Anvendelse af naturgas sammen med fast biobr�ndsel

Asger N. Myken og Niels Bjarne K. Rasmussen, marts 2000, R0004

Sammenfatning og konklusion

De faste biobr�ndsler omfatter prim�rt halm, affald og tr�. Disse br�ndsler er typisk vanskeligere at anvende end de traditionelle (fossile) br�ndsler.

Naturgas kan i nogle tilf�lde med fordel anvendes som supplement til biomassen. De typiske fordele, som kan opn�s, er:

emissionsbegr�nsning
virkningsgradsfor�gelse
forbedring af forbr�ndingskvalitet ved varierende biomasseegenskaber
forsyningssikkerhed
kapacitetsfor�gelse
justering af br�ndv�rdi

I 2012 forventes vedvarende energi at udg�re ca. 120 PJ, svarende til 15% af Danmarks bruttoenergiforbrug p� ca. 820 PJ. Ud af det samlede forbrug af vedvarende energi forventes biomassen, dvs. halm (ca. 23 PJ), affald (ca. 32 PJ) og tr� (ca. 23 PJ), at udg�re ca. 80 PJ. Den �gede anvendelse af faste biobr�ndsler kan fortr�nge/konkurrere med naturgasanvendelse til fx kraftvarmeproduktion. I 2012 forventes naturgasforbruget at v�re steget fra det eksisterende niveau p� ca. 157 PJ (1998) til ca. 200 PJ.

Som et groft sk�n for det samlede tekniske potentiale for kombineret anvendelse af faste biobr�ndsler og naturgas antages, at naturgassen i gennemsnit kan udg�re op til ca. 5% af det samlede biomassepotentiale. Dermed f�s et potentiale for naturgasforbruget til de kombinerede anvendelser p� ca. 4 PJ (2012). Det svarer til 100 mio. kubikmeter naturgas, eller 2,5% af det nuv�rende indenlandske forbrug (1998).

Disse sk�n er baseret p� de metoder og teknologier for anvendelse af biomasse, som anses for teknisk og �konomisk realistiske indenfor den n�rmeste �rr�kke. Der forskes intensivt i forgasning af biomasse, og p� l�ngere sigt er det muligt, at udviklingen inden for dette omr�de �ndrer billedet for anvendelse af biomasse. Dermed �ndres ogs� mulighederne og potentialet for kombineret anvendelse af naturgas og biomasse.

For at opn� maksimal elvirkningsgrad ved elproduktion p� dampkredse anvendes h�jest mulige dampdata (tryk/temperatur). Naturgas kan i visse tilf�lde med fordel anvendes som supplement til biomassebaserede v�rker til overhedning af dampen efter opvarmning i den prim�re kedel.

Ved fyring med halm kan der dannes massive bel�gninger i kedlens konvektionsdel. S�danne bel�gninger p� overhederr�rene kan medf�re en accelereret korrosion ved damptemperaturer over 520�C. Det kan derfor v�re en fordel eller n�dvendigt at operere med lavere udgangstemperaturer fra halmkedlen, og derefter anvende 100% naturgasfyring i en efterf�lgende overheder.

P� fjernvarmev�rker etableres normalt en olie- eller gaskedel, der kan d�kke hele effektbehovet til brug ved spidslast, reparation eller havari p� biomassekedlen. For et typisk fjernvarmeanl�g kan det �rlige gasforbrug udg�re omkring 7% af det samlede energiforbrug.

Foruden spidslast- og backupfunktioner kan der opn�s en r�kke fordele ved samfyring af tr� og naturgas sammenlignet med 100% tr�fyring. I mods�tning til overhedere p� kraftvarmeanl�g foreg�r forbr�ndingen af naturgas i samme forbr�ndingskammer som tr�et. Derved kan der opn�s v�sentlige reduktioner af emissionerne af is�r CO, men ogs� af NO_x og partikler. Der kan endvidere opn�s en r�kke driftsm�ssige fordele ved at anvende naturgas som st�ttebr�ndsel.

