Lars-Erik Åmand

Pyrolysis of poly-l-leucine under combustion-like conditions☆ ☆

The protein poly-L-leucine has been used as a model compound for the nitrogen in biomass fuels. It was pyrolysed in a fluidised bed at 700 and 800C and the pyrolysis gases were analysed with a FT-IR spectrometer. HCN, NH3 and HNCO were identified as the main nitrogen-containing species, while neither NO nor N2O were found among the pyrolysis gases. At 700C, as much as 58% of the nitrogen content was converted into HCN and 31% into NH3. The HCN/NH3 ratio increased from about 1.9 at 700C to above 2.2 at 800C. Pyrolysis of another protein, poly-L-proline, at 800C gave a HCN/NH3 ratio close to 10. This revealed that the protein’s amino acid composition has a marked impact on the composition of the pyrolysate.

Influence of fuel on the emission of nitrogen oxides (NO and N2O) from an 8-MW fluidized bed boiler☆

Emissions of nitric oxide (NO) and nitrous oxide (N20) from an 8-MW circulating fluidized bed boiler have been measured. The influence of the volatile content of the fuel was investigated by using three different fuels and operating the boiler at different bed temperatures, primary air stoichiometries, and excess air ratios. The measurements show that the emissions of NO are extremely low and dependent upon the char loading of the boiler as well as on the existence of unburned combustible matter such as CO and H2 in the gas phase. The char content and CO levels are influenced by the fuel, the bed temperature, and the air-to-fuel ratio in the combustion chamber. The levels of N20 emission are high and caused by the low temperatures used in fluidized beds. The N20 emission is influenced by bed temperature and oxygen concentration as well, but the influence of bed temperature is opposite compared to the dependence of bed temperature on the NO emission.

The temperature's influence on the selectivity between HNCO and HCN from pyrolysis of 2,5-diketopiperazine and 2-pyridone ☆

Two cyclic amides, 2-pyridone and 2,5-diketopiperazine (DKP), were pyrolysed at temperatures ranging from 700 to 1100C. Pyridone is the only one of the four main nitrogen functionalities found in coal that is likely to form HNCO under pyrolysis. DKP is a primary pyrolysis product from proteins, which are the main nitrogen source in biomass. The formation of HNCO from biomass has been suggested to originate from DKP and other cyclic amides. The aromatic 2-pyridone was thermally more stable than the non-aromatic DKP. Both amides formed HCN, HNCO and NH3. The NH3 yields, about 3–4% for 2-pyridone and 10% for DKP, were almost independent of temperature. The HCN yield on the other hand showed strong temperature dependence and increased with temperature for both of the cyclic amides. The HNCO yield decreased with increasing temperature for DKP over the whole temperature interval. For 2-pyridone, the pyrolysis was incomplete at the lowest temperature in the investigation. Between 900 and 1100C, the pyrolysis of 2-pyridone was complete and the HNCO yield decreased with increasing temperature. The HNCO/HCN ratio for both of the cyclic amides decreased with increasing temperature over the whole investigated temperature range. The finding in literature that the HNCO formation from cracking of coal tars produced a maximum HNCO yield at an intermediate temperature, is explained by the thermal stability of pyridone at low temperatures and the selectivity towards HCN at high temperatures.

Influence of SO2 on the NO/N2O chemistry in fluidized bed combustion 2. Interpretation of full-scale observations based on laboratory experiments

In circulating fluidized bed combustion of coal, significant interactions between desulfurization by limestone and emissions of nitrogen oxides (NO, N2O) are generally observed. In order to facilitate interpretation of experimental results obtained in a 12 MW circulating fluidized bed boiler, a series of laboratory tests have been carried out. The tests include homogeneous CO and HCN oxidation in the presence of different concentrations of SO2, CO oxidation catalysed by calcined and partly sulfated limestone particles, and formation and reduction of NO and N20 over bed material containing char. The presence of SO2 in the gas is shown to decrease the rate of homogeneous CO and HCN oxidation and thereby change the product distribution of the nitrogen-containing species. Unlike sulfated limestone, calcined limestone (CaO) is shown to be a good catalyst for oxidation of CO. A lower mean CO concentration is therefore expected in fluidized bed combustors during injection oflimestone. This indirectly influences the nitrogen chemistry. Finally, the influence of NO and O2 on the formation of N2O from char was studied.

Influence of SO2 on the NO/ N2O chemistry in fluidized bed combustion 1. Full-scale experiments

The lime used for sulfur capture in circulating fluidized bed (CFB) boilers interferes with the NO/N2O chemistry. This has been explained as an effect of the surface of the lime. However, homogeneous reactions involving SO2 could also be important. Large-scale experiments were carried out in a CFB boiler with the aim of separating the effect of lime from that of SO2. This was achieved by comparing a test involving removal of SO2 by lime addition, while burning a high sulfur coal, with a test where S02 was added to the combustion air for a low sulfur coal in a quantity corresponding to the emission of SO2 from the high sulfur coal. The tests show that the SO2 itself influences the NO/N2O chemistry independent of the catalytic effect of the lime surface. Carbon monoxide is believed to be an important intermediary species in the reactions, but laboratory investigations are needed to interpret the results further. The connection between NO reduction and N20 formation is further studied by supply of NO to the primary air duct.

