CO2 is going to be neutral for the peels. You're just transporting it from where the oranges grew to where they were dumped. The CO2 benefit is purely from the trees and other biomass that grow where they wouldn't be growing before.

As for methane, that's a good question. Orange peels are better than most things because the limonene inhibits methane producing bacteria. But you'd still get quite a bit in the deeper piles (that produce the anaerobic conditions needed for methane production).

Spreading them out more would help, but might interfere with the beneficial effects.

The orange peel is going to decompose and produce CO2 either way. Methane is produced when there is not enough oxygen available while decomposing, which certainly seems a possibility if it's dumped in big piles.

You're getting downvoted but it's a reasonable question if posed in good faith. The tl;dr is that there are really a few options for what could happen to those orange peels:

(1) Landfill burial

   (1a) Without methane capture and use: Produces methane, relatively high short term warming potential.
   (1b) With methane capture and use: Ends up as CO2 after burning the methane.

(2) Composting (this approach)

   (2a) Mostly aerobic: Produces CO2
   (2b) Mostly anaerobic: Produces methane

A deep pile that is never turned will decompose anaerobically, resulting in fairly undesirable methane. A shallower pile or one that is mixed well will result in mostly aerobic decomposition. The aerobic decomposition will produce CO2 but not huge amounts of it. Each hectare of land could absorb something like ~8 tons of CO2 per year; with 7 hectares, the CO2 emitted by composting 12t of oranges is going to be dwarfed by the new vegetation. After a few years when you're growing big trees, the rate of CO2 absorption might rise as high as 20-30t/year/hectare in costa rica's environment. And this is probably an underestimate, as the soil amendment of the orange peels seems to have stimulated faster regrowth than would have happened otherwise.

And perhaps more to the point: There isn't really a purely "no co2" way of disposing of organic matter other than perhaps burying it at the bottom of a deep mineshaft (but the co2 or methane will be produced anyway). Landfilling it is strictly worse - you still get the decomposition products, _or worse_ because you'll mostly get methane, but without producing useful soil byproducts.

Overall this project is a huge win on a carbon perspective and a waste reduction perspective.