OCR Text |
Show COMBUSTION MODIFICATIONS FOR COAL-FIRED STOKER BOILERS INTRODUCTION Stoker boilers suffer from poor operating efficiency and inadequate air emission control to be attractive in today's market. They are however relatively inexpensive to purchase and can operate on inexpensive fuel. The work presented in this paper shows how the stoker boiler can have its fuel efficiency improved and the emission of particulate matter, nitrogen oxides (NO ), and sulfur oxides (SO ) reduced. These combustion modifications should make the stoker boiler the coal-fired combustion system of choice for the industrial client. KVB has applied for and received a U.S. patent (No. 4,335,660) covering the Stoker Gas Recirculation* systems that are discussed in this paper. DISCUSSION The Concept The fundamental problem hampering boiler efficiency in stoker-fired systems is that of inadequate mixing of the fuel and air. Attempts have been made to improve this mixing by the use of overfire air and different grate designs but the problem remained. Simply stated, '•the fuel and air do not have adequate time to mix properly within the bed dimensions.'* The fuel bed has the requirement that a certain mass flow of combustion air (gas) be supplied in order to cool the fuel bed so that the ash in the bed does not fuse into what is called a clinker (slag). The requirements of mixing and cooling can be supplied if a portion of the flue gas is returned and mixed with the fresh combustion air. The Reg. Trademark of KVB. Kenneth L. Maloney, Ph.D., and Nick Bayard DeVolo are co-inventors of the SGR process. 23-2 |