Comparison of large- and small-scale circulating fluidized bed combustors with respect to pollutant formation and reduction for different fuels

To investigate the scale-up problem of circulating fluidized bed combustors with particular respect to emissions, comparative combustion experiments have been performed in an industrial-size combustor (12 MWth, height 14 m, cross-sectional area 1.6 m X 1.6 m) and in a lab-scale facility (height 16 m, inner diameter 100 mm). A comparison of the axial concentration profiles of oxygen, carbon monoxide, nitric oxide, nitrous oxide and ammonia along the riser height, obtained during the combustion of wood, peat and coal under conditions of equal fuel bed material, solid holdup and gas residence time, shows a basic similarity. This indicates that suitably sized and operated lab-scale combustors may indeed be valuable tools for the investigation of combustion phenomena. However, some significant deviations of the profiles can be recognized, too. These deviations are caused by three-dimensional effects in the large-scale combustor and indicate the limitations of small-scale experiments.

Reversed air staging — a method for reduction of N2O emissions from fluidized bed combustion of coal

Reversed air staging, a method for reduction of N2O emissions, was studied in a 12 MW circulating fluidized bed boiler. The effect of combustor air-ratio, bed temperature, load and limestone addition ratio was investigated. The results indicate that if only the air distribution is changed, the emission of N2O can be decreased to one fourth compared to normal air staging at a temperature of 850°C, with maintained low emissions of NO,´SO2 and CO. With increased bed temperature, 870°C, in combination with increased limestone addition, from Ca/S molar ratio = 3 to 4, it was possible to reduce N2O by 90 vol% with low emissions of NO, SO2 and CO. The method was less efficient at low load, because of the lower cyclone temperature. Also the effect of redistribution of some of the air added in the cyclone outlet to the cyclone inlet was studied. This did not, however, give any improvement in the boiler used compared to adding all of the air in the cyclone outlet. There was no significant difference in combustion efficiency between reversed air staging and normal air staging.

The effect of temperature on the catalytic conversion of Kraft lignin using near-critical water

The catalytic conversion of suspended LignoBoost Kraft lignin was performed in near-critical water using ZrO2/K2CO3 as the catalytic system and phenol as the co-solvent and char suppressing agent. The reaction temperature was varied from 290 to 370°C and its effect on the process was investigated in a continuous flow (1kg/h). The yields of water-soluble organics (WSO), bio-oil and char (dry lignin basis) were in the ranges of 5-11%, 69-87% and 16-22%, respectively. The bio-oil, being partially deoxygenated, exhibited higher carbon content and heat value, but lower sulphur content than lignin. The main 1-ring aromatics (in WSO and diethylether-soluble bio-oil) were anisoles, alkylphenols, catechols and guaiacols. The results show that increasing temperature increases the yield of 1-ring aromatics remarkably, while it increases the formation of char moderately. An increase in the yields of anisoles, alkylphenols and catechols, together with a decrease in the yield of guaiacols, was also observed.

Reduction of N2O in a circulating fluidized-bed combustor☆

Nitrous oxide was injected at various locations in the combustion chamber of a circulating fluidized-bed boiler during combustion of fuels of different volatiles matter: coke, bitumonous coal and wood. The reduction of N2O injected was measured and the results were compared with calculations using available kinetic data. The reductionofthe N2O introduced into the lower part of the combustion chamber was >80%, but also, especially in wood combustion, at the top of the combustion chamber a certain degree of reduction occurred, which is interpreted as being caused by volatiles combustion. The available kinetic data are still not sufficiently accurate for definite conclusions from calulations

Decreased PCDD/F formation when co-firing a waste fuel and biomass in a CFB boiler by addition of sulphates or municipal sewage sludge.

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are formed during waste incineration and in waste-to-energy boilers. Incomplete combustion, too short residence times at low combustion temperatures (<700 °C), incineration of electronic waste and plastic waste containing chlorine are all factors influencing the formation of PCDD/Fs in boilers. The impact of chlorine and catalysing metals (such as copper and iron) in the fuel on PCDD/F formation was studied in a 12 MW(th) circulating fluidised bed (CFB) boiler. The PCDD/F concentrations in the raw gas after the convection pass of the boiler and in the fly ashes were compared. The fuel types were a so-called clean biomass with low content of chlorine, biomass with enhanced content of chlorine from supply of PVC, and solid recovered fuel (SRF) which is a waste fuel containing higher concentrations of both chlorine, and catalysing metals. The PCDD/F formation increased for the biomass with enhanced chlorine content and it was significantly reduced in the raw gas as well as in the fly ashes by injection of ammonium sulphate. A link, the alkali chloride track, is demonstrated between the level of alkali chlorides in the gas phase, the chlorine content in the deposits in the convection pass and finally the PCDD/F formation. The formation of PCDD/Fs was also significantly reduced during co-combustion of SRF with municipal sewage sludge (MSS) compared to when SRF was fired without MSS as additional fuel.