NOx emissions and thermal efficiencies of small scale biomass-fuelled combustion plant with reference to process industries in a developing country

Abstract

Solid biomass materials are an important industrial fuel in many developing countries and also show good potential for usage in Europe within a future mix of renewable energy resources. The sustainable use of wood fuels for combustion relies on operation of plant with acceptable thermal efficiency. There is a clear link between plant efficiency and environmental impacts due to air pollution and deforestation. To supplement a somewhat sparse literature on thermal efficiencies and nitrogen oxide emissions from biomass-fuelled plants in developing countries, this paper presents results for tests carried out on 14 combustion units obtained during field trials in Sri Lanka. The plants tested comprised steam boilers and process air heaters. Biomass fuels included: rubber-wood, fuelwood from natural forests; coconut shells; rice husks; and sugar cane bagasse. Average NO, (NO and NO,) emissions for the plants were found to be 47 gNO, GJ-' with 18% conversion of fuel nitrogen. The former value is the range of NO, emission values quoted for combustion of coal in grate-fired systems; some oil-fired systems and systems operating on natural gas, but is less than the emission levels for the combustion of pulverized fuel and heavy fuel oil. This value is significantly within current European standards for NO, emission from large combustion plants. Average thermal efficiency of the plants was found to be 50%. Observations made on operational practices demonstrated that there is considerable scope for the improvement of this thermal efficiency value by plant supervisor training, drying of fuelwood and the use of simple instruments for monitoring plant performance.

NOx emission reduction by staged combustion in grate combustion of biomass fuels and fuel mixtures

“…However, it is a concern that combustion of biomass will lead to high levels of nitrogen oxides emissions (NO x ; collective term for NO and NO 2 ) from oxidation of nitrogen in the fuel (fuel nitrogen) or potentially conversion of nitrogen in the air (air nitrogen). NO x are contributing to environmental problems such as acid rain, photochemical smog formation and depletion of stratospheric ozone [2]. Also nitrous oxide (N 2 O) may be formed, having a strong greenhouse gas effect.…”

“…Theoretical and experimental studies have shown that these three mechanisms are contributing much less to NO x formation in biomass combustion than fuel-N conversion. Regarding the first route, this is because the Zeldovich mechanism is predominant at temperatures greater than 1500°C [2,3], while the temperatures during biomass combustion for power production are typically lower. Contributions from the prompt mechanism are mainly found for fuel rich and high temperature conditions which are also not relevant in biomass combustion.…”

Industrial Use of Biomass Energy in Sri Lanka

“…Discussions are underway, however, to amend this time limitation and to extend authorization to a three-year period. Although the National Environmental Act is the major item of legislation concerning industrial pollution, government environmental priorities are outlined in the National Environmental Action Plan 1992-1996(Government of Sri Lanka, 1991. This is an ambitious programme to address a whole range of environmental concerns.…”

“…Under the Climate Change Agreement, Sri Lanka has agreed to reduce greenhouse gas emissions to 1990 levels by the year 2000, although the actual level of emissions in 1990 is unknown. The data in this paper provides part of the information required, and the procedures developed (Tariq and Purvis, 1996) can be used to estimate total national emissions from the national energy consumption data. In the case of the Montreal protocol, Sri Lanka has agreed to phase out the use of CFCs by the year 2000 -except for the maintenance of existing units.…”

“…The first three items are relevant to the content of this paper. The results of the technical study are reported elsewhere (Tariq and Purvis, 1996). In brief, the technical study concerned: (1) measurements of the emissions of nitrogen oxides (as NO and NO ) and the thermal efficiencies of nine fuelwood-fired air heaters used in the desiccated coconut and tea industries; and (2) five steam boilers fired with fuelwood, coconut shells and sugercane bagasse.…”