Gdtyjr

I wish I could have the groups inside nomenclature in different pages each and their titles centered. So far I tried the following MWE:

documentclass[10pt,a4paper]article

usepackage[utf8]inputenc
usepackageetoolbox
usepackagenomencl
makenomenclature

renewcommandnomgroup[1]%
 item[centeringnormalsizebfseries
 ifstrequal#1A%
 MakeUppercaseList of Abbreviations and Acronyms%
 %
 ifstrequal#1SMakeUppercaseList of Symbols%
 ]thispagestyleempty

renewcommandnomname

begindocument
 nomenclature[A]H2OWater
 nomenclature[A]CO2Chemical 1
 nomenclature[A]COChemical 2
 nomenclature[A]HCChemical 3
 nomenclature[A]kWpkilowatt-peak
 nomenclature[S]EPEResearch Electrical Company

 printnomenclature[1cm]
enddocument

I got this:

But I wish I have them in different pages and the titles centered like this:

The solution here seem to solve the different pages problem but it doesn't solve the title centered. Could anyone help me please?

The issue here is that command nomgroup needs to be an item to be printed in the nomenclature. Therefore your try with centering (that is inside item[] could not work.

A workaround is to use the following code (see both used hfill outside the label definition of item; important code marked with <======):

 item[]% <=============================================================
 hfilnormalsizebfseries % <======================================
 ifstrequal#1A%
 MakeUppercaseList of Abbreviations and Acronyms%
 %
 ifstrequal#1SMakeUppercaseList of Symbols%
 %
 hfil % <==========================================================

Please see that this kind of centering is not really perfect because the length of the label is not included ... That is the reason I call it a workaround ...

With the following complete code

documentclass[10pt,a4paper]article

usepackage[utf8]inputenc
usepackageetoolbox

usepackageshowframe% <============ to visualize text area and margins

usepackagenomencl
makenomenclature

newififfirstglossaryfirstglossarytrue
renewcommandnomgroup[1]%
 iffirstglossary
 firstglossaryfalse
 else
 clearpage
 fi
 item[]% <=============================================================
 hfilnormalsizebfseries % <======================================
 ifstrequal#1A%
 MakeUppercaseList of Abbreviations and Acronyms%
 %
 ifstrequal#1SMakeUppercaseList of Symbols%
 %
 hfil % <==========================================================
%] % <==================================================================
 thispagestyleempty

renewcommandnomname


begindocument

nomenclature[A]H2OWater
nomenclature[A]CO2Chemical 1
nomenclature[A]COChemical 2
nomenclature[A]HCChemical 3
nomenclature[A]kWpkilowatt-peak
nomenclature[S]EPEResearch Electrical Company

printnomenclature[1cm]
enddocument

you get the following result: