EXCESS AIR ALWAYS HELPS COMBUSTION ? OFCOURSE NOT, if excess air is very much it reduces the combustion temperature. BUT HOw!! Air is composed of 21 % oxygen and 79 % nitrogen (other gases are such as Ar, CO2, H2, Xe, Kr etc but comparably neglegible), on which only oxgen takes part in combustion. Only less than one third of air is useful for combustion. Nitrogen, inspite of helping, absorbing energy from the combustion zone, resulting lower combustion temperature and dilution which effects on the reaction of oxygen and volatile matters. if 1 kg of pinewood is burned with stoichiometric air only 1.2 kg of oxygen takes parts in reaction and remaining 4.5 kg of Nitrogen absorbs energy from combustion zone. For 150 % excess air to burn 1 kg of firewood requires 14.3 kg of air, in which only 2.98 kg of oxygen takes part in reaction. THINK!!!! what does the remaining 11.32 kg Nitrogen contributes for the combustion (only negative effect). The specific heat of Oxygen and nitrogen is 0.795 kJ/kg and 1.04 kJ/kg respectively. It is known that to burn out CO, CH4 we need preheated air above 600 C. Assuming air is entering at room temp (no prehaeating) let 20 C. Nitrogen (11.32 kg) requires 6828 kJ of energy, when air is heated from 20 C to 600 C) and O2 (2.98 kg) requires 1375 kJ/kg of energy. The heating value of pinewood with 15 % moisture is 17.6 MJ/kg. Hence heating air (20 C - 600C) consumes 46 % of fuel energy, and increase with increase in excess air. Hence to much excess is always harmful for clean combustion. Calcultions are based on Excel sheet of Combustion Analysis, developed by myself and it works fine in my case. Happy new year 2004 to all the stovers'. kanchan |