Modern Method to Determine Recovery Boiler Efficiency.

Recovery boiler mass and energy balances are needed for the performance testing of recovery boilers, mill energy wide control systems, and in calculating the air emission data. Typically, recovery boiler balances are mainly based on 1996 TAPPI publication “Performance Test Procedure: Sodium Base Recovery Units”, the use of which is problematic in Europe because of its view that the losses from the recovery of process chemicals are counted when determining the recovery boiler steam generation efficiency. A low efficiency leads to taxation and legislative problems in some European countries. There is a newly accepted European standard for steam generator acceptance tests: EN 12952-15:2003 “Water-tube boilers and auxiliary installations Part 15: Acceptance tests”. This paper examines how the standard could be applied to recovery boilers to find out the net efficiency which is shown to be approximately the same as for other biomass boilers, not 10% lower. A new and more accurate method to calculate losses from convection and conduction is discussed. In addition, a suggestion on how to handle borate autocausticization in the recovery boiler furnace is made. INTRODUCTION Recovery boiler mass and energy balances are needed for the performance testing of recovery boilers, mill energy wide control systems and in calculating the air emission data. There is no suitable procedure for modern recovery boilers. Current recovery boiler balances are mainly based on 1996 TAPPI publication “Performance Test Procedure: Sodium Base Recovery Units” [1] the use of which is problematic in Europe because of US units and its view that the losses from the recovery of process chemicals are counted when determining the recovery boiler steam generation efficiency. General steam generator balances have been based on the outdated DIN 1942 “Acceptance Test on Steam Generators” [2], which fails to mention recovery boilers. There is a newly accepted European standard for steam generator acceptance tests: EN 12952-15:2003 “Water-tube boilers and auxiliary installations Part 15: Acceptance tests” [3]. This paper examines how the standard can be applied to recovery boilers. Because this standard does not give answers to kraft recovery boiler specific questions, some additional definitions have been made to e.g. calculate some heat losses. Borate autocausticization in the recovery boiler furnace has been included. This type of operation takes currently place in some Scandinavian mills. The balances are thus based on the as-fired liquor flow and as-fired liquor dry solids. The mass and energy balances are made for the unit mass flow of the as-fired liquor. BALANCE CALCULATIONS Recovery boiler mass and energy balances have been presented by Gullichsen [4], Clement et al. [5], Adams and Frederick [6], and Vakkilainen [7]. The conventions used to express the heat used in the reduction and the heat losses as smelt are for every author similar to the Tappi standard [1] that covers exclusively the recovery boiler mass and energy balance calculation. Boiler efficiency The boiler efficiency is a measure of the goodness of the chosen process and equipment to transfer the combustion heat to the heat in steam. For the best solid fuel boilers, the boiler efficiency is 86 ... 90% [8]. Oil and natural gas fired boilers can achieve 90 ... 94% efficiency. The boiler efficiency can be defined as the ratio of the useful heat output to the total energy input.