Mindre kedler til individuelle anl�g/blokcentraler m.m. har ofte h�je emissioner af uforbr�ndte kulbrinter, tj�re, PAH og organiske komponenter. Ved kombineret drift med naturgas kan emissionerne reduceres og �rsnyttevirkningen forbedres, idet gaskedlen kan d�kke enten lavlast- eller spidslastdrift, s�ledes at modulationsomr�det for biobr�ndselsenheden mindskes. Brugerne opn�r endvidere en forbedret komfort og forsyningssikkerhed, idet varmeforsyningen er sikret, selvom der ikke fyres manuelt hver dag.

Abstract in English

Solid biomass fuels comprise primarily straw, waste and wood. These fuels are typically more difficult to exploit than traditional (fossil) fuels. In some cases natural gas can be used as supplement to the biomass fuels. Typical advantages are:

reduction of emissions
efficiency improvement
improvement of combustion quality at unstable biomass properties
reliability of supply
increasement of capacity
adjustment of calorific value

By 2012 sustainable energy resources are expected to account for approximately 120 PJ, corresponding to 15% of Denmark�s gross energy consumption of approximately 820 PJ. Among the sustainable energy resources, the consumption of solid biomass, i.e. straw (appr. 23 PJ), waste (appr. 32 PJ) and wood (appr. 23 PJ), is expected to make up approximately 80 PJ. The increased consumption of solid biomass can supersede/compete with natural gas utilisation for e.g. combined heat and power (CHP) production. The consumption of natural gas is expected to increase from the present level of 157 PJ (1998) to approximately 200 PJ by 2012.

As a rough estimate of the technical potential for combined utilisation of solid biomass fuels and natural gas, it is assumed that natural gas can account for up to 5% in average of the total biomass potential. This means that the potential natural gas consumption in combined applications is approximately 4 PJ, corresponding to 100 million cubic metres or 2.5% of the present domestic annual natural gas consumption (1998).

These estimates are based on the methods and technology for biomass utilisation that is considered as technical and economic feasible within a number of years. Intensive research is conducted on gasification of biomass, and the development in this field may change the future situation of biomass utilisation. Hence, also the options and potential for combined utilisation of natural gas and biomass is changed.

When producing power based on steam cycles, the steam temperature and pressure is maximised in order to obtain the highest possible electrical efficiency. In some cases it is advantageous to use natural gas for superheating of the steam after the primary boiler in biomass based CHP plants.

In boilers heated with straw deposits can occur in the boiler. These deposits can accelerate corrosion at steam temperatures above 520�C. As a consequence, it can be an advantage or necessary to operate the boiler at lower exit temperatures from the straw fired boiler and then use 100% natural gas firing in a superheater following the primary boiler.

A oil- or gasfired boiler is normally installed at district heating plants and used as supplement to the biomass fired boiler during peak load and as backup during overhaul and breakdown. For a typical district heating plant the annual natural gas consumption accounts for approximately 7% of the total energy consumption.

Besides peak load and backup functions, a number of advantages can be obtained by co-firing of wood and natural gas compared to 100% wood firing. Unlike superheaters at CHP plants, the combustion of natural gas and wood takes place in the same combustion chamber. As a consequence, substantial reductions of the emissions of in particularly CO, but also NOx and particles, can be obtained. Furthermore, several operational advantages can be obtained using co-firing with natural gas.

The emissions of unburned hydrocarbons, tar, PAH and VOC from small boilers for e.g. domestic applications and central heating is often high. Combined operation with natural gas can reduce the emissions and the annual average efficiency. Natural gas combustion can be used for operation at either low load or peak load, which means that the required modulation range of biomass boiler is reduced. Furthermore, the user obtains an improved comfort and supply of reliability, since the heating system is not dependent on manual firing of e.g. wood on a daily basis.